''Galileo'' was an American robotic space program that studied the planet
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
and
its moons, as well as several other
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
bodies. Named after the Italian astronomer
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
, the
''Galileo'' spacecraft consisted of an orbiter and an entry probe. It was delivered into Earth orbit on October 18, 1989 by on the
STS-34 mission, and arrived at Jupiter on December 7, 1995, after
gravitational assist flybys of
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
and
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, and became the first spacecraft to orbit Jupiter. It launched the first probe into Jupiter, directly measuring its
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
. Despite suffering major antenna problems, ''Galileo'' achieved the first
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
flyby, of
951 Gaspra
951 Gaspra is an S-type asteroid that orbits very close to the inner edge of the asteroid belt. Gaspra was discovered by Russian astronomer G. N. Neujmin in 1916. Neujmin named it after Gaspra, a Black Sea retreat that was visited by his contemp ...
, and discovered the first
asteroid moon
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important ...
, Dactyl, around
243 Ida
Ida, minor planet designation 243 Ida, is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after a nymph from Greek mythology. ...
. In 1994, ''Galileo'' observed
Comet Shoemaker–Levy 9
Comet Shoemaker–Levy 9 ( formally designated D/1993 F2) broke apart in July 1992 and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. This generated a l ...
's collision with Jupiter.
Jupiter's atmospheric composition and
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
clouds were recorded.
Io's
volcanism
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...
and
plasma
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
interactions with Jupiter's atmosphere were also recorded. The data ''Galileo'' collected supported
the theory of a liquid ocean under the icy surface of
Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
, and there were indications of similar liquid-
saltwater
Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish water, ...
layers under the surfaces of
Ganymede and
Callisto Callisto most commonly refers to:
*Callisto (mythology), a nymph
*Callisto (moon), a moon of Jupiter
Callisto may also refer to:
Art and entertainment
*''Callisto series'', a sequence of novels by Lin Carter
*''Callisto'', a novel by Torsten Kro ...
. Ganymede was shown to possess a
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
and the spacecraft found new evidence for
exosphere
The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the densit ...
s around Europa, Ganymede, and Callisto. ''Galileo'' also discovered that Jupiter's faint
ring system
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as ...
consists of dust from impacts on the four small inner moons. The extent and structure of Jupiter's
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
was also mapped.
On September 20, 2003, after 14 years in space and 8 years in the Jovian system, ''Galileo'' mission was terminated by sending it into Jupiter's atmosphere at a speed of over , completely eliminating the possibility of
contaminating local moons with terrestrial bacteria.
Background
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
is the largest planet in the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
, with more than twice the mass of all the other planets combined. Consideration of sending a probe to Jupiter began as early as 1959, when the
National Aeronautics and Space Administration
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding th ...
(NASA)
Jet Propulsion Laboratory
The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States.
Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...
(JPL) developed four mission concepts:
* Deep space flights would fly through interplanetary space;
* Flyby missions would fly past planets, and could visit multiple planets on a single mission;
* Orbiter missions would place a probe in orbit around a planet for detailed study;
* Planetary entry and lander missions, which would explore the atmosphere and surface.
Two missions to Jupiter, ''
Pioneer 10
''Pioneer 10'' (originally designated Pioneer F) is an American space probe, launched in 1972 and weighing , that completed the first mission to the planet Jupiter. Thereafter, ''Pioneer 10'' became the first of five artificial objects to ach ...
'' and ''
Pioneer 11
''Pioneer 11'' (also known as ''Pioneer G'') is a robotic space probe launched by NASA on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays. It was the first probe to encounter ...
'', were approved in 1969, with NASA's
Ames Research Center
The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laborat ...
given responsibility for planning the missions. ''Pioneer 10'' was launched in March 1972 and passed within of Jupiter in December 1973. It was followed by ''Pioneer 11'', which was launched in April 1973, and passed within of Jupiter in December 1974, before heading on to an encounter with
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
. They were followed by the more advanced ''
Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
'' and ''
Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
'' spacecraft, which were launched on 5 September and 20 August 1977 respectively, and reached Jupiter in March and July 1979.
Planning
Initiation
Following the approval of the ''Voyager'' missions, NASA's Scientific Advisory Group (SAG) for Outer Solar System Missions considered the requirements for Jupiter orbiters and atmospheric probes. It noted that the technology to build a
heat shield
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
for an atmospheric probe did not yet exist, and indeed facilities to test one under the conditions found on Jupiter would not be available until 1980. There was also concern about the effects of radiation on spacecraft components, which would be better understood after ''Pioneer 10'' and ''Pioneer 11'' had conducted their flybys. These indicated that the effects were less severe than feared. NASA management designated the JPL as the lead center for the Jupiter Orbiter Probe (JOP) Project.
John R. Casani, who had headed the Mariner and Voyager projects, became the first project manager. The JOP would be the fifth spacecraft to visit Jupiter, but the first to orbit it, and the probe would be the first to enter its atmosphere.
![Galileo Preparations - GPN-2000-000672](https://upload.wikimedia.org/wikipedia/commons/1/1d/Galileo_Preparations_-_GPN-2000-000672.jpg)
An important decision made at this time by Ames and the JPL was to use a
Mariner program
The Mariner program was conducted by the American space agency NASA to explore other planets. Between 1962 and late 1973, NASA's Jet Propulsion Laboratory (JPL) designed and built 10 robotic interplanetary probes named Mariner to explore the in ...
spacecraft like that used for Voyager for the Jupiter orbiter, rather than a Pioneer. Pioneer was stabilized by spinning the spacecraft at 60
rpm
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
, which gave a 360-degree view of the surroundings, and did not require an attitude control system. By contrast, Mariner had an attitude control system with three
gyroscopes
A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rotat ...
and two sets of six
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
jet thrusters. Attitude was determined with reference to the Sun and
Canopus
Canopus is the brightest star in the southern constellation of Carina (constellation), Carina and the list of brightest stars, second-brightest star in the night sky. It is also Bayer designation, designated α Carinae, which is Lat ...
, which were monitored with two primary and four secondary sensors. There was also an
inertial reference unit
An inertial reference unit (IRU) is a type of inertial sensor which uses gyroscopes (electromechanical, ring laser gyro or MEMS) and accelerometers (electromechanical or MEMS) to determine a moving aircraft’s or spacecraft’s change in rotatio ...
and an
accelerometer
An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is accele ...
. This allowed it to take high resolution images, but the functionality came at a cost of increased weight. A Mariner weighed compared to just for a Pioneer.
The Voyager spacecraft had been launched by
Titan IIIE
The Titan IIIE or Titan 3E, also known as the Titan III-Centaur, was an American expendable launch system. Launched seven times between 1974 and 1977, it enabled several high-profile NASA missions, including the Voyager program, Voyager and Viki ...
rockets with a
Centaur
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as being ...
upper stage, but Titan was retired afterwards. In the late 1970s, NASA was focused on the development of the reusable
Space Shuttle
The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...
, which was expected to make expendable rockets obsolete. In late 1975, NASA decreed that all future planetary missions would be launched by the Space Shuttle. The JOP would be the first to do so. The Space Shuttle was supposed to have the services of a
space tug
''Space Tug'' is a young adult science fiction novel by author Murray Leinster. It was published in 1953 by Shasta Publishers in an edition of 5,000 copies. It is the second novel in the author's Joe Kenmore series. Groff Conklin gave it a mixe ...
to launch payloads requiring something more than a
low Earth orbit
A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never mor ...
, but this was never approved. The
United States Air Force
The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Signal ...
then developed the
solid-fueled
Solid fuel refers to various forms of solid material that can be burnt to release energy, providing heat and light through the process of combustion. Solid fuels can be contrasted with liquid fuels and gaseous fuels. Common examples of solid fuel ...
Interim Upper Stage (IUS), later renamed the
Inertial Upper Stage
The Inertial Upper Stage (IUS), originally designated the Interim Upper Stage, was a two-stage, solid-fueled space launch system developed by Boeing for the United States Air Force beginning in 1976 for raising payloads from low Earth orbit to h ...
(with the same acronym), for the purpose.
The IUS was not powerful enough to launch a payload to Jupiter without resorting to using a series of
gravitational slingshot
In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the ...
maneuvers around planets to garner additional speed, something most engineers regarded as inelegant, and which planetary scientists at JPL disliked because it meant that the mission would take months or years longer to reach Jupiter. Longer travel times meant that components would age and the onboard power supply and propellant would be depleted. Some of the gravity assist options also meant flying closer to the Sun, which would induce thermal stresses. However, the IUS was constructed in a modular fashion, with two stages, a large one with of propellant, and a smaller one with . This was sufficient for most satellites. It could also be configured with two large stages to launch multiple satellites. A configuration with three stages, two large and one small, would be enough for a planetary mission, so NASA contracted with
Boeing
The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and product ...
for the development of a three-stage IUS.
It was estimated that the JOP would cost $634 million (equivalent to $ million in ), and it had to compete for
fiscal year
A fiscal year (or financial year, or sometimes budget year) is used in government accounting, which varies between countries, and for budget purposes. It is also used for financial reporting by businesses and other organizations. Laws in many ...
1978 funding with the Space Shuttle and the
Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
. A successful lobbying campaign secured funding for both JOP and Hubble over the objections of
Senator
A senate is a deliberative assembly, often the upper house or chamber of a bicameral legislature. The name comes from the ancient Roman Senate (Latin: ''Senatus''), so-called as an assembly of the senior (Latin: ''senex'' meaning "the el ...
William Proxmire
Edward William Proxmire (November 11, 1915 – December 15, 2005) was an American politician. A member of the Democratic Party, he served as a United States Senator from Wisconsin from 1957 to 1989. He holds the record for being the longest-servi ...
, the chairman of the Independent Agencies Appropriations Subcommittee. The
United States Congress
The United States Congress is the legislature of the federal government of the United States. It is bicameral, composed of a lower body, the House of Representatives, and an upper body, the Senate. It meets in the U.S. Capitol in Washing ...
approved funding for the Jupiter Orbiter Probe on July 12, 1977, and JOP officially commenced on October 1, 1977, the start of the fiscal year. Casani solicited suggestions for a more inspirational name for the project, and the most votes went to "Galileo" after
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
, the first person to view Jupiter through a telescope, and the discoverer of what is now known as the
Galilean moons
The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter ...
in 1610. It was noted that the name was also that of a
spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
in the ''
Star Trek
''Star Trek'' is an American science fiction media franchise created by Gene Roddenberry, which began with the eponymous 1960s television series and quickly became a worldwide pop-culture phenomenon. The franchise has expanded into vari ...
'' television show. The new name was adopted in February 1978.
Preparation
Early plans called for a launch on on
STS-23 sometime between 2 and 12 January 1982, this being the launch window when Earth, Jupiter and
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
were aligned in such a way as to permit Mars to be used for a
gravitational slingshot
In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the ...
maneuver. To enhance reliability and reduce costs, the ''Galileo'' project's engineers decided to switch from a pressurized atmospheric entry probe to a vented one. This added to its weight. Another was added in structural changes to improve reliability. This would require additional fuel in the IUS. But the three-stage IUS was itself overweight, by about .
![Model of Centaur G with Galileo probe (upright)](https://upload.wikimedia.org/wikipedia/commons/d/df/Model_of_Centaur_G_with_Galileo_probe_%28upright%29.jpg)
Lifting ''Galileo'' and the IUS would require the use of the special lightweight version of the
Space Shuttle external tank
The Space Shuttle external tank (ET) was the component of the Space Shuttle launch vehicle that contained the liquid hydrogen fuel and liquid oxygen oxidizer. During lift-off and ascent it supplied the fuel and oxidizer under pressure to the ...
, the
Space Shuttle orbiter
The Space Shuttle orbiter is the spaceplane component of the Space Shuttle, a partially reusable orbital spacecraft system that was part of the discontinued Space Shuttle program. Operated from 1977 to 2011 by NASA, the U.S. space agency, thi ...
stripped of all non-essential equipment, and the
Space Shuttle main engine
The Aerojet Rocketdyne RS-25, also known as the Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is currently used on the Space Launch System (SLS).
Designed and manufacture ...
s (SSME) running at full power—109 percent of their rated power level. Running at this power level necessitated the development of a more elaborate engine cooling system. By 1980, delays in the Space Shuttle program pushed the launch date for ''Galileo'' back to 1984. While a Mars slingshot was still possible in 1984, it would no longer be sufficient.
NASA decided to split ''Galileo'' into two separate spacecraft, an
atmospheric probe and a Jupiter orbiter, with the orbiter launched in February 1984 and the probe following a month later. The orbiter would be in orbit around Jupiter when the probe arrived, allowing it to perform its role as a relay. Separating the two spacecraft required a second mission and a second carrier to be built for the probe, and was estimated to cost an additional $50 million (equivalent to $ million in ), but NASA hoped to be able to recoup some of this through separate completive bidding on the two. The problem was that while the atmospheric probe was light enough to launch with the two-stage IUS, the Jupiter orbiter was too heavy to do so, even with a gravity assist from Mars, so the three-stage IUS was still required.
By late 1980, the price tag for the IUS had risen to $506 million (equivalent to $ million in ). The USAF could absorb this cost overrun (and indeed had anticipated that it might cost far more), but NASA was faced with a quote of $179 million (equivalent to $ million in ) for the development of the three-stage version, which was $100 million (equivalent to $ million in ) more than it had budgeted for. At a press conference on January 15, 1981,
NASA Administrator
The Administrator of the National Aeronautics and Space Administration is the highest-ranking official of NASA, the national space agency of the United States. The administrator is NASA's chief decision maker, responsible for providing clarity to ...
Robert A. Frosch
Robert Alan Frosch FREng (May 22, 1928 – December 30, 2020) was an American scientist who was the fifth administrator of NASA. He was the administrator from 1977 to 1981 during the Carter administration.
Biography
Born in New York City, Fro ...
announced that NASA was withdrawing support for the three-stage IUS, and going with a
Centaur G Prime upper stage because "no other alternative upper stage is available on a reasonable schedule or with comparable costs."
Centaur provided many advantages over the IUS. The main one was that it was far more powerful. The probe and orbiter could be recombined, and the probe could be delivered directly to Jupiter in two years' flight time. The second was that despite this, it was also more gentle than the IUS, as it had lower thrust, thereby minimizing the chance of damage to the payload. Thirdly, unlike solid-fuel rockets which burned to completion once ignited, Centaur could be switched off and on again. This gave it flexibility, which increased the chances of a successful mission, and permitted options like asteroid flybys. Centaur was proven and reliable, whereas the IUS had not yet flown. The only concern was about safety; solid-fuel rockets were considered safer than liquid-fuel ones, especially ones containing liquid hydrogen. NASA engineers estimated that additional safety features might take up to five years to develop and cost up to $100 million (equivalent to $ million in .
In February 1981, the JPL learned that the
Office of Management and Budget
The Office of Management and Budget (OMB) is the largest office within the Executive Office of the President of the United States (EOP). OMB's most prominent function is to produce the president's budget, but it also examines agency programs, pol ...
(OMB) was planning major cuts to NASA's budget, and was considering cancelling ''Galileo''. What saved it from cancellation was the intervention of the USAF. The JPL had considerable experience with autonomous spacecraft. This was a necessity for deep space probes, since a signal from Earth takes anything from 35 to 52 minutes to reach Jupiter. The USAF was interested in providing this capability for its satellites so that they would be able to determine their attitude using onboard systems rather than relying on
ground station
A ground station, Earth station, or Earth terminal is a terrestrial radio station designed for extraplanetary telecommunication with spacecraft (constituting part of the ground segment of the spacecraft system), or reception of radio waves fro ...
s, which were not "hardened" against nuclear attacks, and could take evasive action in the face of anti-satellite weapons. It was also interested in the manner in which the JPL was designing ''Galileo'' to withstand the intense radiation of the
magnetosphere of Jupiter
The magnetosphere of Jupiter is the cavity created in the solar wind by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosp ...
. On February 6, 1981
Strom Thurmond
James Strom Thurmond Sr. (December 5, 1902June 26, 2003) was an American politician who represented South Carolina in the United States Senate from 1954 to 2003. Prior to his 48 years as a senator, he served as the 103rd governor of South Caro ...
, the
President pro tempore of the Senate
A president pro tempore or speaker pro tempore is a constitutionally recognized officer of a legislative body who presides over the chamber in the absence of the normal presiding officer. The phrase ''pro tempore'' is Latin "for the time being".
...
, wrote directly to
David Stockman
David Alan Stockman (born November 10, 1946) is an American politician and former businessman who was a Republican U.S. Representative from the state of Michigan (1977–1981) and the Director of the Office of Management and Budget (1981–1985) ...
, the Director of the OMB, arguing that ''Galileo'' was vital to the nation's defense.
![Astronauts John Fabian and Dave Walker pose in front of a model of the Shuttle-Centaur](https://upload.wikimedia.org/wikipedia/commons/a/a6/Astronauts_John_Fabian_and_Dave_Walker_pose_in_front_of_a_model_of_the_Shuttle-Centaur.jpg)
In December 1984 Casani proposed adding a flyby of asteroid
29 Amphitrite
Amphitrite (minor planet designation: 29 Amphitrite) is one of the largest S-type asteroids, approximately in diameter, and probably fifth largest after Eunomia, Juno, Iris and Herculina.
Discovery
Amphitrite was discovered by Albert Marth o ...
to the ''Galileo'' mission. In plotting a course to Jupiter, the engineers were concerned to avoid asteroids. Little was known about them at the time, and it was suspected that they could be surrounded by dust particles. Flying through a dust cloud could damage the spacecraft's optics and possibly the spacecraft itself. To be safe, the JPL wanted to avoid asteroids by at least . Most of the asteroids in the vicinity of the flight path like
1219 Britta and
1972 Yi Xing were only a few kilometers in diameter and posed little value when observed from a safe distance, but 29 Amphitrite was one of the largest of the asteroids, and a flyby at even could have great scientific value. The flyby would delay the spacecraft's arrival in Jupiter orbit from August 29 to December 10, 1988, and the expenditure of propellant would reduce the number of orbits of Jupiter from eleven to ten. This was expected to add $20 to $25 million (equivalent to $ to $ million in ) to the cost of the ''Galileo'' project. The 29 Amphitrite flyby was approved by NASA Administrator
James M. Beggs
James Montgomery Beggs (January 9, 1926 – April 23, 2020) was an American businessman and consultant who served as the 6th Administrator of NASA from 1981 to 1986.
Early life and education
James Montgomery Beggs was born in Pittsburgh, Pe ...
on December 6, 1984.
During testing, contamination was discovered in the system of metal
slip ring
A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure. A slip ring can be used in any electromechanical system that requires rotation while transmitting p ...
s and brushes used to transmit electrical signals around the spacecraft, and they were returned to be refabricated. The problem was traced back to a
chlorofluorocarbon
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and prop ...
used to clean parts after soldering. It had been absorbed, and was then released in a vacuum environment. It mixed with debris generated as the brushes wore down, and caused intermittent problems with electrical signal transmission. Problems were also detected in the performance of memory devices in an electromagnetic radiation environment. The components were replaced, but then a
read disturb
Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use ...
problem arose, in which reads from one memory location disturbed those in adjacent locations. This was found to have been caused by the changes made to make the components less sensitive to electromagnetic radiation. Each component had to be removed, retested, and replaced. All of the spacecraft components and spare parts received a minimum of 2,000 hours of testing. The spacecraft was expected to last for at least five years—long enough to reach Jupiter and perform its mission. On December 19, 1985, it departed the JPL in
Pasadena, California
Pasadena ( ) is a city in Los Angeles County, California, northeast of downtown Los Angeles. It is the most populous city and the primary cultural center of the San Gabriel Valley. Old Pasadena is the city's original commercial district.
I ...
, on the first leg of its journey, a road trip to the
Kennedy Space Center
The John F. Kennedy Space Center (KSC, originally known as the NASA Launch Operations Center), located on Merritt Island, Florida, is one of the National Aeronautics and Space Administration's (NASA) ten field centers. Since December 1968 ...
in
Florida
Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...
. The ''Galileo'' mission was scheduled for
STS-61-G on May 20, 1986, using .
Reconsideration
On January 28, 1986, lifted off on the
STS-51-L
STS-51-L was the 25th mission of the NASA Space Shuttle program and the final flight of Space Shuttle ''Challenger''.
Planned as the first Teacher in Space Project flight in addition to observing Halley's Comet for six days and performing a ...
mission. A failure of the solid rocket booster 73 seconds into flight tore the spacecraft apart, resulting in the deaths of all seven crew members. The
Space Shuttle ''Challenger'' disaster was America's worst space disaster up to that time. The immediate impact on the ''Galileo'' project was that the May launch date could not be met because the Space Shuttles were grounded while the cause of the disaster was investigated. When they did fly again, ''Galileo'' would have to compete with high priority
Department of Defense Department of Defence or Department of Defense may refer to:
Current departments of defence
* Department of Defence (Australia)
* Department of National Defence (Canada)
* Department of Defence (Ireland)
* Department of National Defense (Philipp ...
launches, the
tracking and data relay satellite
A tracking and data relay satellite (TDRS) is a type of communications satellite that forms part of the Tracking and Data Relay Satellite System (TDRSS) used by NASA and other United States government agencies for communications to and fro ...
system, and the Hubble Space Telescope. By April 1986, it was expected that the Space Shuttles would not fly again before July 1987 at the earliest, and ''Galileo'' could not be launched before December 1987.
![Animation of Galileo trajectory](https://upload.wikimedia.org/wikipedia/commons/0/05/Animation_of_Galileo_trajectory.gif)
The
Rogers Commission
The Rogers Commission Report was written by a Presidential Commission charged with investigating the Space Shuttle ''Challenger'' disaster during its 10th mission, STS-51-L. The report, released and submitted to President Ronald Reagan on Jun ...
handed down its report on June 6, 1986. It was critical of NASA's safety protocols and risk management. In particular, it noted the hazards of Centaur-G stage. On June 19, 1986, NASA Administrator
James C. Fletcher
James Chipman Fletcher (June 5, 1919 – December 22, 1991) served as the 4th and 7th Administrator of NASA, first from April 27, 1971 to May 1, 1977, under President Richard M. Nixon, and again from May 12, 1986 to April 8, 1989, under Pres ...
canceled the Shuttle-Centaur project. This was only partly due to the NASA management's increased aversion to risk in the wake of the ''Challenger'' disaster; NASA management also considered the money and manpower required to get the Space Shuttle flying again, and decided that there was insufficient resources to resolve lingering issues with Shuttle-Centaur as well. The changes to the Space Shuttle proved more extensive than anticipated, and in April 1987 the JPL was informed that ''Galileo'' could not be launched before October 1989. The ''Galileo'' spacecraft was shipped back to the JPL.
Without Centaur, it looked like there would not be any means of getting the spacecraft to Jupiter, and it looked for a time like its next trip would be to the
Smithsonian Institution
The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Founded ...
.
The cost of keeping it ready to fly in space was reckoned at $40 to $50 million per year (equivalent to $ to $ million in ), and the estimated cost of the whole project had blown out to $1.4 billion (equivalent to $ billion in ).
At the JPL, the ''Galileo'' Mission Design Manager and Navigation Team Chief, Robert Mitchell, assembled a team that consisted of Dennis Byrnes, Louis D'Amario, Roger Diehl and himself, to see if they could find a trajectory that would get ''Galileo'' to Jupiter using only a two-stage IUS. Roger Diehl came up with the idea of using a series of gravitational slingshots to provide the additional velocity required to reach Jupiter. This would require ''Galileo'' to fly past Venus, and then past Earth twice. This was referred to as the Venus-Earth-Earth Gravity Assist (VEEGA) trajectory.
The reason no one had thought of it before was that the second encounter with Earth would not give the spacecraft any extra energy. Diehl realised that this was not necessary; the second encounter with Earth would merely change its direction to put it on a course for Jupiter. In addition to increasing the flight time to six years, the VEEGA trajectory had an additional drawback from the point of view of
NASA Deep Space Network
The NASA Deep Space Network (DSN) is a worldwide network of American spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA's interplanetary ...
(DSN): ''Galileo'' would arrive at Jupiter when it was at the maximum range from Earth, and maximum range meant minimum signal strength. Furthermore, it would have a southerly declination of −23 degrees instead of a northerly one of +18 degrees, so the main tracking station would be the
Canberra Deep Space Communication Complex
The Canberra Deep Space Communication Complex (CDSCC) is a satellite communication station, part of the Deep Space Network of NASA's Jet Propulsion Laboratory (JPL), located at Tidbinbilla in the Australian Capital Territory. Opened in 19 ...
in Australia, with its two 34-meter and one 70-meter antennae. This was supplemented the 64-meter antenna at the
Parkes Observatory
Parkes Observatory is a radio astronomy observatory, located north of the town of Parkes, New South Wales, Australia. It hosts Murriyang, the 64 m CSIRO Parkes Radio Telescope also known as "The Dish", along with two smaller radio telescopes. T ...
.
![Galileo probe deployed (large)](https://upload.wikimedia.org/wikipedia/commons/6/65/Galileo_probe_deployed_%28large%29.jpg)
Initially it was thought that the VEEGA trajectory demanded a November launch, but D'Amario and Byrnes calculated that a mid-course correction between Venus and Earth would permit an October launch as well. Taking such a roundabout route meant that ''Galileo'' would require sixty months to reach Jupiter instead of just thirty, but it would get there.
Consideration was given to using the USAF's
Titan IV
Titan IV was a family of heavy-lift space launch vehicles developed by Martin Marietta and operated by the United States Air Force from 1989 to 2005. Launches were conducted from Cape Canaveral Air Force Station, Florida and Vandenberg Air For ...
launch system with its Centaur G Prime upper stage. This was retained as a backup for a time, but in November 1988 the USAF informed NASA that it could not provide a Titan IV in time for the May 1991 launch opportunity, owing to the backlog of high priority Department of Defense missions. However, the USAF supplied IUS-19, which had originally been earmarked for a Department of Defense mission, for use by the ''Galileo'' mission.
As the launch date of ''Galileo'' neared,
anti-nuclear groups
Anti-nuclear organizations may oppose uranium mining, nuclear power, and/or nuclear weapons. Anti-nuclear groups have undertaken public protests and acts of civil disobedience which have included occupations of nuclear plant sites. Some of the mo ...
, concerned over what they perceived as an unacceptable risk to the public's safety from the
plutonium
Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
in the ''Galileo''
radioisotope thermoelectric generator
A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioacti ...
s (RTGs) and General Purpose Heat Source (GPHS) modules, sought a court injunction prohibiting ''Galileo'' launch.
RTGs were necessary for deep space probes because they had to fly distances from the Sun that made the use of solar energy impractical.
They had been used for years in planetary exploration without mishap: the Department of Defense's
Lincoln Experimental Satellite
The Lincoln Experimental Satellite series was designed and built by Lincoln Laboratory at Massachusetts Institute of Technology between 1965 and 1976, under USAF sponsorship, for testing devices and techniques for satellite communication.
Developm ...
s 8/9 had 7 percent more plutonium on board than ''Galileo'', and the two
''Voyager'' spacecraft each carried 80 percent of plutonium.
By 1989, plutonium had been used in 22 spacecraft.
Activists remembered the crash of the
Soviet Union
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national ...
's nuclear-powered
Kosmos 954
Kosmos 954 (russian: Космос 954) was a reconnaissance satellite launched by the Soviet Union in 1977. A malfunction prevented safe separation of its onboard nuclear reactor; when the satellite reentered the Earth's atmosphere the follow ...
satellite in Canada in 1978, and the ''Challenger'' disaster, while it did not involve nuclear fuel raised public awareness about spacecraft failures. No RTGs had ever done a non-orbital swing past the Earth at close range and high speed, as ''Galileo'' VEEGA trajectory required it to do. This created a novel mission failure modality that might plausibly have entailed dispersal of ''Galileo'' plutonium in the Earth's atmosphere. Scientist
Carl Sagan
Carl Edward Sagan (; ; November 9, 1934December 20, 1996) was an American astronomer, planetary scientist, cosmologist, astrophysicist, astrobiologist, author, and science communicator. His best known scientific contribution is research on ext ...
, a strong supporter of the ''Galileo'' mission, wrote that "there is nothing absurd about either side of this argument."
Before the ''Challenger'' disaster, the JPL had conducted shock tests on the RTGs that indicated that they could withstand a pressure of without a failure, which would have been sufficient to withstand an explosion on the launch pad. The possibility of adding additional shielding was considered but rejected, mainly because it would add an unacceptable amount of extra weight. After the ''Challenger'' disaster, NASA commissioned a study on the possible effects if such an event occurred with ''Galileo'' on board. Angus McRonald, a JPL engineer, concluded that what would happen would depend on the altitude at which the Space Shuttle broke up. If the ''Galileo''/IUS combination fell free of the orbiter at , the RTGs would fall to Earth without melting, and drop into the Atlantic Ocean about from the Florida coast. On the other hand, if the orbiter broke up at an altitude of it would be traveling at and the RTG cases and GPHS modules would melt before falling into the Atlantic off the Florida coast. NASA concluded that the chance of such a disaster was 1 in 2,500, although anti-nuclear groups thought it might be as high as 1 in 430.
The risk to an individual would be 1 in 100 million, about two orders of magnitude less than the danger of being killed by lightning. The prospect of an inadvertent re-entry into the atmosphere during the VEEGA maneuvers was reckoned at less than one in two million,
but an accident might have released up to .
Launch
STS-34 was the mission designated to launch ''Galileo'', scheduled for October 12, 1989, in the Space Shuttle ''Atlantis''. The spacecraft was delivered to the Kennedy Space Center by a high-speed truck convoy that departed the JPL in the middle of the night. There were fears that the spacecraft might be hijacked by anti-nuclear activists or terrorists, so the route was kept secret from the drivers, who drove through the night and the following day and only stopped for food and fuel.
Last minute efforts by three environmental groups to halt the launch were rejected by the
District of Columbia Circuit. In a concurring opinion, Chief Justice
Patricia Wald
Patricia Ann McGowan Wald (September 16, 1928 – January 12, 2019) was an American judge who served as the Chief United States circuit judge of the United States Court of Appeals for the District of Columbia Circuit (D.C. Circuit) and as a judg ...
wrote that while the legal challenge was not
frivolous, there was no evidence that NASA had acted improperly in compiling the mission's environmental assessment, and the appeal was therefore denied on technical grounds. On October 16, eight protesters were arrested for trespassing at the Kennedy Space Center; three were jailed and the remaining five released.
The launch was twice delayed; first by a faulty main engine controller that forced a postponement to October 17, and then by inclement weather, which necessitated a postponement to the following day,
but this was not a concern since the launch window extended until November 21.
''Atlantis'' finally lifted off at 16:53:40
UTC on October 18, and went into a orbit.
''Galileo'' was successfully deployed at 00:15 UTC on October 19. Following the IUS burn, the ''Galileo'' spacecraft adopted its configuration for solo flight, and separated from the IUS at 01:06:53 UTC on October 19.
The launch was perfect, and ''Galileo'' was soon headed towards Venus at over . ''Atlantis'' returned to Earth safely on October 23.
Venus encounter
The encounter with
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
on February 9 was in view of the DSN's Canberra and
Madrid Deep Space Communications Complexes. ''Galileo'' flew by at 05:58:48 UTC on February 10, 1990, at a range of .
Doppler data collected by the DSN allowed the JPL to verify that the gravitational assist maneuver had been successful, and the spacecraft had obtained the expected increase in speed. Unfortunately, three hours into the flyby, the tracking station at
Goldstone had to be shut down due to high winds, and Doppler data was lost.
![Galileo Venus global view](https://upload.wikimedia.org/wikipedia/commons/1/1f/Galileo_Venus_global_view.jpg)
Because Venus was much closer to the Sun than the spacecraft had been designed to operate, great care was taken to avoid thermal damage. In particular, the
X-band
The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is rather indefinitely set at approxim ...
high gain antenna
A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance ...
(HGA) was not deployed, but was kept folded up like an umbrella and pointed away from the Sun to keep it shaded and cool. This meant that the two small
S-band
The S band is a designation by the Institute of Electrical and Electronics Engineers (IEEE) for a part of the microwave band of the electromagnetic spectrum covering frequencies from 2 to 4 gigahertz (GHz). Thus it crosses the conventional ...
low gain antennae (LGA) had to be used instead. They had a maximum bandwidth of 1,200
bits per second
In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time.
The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
compared to the 134,000 bit/s expected from the HGA. As the spacecraft moved further from Earth, it also necessitated the use of the DSN's dishes, to the detriment of other users, who had lower priority than ''Galileo''. Even so, the downlink telemetry rate fell to 40 bit/s within a few days of the Venus flyby, and by March it was down to just 10 bit/s.
Venus had been the focus of many automated flybys, probes, balloons and landers, most recently the ''
Magellan
Ferdinand Magellan ( or ; pt, Fernão de Magalhães, ; es, link=no, Fernando de Magallanes, ; 4 February 1480 – 27 April 1521) was a Portuguese people, Portuguese explorer. He is best known for having planned and led the Magellan expeditio ...
'' spacecraft, and ''Galileo'' had not been designed with Venus in mind. Nonetheless, there were useful observations that it could make, as it carried some instruments that had never flown on spacecraft to Venus, such as the near-infrared mapping spectrometer (NIMS). Telescopic observations of Venus had revealed that there were certain parts of the infrared spectrum that the
greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
es in the Venusian atmosphere did not block, making them transparent on these wavelengths. This permitted the NIMS to both view the clouds and obtain maps of the equatorial and mid-latitudes of the night side of Venus with three to six times the resolution of Earth-based telescopes. The ultraviolet spectrometer (UVS) was also deployed to observe the Venusian clouds and their motions.
Another set of observations was conducted using Galileo's energetic particles detector (EPD) when ''Galileo'' moved through the
bow shock
In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of ...
caused by Venus's interaction with the
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
. Earth's strong magnetic field causes this to occur at around from its center, but Venus's weak magnetic field causes the bow wave to occur nearly on the surface, so the solar wind interacts with the atmosphere. A search for
lightning
Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
on Venus was conducted using the plasma wave detector, which noted nine bursts which were likely caused by lightning, but efforts to capture an image of lightning with the solid-state imaging system (SSI) were unsuccessful.
Earth encounters
Flybys
''Galileo'' made two small course corrections on 9 to 12 April and 11 to 12 May 1990. The spacecraft flew by
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
twice; the first time at a range of at 20:34:34 UTC on December 8, 1990.
This was only higher than predicted, and the time of the closest approach was only a second off. It was the first time that a deep space probe had returned to Earth from interplanetary space. A second flyby of Earth was at at 15:09:25 UTC on December 8, 1992.
This time the spacecraft passed within a kilometer of its aiming point over the South Atlantic. This was so accurate that a scheduled course correction was cancelled, thereby saving of propellant.
Earth's bow shock and the solar wind
![Galileo Earth - PIA00114](https://upload.wikimedia.org/wikipedia/commons/0/0b/Galileo_Earth_-_PIA00114.jpg)
The opportunity was taken to conduct a series of experiments. A study of Earth's bow shock was conducted as ''Galileo'' passed by Earth's day side. The solar wind travels at and is deflected by
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...
, creating a
magnetic tail
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dyn ...
on Earth's dark side over a thousand times the radius of the planet. Observations were made by ''Galileo'' when it passed through the magnetic tail on Earth's dark side at a distance of from the planet. The magnetosphere was quite active at the time, and ''Galileo'' detected magnetic storms and
whistlers
Whistler may refer to:
* Someone who whistles
Places
Canada
* Whistler, British Columbia, a resort town
** Whistler railway station
** Whistler Secondary School
* Whistler Blackcomb, a ski resort in British Columbia
* Whistler Mountain, Briti ...
caused by lightning strikes. The NIMS was employed to look for
mesospheric clouds, which are believed to be caused by
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
released by industrial processes. Normally they are only seen in September or October, but ''Galileo'' was able to detect them in December, an indication of damage to Earth's ozone layer.
Remote detection of life on Earth
The astronomer Carl Sagan, pondering the question of whether life on Earth could be easily detected from space, devised a set of experiments in the late 1980s using ''Galileo'' remote sensing instruments during the mission's first Earth flyby in December 1990. After data acquisition and processing, Sagan published a paper in ''
Nature
Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...
'' in 1993 detailing the results of the experiment. ''Galileo'' had indeed found what are now referred to as the "Sagan criteria for life". These included strong absorption of light at the red end of the visible spectrum (especially over
continents
A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...
) which was caused by absorption by chlorophyll in photosynthesizing plants, absorption bands of molecular oxygen which is also a result of plant activity, infrared absorption bands caused by the ~1 micromole per
mole
Mole (or Molé) may refer to:
Animals
* Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America
* Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
(μmol/mol) of methane in Earth's atmosphere (a gas which must be replenished by either volcanic or biological activity), and modulated narrowband radio wave transmissions uncharacteristic of any known natural source. ''Galileo'' experiments were thus the first ever controls in the newborn science of
astrobiological
Astrobiology, and the related field of exobiology, is an interdisciplinary scientific field that studies the origins, early evolution, distribution, and future of life in the universe. Astrobiology is the multidisciplinary field that investig ...
remote sensing.
''Galileo'' Optical Experiment
In December 1992, during ''Galileo'' second gravity-assist
planetary flyby
A planetary flyby is the act of sending a space probe past a planet or a dwarf planet close enough to record scientific data. This is a subset of the overall concept of a flyby in spaceflight.
The first flyby of another planet with a functioni ...
of Earth, another groundbreaking experiment was performed. Optical communications in space were assessed by detecting light pulses from powerful lasers with ''Galileo'' CCD. The experiment, dubbed ''Galileo'' Optical Experiment or GOPEX, used two separate sites to beam laser pulses to the spacecraft, one at
Table Mountain Observatory
Table Mountain Observatory (TMO) is an astronomical observation facility operated by NASA's Jet Propulsion Laboratory (California Institute of Technology). It is located in Big Pines, California, in Angeles National Forest near Wrightwood, north- ...
in California and the other at the
Starfire Optical Range
Starfire Optical Range (SOR - Pronounced as an initialism) is a United States Air Force research laboratory on the Kirtland Air Force Base in Albuquerque, New Mexico. Its primary duty, according to the official website, is to "develop and demo ...
in
New Mexico
)
, population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano)
, seat = Santa Fe
, LargestCity = Albuquerque
, LargestMetro = Tiguex
, OfficialLang = None
, Languages = English, Spanish ( New Mexican), Navajo, Ker ...
. The Table Mountain site used a
frequency doubled
Second-harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of ...
Nd:YAG laser operating at a wavelength of 532 nm, with a repetition rate of ~15 to 30 Hz and a pulse power
full width at half maximum
In a distribution, full width at half maximum (FWHM) is the difference between the two values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve mea ...
(FWHM) in the tens of megawatts range, which was coupled to a Cassegrain telescope for transmission to ''Galileo''. The Starfire range site used a similar setup with a larger, , transmitting telescope. Long exposure (~0.1 to 0.8 s) images using ''Galileo'' 560 nm centered green filter produced images of Earth clearly showing the laser pulses even at distances of up to .
Adverse weather conditions, restrictions placed on laser transmissions by the U.S.
Space Defense Operations Center
The Space Defense Center (SDC) was a space operation center of the North American Aerospace Defense Command. It was successively housed at two Colorado locations, Ent Air Force Base, followed by Cheyenne Mountain Complex#Improvements, Cheyenne Mou ...
(
SPADOC) and a pointing error caused by the scan platform acceleration on the spacecraft being slower than expected (which prevented laser detection on all frames with less than 400 ms exposure times) all contributed to the reduction of the number of successful detections of the laser transmission to 48 of the total 159 frames taken. Nonetheless, the experiment was considered a resounding success and the data acquired will likely be used in the future to design laser downlinks that will send large volumes of data very quickly from spacecraft to Earth. The scheme was studied in 2004 for a data link to a future Mars orbiting spacecraft.
Lunar observations
File:Moon-galileo-color.jpg, Showing Mare Orientale
Mare Orientale (Latin ''orientāle'', the "eastern sea") is a lunar mare. It is located on the western border of the near side and far side of the Moon, and is difficult to see from an Earthbound perspective. Images from spacecraft have reveale ...
File:The Moon from Galileo - GPN-2000-000473.jpg, ''Galileo'' shot of the north pole of Earth's Moon
File:Moon Crescent - False Color Mosaic.jpg, False color
False color (or pseudo color) refers to a group of color rendering methods used to display images in color which were recorded in the visible or non-visible parts of the electromagnetic spectrum. A false-color image is an image that depicts ...
mosaic by ''Galileo'' showing compositional variations of the Moon's surface
High gain antenna problem
![Galileo orbiter arrival at Jupiter (cropped)](https://upload.wikimedia.org/wikipedia/commons/8/80/Galileo_orbiter_arrival_at_Jupiter_%28cropped%29.jpg)
Once ''Galileo'' headed beyond Earth, it was no longer risky to employ the HGA, so on April 11, 1991, ''Galileo'' was ordered to unfurl it. This was done using two small dual drive actuator (DDA) motors, and was expected to take 165 seconds, or 330 seconds if one failed. They would drive a
worm gear
A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm wheel (which is similar in appearance to a spur gear). The two elements are also called the worm screw and worm gear. The terminolo ...
. The antenna had 18 graphite-epoxy ribs, and when the driver motor started and put pressure on the ribs, they were supposed to pop out of the cup their tips were held in, and the antenna would unfold like an umbrella. When it reached the fully deployed configuration, redundant
microswitch
A miniature snap-action switch, also trademarked and frequently known as a micro switch, is an electric switch that is actuated by very little physical force, through the use of a tipping-point mechanism, sometimes called an "over-center" mechan ...
es would shut down the motors. Otherwise they would run for eight minutes before being automatically shut down to prevent them from overheating.
Through telemetry from ''Galileo'', investigators determined that the electric motors had stalled at 56 seconds, the spacecraft's spin rate had decreased and its wobble had increased. Only 15 ribs had popped out, leaving the antenna looking like a lop-sided, half-open umbrella. The first suggestion was to re-fold the antenna and try the opening sequence again. This was not possible; although the motors were capable of running in reverse, the antenna was not designed for this, and human assistance was required when it was done on Earth to ensure that the wire mesh did not snag. It was later discovered that less torque was available from the DDA each time, so after five deploy and stow operations, the DDA torque was half its original value.
The first thing the ''Galileo'' team tried was to rotate the spacecraft away from the Sun and back again on the assumption that the problem was with friction holding the pins in their sockets. If so, then heating and cooling the ribs might cause them to pop out of their sockets. This was done seven times, but with no result. They then tried swinging LGA-2 (which faced in the opposite direction to the HGA and LGA-1) 145 degrees to a hard stop, thereby shaking the spacecraft. This was done six times with no effect. Finally, they tried shaking the antenna by pulsing the DDA motors at 1.25 and 1.875 Hertz. This increased the torque by up to 40 percent. The motors were pulsed 13,000 times over a three-week period in December 1992 and January 1993, but only managed to move the ballscrew by one and a half revolutions beyond the stall point.
![Galileo in 1983](https://upload.wikimedia.org/wikipedia/commons/4/40/Galileo_in_1983.jpg)
Investigators concluded that during the 4.5 years that ''Galileo'' spent in storage after the ''Challenger'' disaster, the
lubricant
A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...
s between the tips of the ribs and the cup were eroded and worn by
vibration
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...
during the three cross-country journeys by truck between California and Florida for the spacecraft. The failed ribs were those closest to the flat-bed trailers carrying ''Galileo'' on these trips. The use of land transport was partly to save costs—air transport would have cost an additional $65,000 () or so per trip—but also to reduce the amount of handling required in loading and unloading the aircraft, which was considered a major risk of damage. The spacecraft was also subjected to severe vibration in a vacuum environment by the IUS. Experiments on Earth with the test HGA showed that having a set of stuck ribs all on one side reduced the DDA torque produced by up to 40 percent.
The antenna lubricants were applied only once, nearly a decade before launch. Furthermore, the HGA was not subjected to the usual rigorous testing, because there was no backup unit that could be installed in ''Galileo'' in case of damage. The flight-ready HGA was never given a thermal evaluation test, and was unfurled only a half dozen or so times before the mission. But testing might not have revealed the problem; the
Lewis Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facilit ...
was never able to replicate the problem on Earth, and it was assumed to be the combination of loss of lubricant during transportation, vibration during launch by the IUS, and a prolonged period of time in the vacuum of space where bare metal touching could undergo
cold welding
Cold welding or contact welding is a solid-state welding process in which joining takes place without fusion or heating at the interface of the two parts to be welded. Unlike in fusion welding, no liquid or molten phase is present in the joint. ...
.
Fortunately, LGA-1 was capable of transmitting information back to Earth, although since it transmitted a signal
isotropic
Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...
ally, its
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
was significantly less than what the high-gain antenna's would have been; the high-gain antenna was to have transmitted at 134
kilobit
The kilobit is a multiple of the unit bit for digital information or computer storage. The prefix ''kilo-'' (symbol k) is defined in the International System of Units (SI) as a multiplier of 103 (1 thousand), and therefore,
:1 kilobit = = 1000& ...
s per second, whereas LGA-1 was only intended to transmit at about 8 to 16 bits per second. LGA-1 transmitted with a power of about 15 to 20 watts, which by the time it reached Earth and had been collected by one of the large aperture 70-meter DSN antennas, had a total power of about 10 zeptowatts. Through the implementation of sophisticated technologies, the arraying of several Deep Space Network antennas and sensitivity upgrades to the receivers used to listen to ''Galileo'' signal, data throughput was increased to a maximum of 160 bits per second.
By further using data compression, the effective bandwidth could be raised to 1,000 bits per second.
The data collected on Jupiter and its moons was stored in the spacecraft's onboard tape recorder, and transmitted back to Earth during the long
apoapsis
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
portion of the probe's orbit using the low-gain antenna. At the same time, measurements were made of Jupiter's magnetosphere and transmitted back to Earth. The reduction in available bandwidth reduced the total amount of data transmitted throughout the mission,
but William J. O'Neil, ''Galileo'' project manager from 1992 to 1997, expressed confidence that 70 percent of ''Galileo'' science goals could still be met. The decision to use magnetic tape for storage was a conservative one, taken in the late 1970s when the use of tape was common. But conservatism was not restricted to engineers; a 1980 suggestion that the results of ''Galileo'' could be distributed electronically instead of on paper was regarded as ridiculous by geologists, on the grounds that storage would be prohibitively expensive; some of them thought that taking measurements on a computer involved putting a wooden ruler up to the screen.
Asteroid encounters
951 Gaspra
![951 Gaspra](https://upload.wikimedia.org/wikipedia/commons/8/81/951_Gaspra.jpg)
Two months after entering the asteroid belt, ''Galileo'' performed the first asteroid encounter by a spacecraft, passing the
S-type asteroid
S-type asteroids are asteroids with a spectral type that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are of this type, making it the second ...
951 Gaspra
951 Gaspra is an S-type asteroid that orbits very close to the inner edge of the asteroid belt. Gaspra was discovered by Russian astronomer G. N. Neujmin in 1916. Neujmin named it after Gaspra, a Black Sea retreat that was visited by his contemp ...
to a distance of at 22:37 UTC on October 29, 1991 at a relative speed of about .
In all, 57 images of Gaspra were taken with the SSI, covering about 80% of the asteroid. Without the HGA, the bit rate was only about 40 bit/s, so an image took up to 60 hours to transmit back to Earth. The ''Galileo'' project was able to secure 80 hours of the Canberra's 70-meter dish time between 7 and 14 November 1991, but most of images taken, including low-resolution images of more of the surface, were not transmitted to Earth until November 1992.
The imagery revealed a cratered and irregular body, measuring about . Its shape was not remarkable for an asteroid of its size. Measurements were taken using the NIMS to indicate the asteroid's composition and physical properties. While Gaspra has plenty of small craters—over 600 of them ranging in size from —it lacks large ones, hinting at a relatively recent origin. However, it is possible that some of the depressions were eroded craters. Perhaps the most surprising feature was several relatively flat planar areas. Measurements of the solar wind in the vicinity of the asteroid showed it changing direction a few hundred kilometers from Gaspra, which hinted that it might have a magnetic field, but this was not certain.
243 Ida and Dactyl
![243 ida](https://upload.wikimedia.org/wikipedia/commons/7/7e/243_ida.jpg)
Following the second Earth encounter, ''Galileo'' performed close observations of another asteroid,
243 Ida
Ida, minor planet designation 243 Ida, is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after a nymph from Greek mythology. ...
, at 16:52:04 UTC on August 28, 1993, at a range of . Measurements were taken from ''Galileo'' using SSI and NIMS. The images revealed that Ida had a small moon measuring around in diameter, which appeared in 46 images.
A competition was held to select a name for the moon, which was ultimately dubbed Dactyl after the legendary
Dactyloi; craters on Dactyl were named after individual dactyloi. Regions on 243 Ida were named after cities where
Johann Palisa
Johann Palisa (6 December 1848 – 2 May 1925) was an Austrian astronomer, born in Troppau, Austrian Silesia, now Czech Republic. He was a prolific discoverer of asteroids, discovering 122 in all, from 136 Austria in 1874 to 1073 Gel ...
, the discover of 243 Ida, made his observations, while ridges on 243 Ida were named in honor of deceased ''Galileo'' team members. Dactyl was the first
asteroid moon
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important ...
discovered. Previously moons of asteroids had been assumed to be rare. The discovery of Dactyl hinted that they might in fact be quite common. From subsequent analysis of this data, Dactyl appeared to be an S-type asteroid, and spectrally different from 243 Ida. It was hypothesized that both may have been produced by the breakup of a
Koronis parent body.
The requirement to use the LGA resulted in a bit rate of 40 bit/s, and that only from August 28 to September 29, 1993 and from February to June 1994. ''Galileo'' tape recorder was used to store images, but tape space was also required for the primary Jupiter mission. A technique was developed whereby only image fragments of two or three lines out of every 330 were initially sent. A determination could then be made as to whether the image was of 243 Ida or empty space. Ultimately, only about 16 percent of the SSI data recorded could be sent back to Earth.
Voyage to Jupiter
Comet Shoemaker–Levy 9
![SL9ImpactGalileo](https://upload.wikimedia.org/wikipedia/commons/1/1b/SL9ImpactGalileo.jpg)
''Galileo'' prime mission was a two-year study of the Jovian system, but while it was en route, an unusual opportunity arose. On 26 March 1993, comet-seeking astronomers
Carolyn S. Shoemaker
Carolyn Jean Spellmann Shoemaker (June 24, 1929 – August 13, 2021) was an American astronomer and a co-discoverer of Comet Shoemaker–Levy 9. She discovered 32 comets (then a record for the most by an individual) and more than 500 astero ...
,
Eugene M. Shoemaker
Eugene Merle Shoemaker (April 28, 1928 – July 18, 1997) was an American geologist. He co-discovered Comet Shoemaker–Levy 9 with his wife Carolyn S. Shoemaker and David H. Levy. This comet hit Jupiter in July 1994: the impact was televi ...
and
David H. Levy
David Howard Levy (born May 22, 1948) is a Canadian amateur astronomer, science writer and discoverer of comets and minor planets, who co-discovered Comet Shoemaker–Levy 9 in 1993, which collided with the planet Jupiter in 1994.
Biography
L ...
discovered fragments of a comet orbiting Jupiter. They were the remains of a comet that had passed within the
Roche limit
In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forc ...
of Jupiter, and had been torn apart by
tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
s. It was named
Comet Shoemaker–Levy 9
Comet Shoemaker–Levy 9 ( formally designated D/1993 F2) broke apart in July 1992 and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. This generated a l ...
. Calculations indicated that it would crash into the planet sometime between 16 and 24 July 1994. While ''Galileo'' was still a long way from Jupiter, it was perfectly positioned to observe this event, whereas terrestrial telescopes had to wait to see the impact sites as they rotated into view because it would occur on Jupiter's night side.
Instead of burning up in Jupiter's atmosphere as expected, the first of the 21 comet fragments struck the planet at around and exploded with a fireball high, easily discernible to Earth-based telescopes even though it was on the night side of the planet. The impact left a series of dark scars on the planet, some two or three times as large as the Earth, that persisted for weeks. When ''Galileo'' observed an impact in ultraviolet light, it lasted for about ten seconds, but in the infrared it persisted for 90 seconds or more. When a fragment hit the planet, it increased Jupiter's overall brightness by about 20 percent. The NIMS observed one fragment create a fireball in diameter that burned with a temperature of , which was hotter than the surface of the Sun.
Probe deployment
The ''Galileo'' probe separated from the orbiter at 03:07 UTC on July 13, 1995,
five months before its rendezvous with the planet on December 7. At this point, the spacecraft was still from Jupiter, but from Earth, and telemetry from the spacecraft, travelling at the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
, took 37 minutes to reach the JPL. A tiny
Doppler shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
in the signal of the order of a few centimeters per second indicated that the separation had been accomplished. The ''Galileo'' orbiter was still on a collision course with Jupiter. Previously, course corrections had been made using the twelve thrusters, but with the probe on its way, the ''Galileo'' orbiter could now fire its
Messerschmitt-Bölkow-Blohm
Messerschmitt-Bölkow-Blohm (MBB) was a West German aerospace manufacturer. It was formed during the late 1960s as the result of efforts to consolidate the West German aerospace industry; aircraft manufacturer Messerschmitt AG merged with the civi ...
main engine which had been covered by the probe until then. At 07:38 UTC on July 27, it was fired for the first time to place the ''Galileo'' orbiter on course to enter orbit around Jupiter, whence it would perform as a communications relay for the ''Galileo'' probe. The ''Galileo'' probe's project manager, Marcie Smith at the
Ames Research Center
The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laborat ...
, was confident that this role could be performed by LGA-1. The burn lasted for five minutes and eight seconds, and changed the velocity of the ''Galileo'' orbiter by .
Dust storms
In August 1995, the ''Galileo'' orbiter encountered a severe dust storm from Jupiter that took several months to traverse. Normally the spacecraft's dust detector picked up a dust particle every three days; now it detected up to 20,000 particles a day. Interplanetary dust storms had previously been encountered by the ''
Ulysses
Ulysses is one form of the Roman name for Odysseus, a hero in ancient Greek literature.
Ulysses may also refer to:
People
* Ulysses (given name), including a list of people with this name
Places in the United States
* Ulysses, Kansas
* Ulysse ...
'' space probe, which had passed by Jupiter three years before on its mission to study the Sun's polar regions, but those encountered by ''Galileo'' were more intense. The dust particles were about the same size as those in cigarette smoke, and had speeds ranging from depending on their size. The existence of the dust storms had come as a complete surprise to scientists. While data from both ''Ulysses'' and ''Galileo'' hinted that they originated somewhere in the Jovian system, it was a mystery how they had been created, and how they had escaped from Jupiter's strong gravitational and electromagnetic fields.
Tape recorder anomaly
The failure of ''Galileo'' high-gain antenna meant that data storage to the tape recorder for later compression and playback was absolutely crucial in order to obtain any substantial information from the flybys of Jupiter and its moons. This was a four-track, 114-
megabyte
The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...
digital tape recorder, manufactured by
Odetics Corporation. On October 11, it was stuck in rewind mode for 15 hours before engineers learned what had happened and were able to send commands to shut it off. Although the recorder itself was still in working order, the malfunction had possibly damaged a length of tape at the end of the reel. This section of tape was declared "off limits" to any future data recording, and was covered with 25 more turns of tape to secure the section and reduce any further stresses, which could tear it. Because it happened only weeks before ''Galileo'' entered orbit around Jupiter, the anomaly prompted engineers to sacrifice data acquisition of almost all of the
Io and
Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
observations during the orbit insertion phase, in order to focus on recording data sent from the Jupiter probe descent.
Jupiter
Arrival
The ''Galileo'' Orbiter's magnetometers reported that the spacecraft had encountered the bow wave of Jupiter's magnetosphere on November 16, 1995, when it was still from Jupiter. The bow wave was not stationary, but moved to and fro in responses to solar wind gusts, and was therefore crossed multiple times between 16 and 26 November, by which time it was from Jupiter.
On December 7, 1995, the orbiter arrived in the Jovian system. That day it made a flyby of Europa at 11:09 UTC, and then an flyby of Io at 15:46 UTC, using Io's gravity to reduce its speed, and thereby conserve propellant for use later in the mission. At 19:54 it made its closest approach to Jupiter. The orbiter's electronics had been heavily shielded against radiation, but the radiation exceeded expectations, and nearly the spacecraft's design limits. One of the navigational systems failed, but the backup took over. Most robotic spacecraft respond to failures by entering
safe mode and awaiting further instructions from Earth, but with a minimum of a two-hour signalling delay, this was not possible for ''Galileo''.
Atmospheric probe
![Descent Module](https://upload.wikimedia.org/wikipedia/commons/e/e6/Descent_Module.jpeg)
Meanwhile, the probe awoke in response to an alarm at 16:00 UTC and began powering up its instruments. It passed through the
rings of Jupiter
Ring may refer to:
* Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry
* To make a sound with a bell, and the sound made by a bell
:(hence) to initiate a telephone connection
Arts, entertainment and media Film and ...
and encountered a previously undiscovered belt of radiation ten times as strong as Earth's
Van Allen radiation belt. Before the atmospheric entry, the probe discovered a new
radiation belt above Jupiter's cloud tops. It entered
Jupiter's atmosphere
The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ...
without any braking at 22:04 UTC on December 7, 1995. At this point it was moving at relative to Jupiter.
It had been predicted that the probe would pass through three layers of clouds: an upper one consisting of
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
ice-particles at a pressure of ; a middle one of
ammonium hydrosulphide ice particles at a pressure of ; and one of water vapor at . The atmosphere through which the probe descended was much denser and hotter than expected. Jupiter was also found to have only half the amount of helium expected and the data did not support the three-layered cloud structure theory: only one significant cloud layer was measured by the probe, at a pressure of around but with many indications of smaller areas of increased particle densities along the whole length of the trajectory.
The probe slowed to subsonic speed within two minutes of entry. The rapid flight through the atmosphere produced a plasma with a temperature of about , and the probe's carbon phenolic heat shield lost more than half of its mass during the descent. At the time, this was by far the most difficult
atmospheric entry
Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the ent ...
ever attempted; the probe entered at
Mach 50 and had to withstand a peak
deceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
of . As it passed through Jupiter's cloud tops, it started transmitting data to the orbiter, above. The data was not immediately relayed to Earth, but a single bit was transmitted from the orbiter as a notification that the signal from the probe was being received and recorded, which would then take days to arrive using the LGA. At 22:04 UTC the probe began its plunge into the atmosphere, defined for the purpose as being above the pressure level, since Jupiter has no solid surface.
The probe's heat shield, making up almost half of the probe's total mass, lost during the entry.
The atmospheric probe deployed its
parachute
A parachute is a device used to slow the motion of an object through an atmosphere by creating drag or, in a ram-air parachute, aerodynamic lift. A major application is to support people, for recreation or as a safety device for aviators, who ...
fifty-three seconds later than anticipated, resulting in a small loss of upper atmospheric readings. This was attributed to wiring problems with an accelerometer that determined when to begin the parachute deployment sequence. It then dropped its heat shield, which fell into Jupiter's interior.
The parachute reduced the probe's speed to . The signal from the probe was no longer detected by the orbiter after 61.4 minutes. It was believed that the probe continued to fall at terminal velocity, but the temperature would climb to and the pressure to , completely destroying it.
The probe's seven scientific instruments yielded a wealth of information. The probe detected very strong winds. Scientists had expected to find wind speeds of up to , but winds of up to were detected. The implication was that the winds are not produced by heat generated by sunlight or the condensation of water vapor (the main causes on Earth), but are due to an internal heat source. It was already well known that the atmosphere of Jupiter was mainly composed of hydrogen, but the clouds of
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
and
ammonium sulfide
Ammonium hydrosulfide is the chemical compound with the formula .
Composition
It is the salt derived from the ammonium cation and the hydrosulfide anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an e ...
were much thinner than expected, and clouds of water vapor were not detected. This was the first observation of ammonia clouds in another planet's atmosphere. The atmosphere creates ammonia ice particles from material coming up from lower depths.
The probe detected less lightning, less water, but more winds than expected. The atmosphere was more turbulent and the winds a lot stronger than the expected maximum of . It required a laborious analysis of the initial wind data from the probe to determine the actual measured wind speeds. The results eventually showed that wind speeds in the outermost layers were , in agreement with previous measurements from afar, but that winds increased dramatically at pressure levels of 1–4 bars, then remaining consistently high at around . No solid surface was detected during the downward journey.
[
The abundance of ]nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
was three times that of the Sun, raising the possibility that they had been acquired from other bodies in the Solar system,[ but the low abundance of water cast doubt on theories that Earth's water had been acquired from comets. There was far less lightning activity than expected, only about a tenth of the level of activity on Earth, but this was consistent with the lack of water vapor. More surprising was the high abundance of ]noble gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemi ...
es, argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
, krypton
Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
and xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
, with abundances up to three times that found in the Sun. For Jupiter to trap these gases, it would have had to be much colder than today, around , which suggested that either Jupiter had once been much further from the Sun, or that the interstellar debris that the Solar system had formed from was much colder than had been thought.
File:Galileo Probe - AC81-0174.jpg, Artist's impression of the probe's entry into Jupiter's atmosphere
The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ...
Image:Galileo atmospheric probe.jpg, Timeline of the probe's atmospheric entry.
File:Jupiter's clouds.jpg, Jupiter's clouds - expected and actual results of ''Galileo''s atmospheric probe mission
Orbiter
With the probe data collected, the ''Galileo'' orbiter's next task was to slow down in order to avoid heading off into the outer solar system. A burn sequence commencing at 00:27 UTC on December 8 and lasting 49 minutes reduced the spacecraft's speed by and enter a 198-day parking orbit. The ''Galileo'' orbiter then became the first artificial satellite of Jupiter. Most of its initial 7-month long orbit was occupied transmitting the data from the probe back to Earth. When the orbiter reached its apojove on March 26, 1996, the main engine was fired again to increase the orbit from four times the radius of Jupiter to ten times. By this time the orbiter had received half the radiation allowed for in the mission plan, and the higher orbit was to conserve the instruments for as long as possible by limiting the radiation exposure.
The spacecraft traveled around Jupiter in elongated ellipse
In mathematics, an ellipse is a plane curve surrounding two focus (geometry), focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. It generalizes a circle, which is the special ty ...
s, each orbit lasting about two months. The differing distances from Jupiter afforded by these orbits allowed ''Galileo'' to sample different parts of the planet's extensive magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
. The orbits were designed for close-up flybys of Jupiter's largest moons. A naming scheme was devised for the orbits: a code with the first letter of the moon being encountered on that orbit (or "J" if none was encountered) plus the orbit number.
Io
The innermost of the four Galilean moons, Io is roughly the same size as Earth's moon, with a radius of . It is in orbital resonance with Ganymede and Europa, and tidally locked with Jupiter, so just as the Earth's Moon always has the same side facing Earth, Io always has the same side facing Jupiter. It orbits faster though, with a rotation period of 1.769 days. As a result, rotational and tidal forces are 220 times as great as those on Earth's moon. These frictional forces are sufficient to melt rock and create volcanoes and lava flows. Although only a third of the size of Earth, Io generates twice as much heat. While geological events occur on Earth over periods of thousands or even millions of years, cataclysmic events are common on Io. Visible changes occurred between orbits of ''Galileo''. The colorful surface is a mixture of red, white and yellow sulphur compounds.
''Galileo'' flew past Io on arrival day, but in the interest of protecting the tape recorder, O'Neil decided to forego collecting images. Only the fields and particles instruments were allowed to collect data, as these required the tape recorder to run at slow speeds, and it was believed that it could handle this, whereas the SSI camera required it to operate a high speed, with abrupt stops and starts. It was a crushing blow to scientists, some of whom had waited years for the opportunity. No other Io encounters were scheduled during the prime mission because it was feared that the high radiation levels close to Jupiter would damage the spacecraft. However, valuable information was still obtained; Doppler data used to measure Io's gravitational field revealed that Io had a core of molten iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
and iron sulfide
Iron sulfide or Iron sulphide can refer to range of chemical compounds composed of iron and sulfur.
Minerals
By increasing order of stability:
* Iron(II) sulfide, FeS
* Greigite, Fe3S4 (cubic)
* Pyrrhotite, Fe1−xS (where x = 0 to 0.2) (monocli ...
.
Another opportunity to observe Io arose during the GEM, when ''Galileo'' flew past Io on orbits I24 and I25, and it would revisit Io during the GMM, on orbits I27, I31, I32 and I33. As ''Galileo'' approached Io on I24 at 11:09 UTC on October 11, 1999, it entered safe mode. Apparently, high energy electrons had altered a bit on a memory chip. When it entered safe mode, the spacecraft turned off all non-essential functions. Normally it took seven to ten days to diagnose and recover from a safe mode incident; this time the ''Galileo'' Project team at the JPL had nineteen hours before the encounter with Io. After a frantic effort, they managed to diagnose a problem that had never been seen before, and restore the spacecraft systems with just two hours to spare. Not all of the planned activities could be carried out, but ''Galileo'' obtained a series of high-resolution color images of the Pillan Patera
Pillan Patera is a patera, or a complex crater with scalloped edges, on Jupiter's moon Io. It is located at , south of Pillan Mons and west of Reiden Patera. It is named after the Araucanian thunder, fire, and volcano god. Its name was approved ...
, and Zamama, Prometheus, and Pele volcanic eruption centers.
When ''Galileo'' next approached Io on I25 at 20:40 Pacific Time
The Pacific Time Zone (PT) is a time zone encompassing parts of western Canada, the western United States, and western Mexico. Places in this zone observe standard time by subtracting eight hours from Coordinated Universal Time ( UTC−08:00). ...
on November 25, 1999, the JPL were eating their Thanksgiving dinner
The centerpiece of contemporary Thanksgiving in the United States and in Canada is Thanksgiving dinner (informally called turkey dinner), a large meal generally centered on a large roasted turkey. Thanksgiving could be considered the largest ...
at the ''Galileo'' Mission Control Center when, with the encounter with Io just four hours away, the spacecraft again entered safe mode. This time the problem was traced to a software patch implemented to bring ''Galileo'' out of safe mode during I24. Fortunately, the spacecraft had not shut down as much as it had on I24, and the team at JPL were able to bring it back online. During I24 they had done so with two hours to spare; this time, they had just three minutes. Nonetheless, the flyby was very successful, with ''Galileo'' NIMS and SSI camera capturing an erupting volcano that generated a long plume of lava that was sufficiently large and hot to have also been detected by the NASA Infrared Telescope Facility
The NASA Infrared Telescope Facility (NASA IRTF) is a telescope optimized for use in infrared astronomy and located at the Mauna Kea Observatory in Hawaii. It was first built to support the Voyager missions and is now the US national facility ...
atop Mauna Kea
Mauna Kea ( or ; ; abbreviation for ''Mauna a Wākea''); is a dormant volcano on the island of Hawaii. Its peak is above sea level, making it the highest point in the state of Hawaii and second-highest peak of an island on Earth. The peak is ...
in Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
. While such events were more common and spectacular on Io than on Earth, it was extremely fortuitous to have captured it.
The safe mode incidents on I24 and I25 left some gaps in the data, which I27 targeted. This time ''Galileo'' passed just over the surface of Io. At this time, the spacecraft was nearly at the maximum distance from Earth, and there was a solar conjunction, a period when the Sun blocked the line of sight between Earth and Jupiter. As a consequence, three quarters of the observations were taken over a period of just three hours. NIMS images revealed fourteen active volcanoes in a region thought to contain just four. Images of Loki Patera
Loki Patera is the largest volcanic depression on Jupiter's moon Io, in diameter. It contains an active lava lake, with an episodically overturning crust. The level of activity seen is similar to a superfast spreading mid-ocean ridge on Earth. ...
showed that in the four and half months between I24 and I27, some had been covered in fresh lava. Unfortunately, a series of observations of extreme ultraviolet
Extreme ultraviolet radiation (EUV or XUV) or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 124 nm down to 10 nm, and therefore (by the Planck– ...
(EUV) had to be cancelled due to yet another safe mode event. Radiation exposure caused a transient bus
A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
reset, a computer hardware error resulting in a safe mode event. A software patch implemented after the Europa encounter on orbit E19 guarded against this when the spacecraft was within 15 Jupiter radii of the planet, but this time it occurred at 29 Jupiter radii. The safe mode event also caused a loss of tape playback time, but the project managers decide to carry over some Io data into orbit G28, and play it back then. This limited the amount of tape space available for that Ganymede encounter, but the Io data was considered to be more valuable.
The discovery of Io's iron core raised the possibility that it had a magnetic field. The I24, I25 and I27 encounters had been on equatorial orbits, which made it difficult to determine whether Io had its own magnetic field, or one induced by Jupiter. Accordingly, on orbit I31, ''Galileo'' passed within of the surface of the north pole of Io, and on orbit I32 it flew over the south pole. After examining the magnetometer results, planetary scientist
Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their ...
Margaret G. Kivelson, announced that Io had no intrinsic magnetic field, which meant that its molten iron core did not have the same convective properties as that of Earth. On I31 ''Galileo'' sped through an area that had been in the plume of the Tvashtar Paterae volcano, and it was hoped that the plume could be sampled. This time, Tvashtar was quiet, but the spacecraft flew through the plume of another, previously unknown, volcano away. What had been assumed to be hot ash from the volcanic eruption turned out to be sulphur dioxide snowflakes, each consisting of 15 to 20 molecules clustered together. ''Galileo'' final return to Io on orbit I33 was marred by another safe mode incident. Although the project team worked hard to restore the spacecraft to working order, much of the hoped-for data was lost.
Europa
Although the smallest of the four Galilean moons, with a radius of , Europa is still the sixth largest moon in the solar system. Observations from Earth indicated that it was covered in ice. Like Io, Europa is tidally locked with Jupiter. It is in orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
with Io and Ganymede, with its 85-hour orbit being twice that of Io, but half that of Ganymede. Conjunctions with Io always occur on the opposite side of Jupiter to those with Ganymede. Europa is therefore subject to tidal effects. There is no evidence of volcanism like on Io, but ''Galileo'' revealed that the surface ice was covered in cracks.
Some observations of Europa were made during orbits G1 and G2. On C3, ''Galileo'' conducted a "nontargeted" encounter of Europa on 6 November 1996. A "nontargeted" encounter is defined as a secondary flyby up to a distance of . During E4 from 15 to 22 December 1996, ''Galileo'' flew within of Europa, but data transmission was hindered by a Solar occultation that blocked transmission for ten days.
''Galileo'' returned to Europa on E6 in January 1997, this time at a height of to analyze oval-shaped features in the infrared and ultraviolet spectra. Occultations by Europa, Io and Jupiter provided data on the atmospheric profiles of Europa, Io and Jupiter, and measurements were made of Europa's gravitational field. On E11 from 2 to 9 November 1997, data was collected on the magnetosphere. Due to the problems with the HGA, only about two percent of the anticipated number of images of Europa were obtained by the primary mission. On the GEM, the first eight orbits, E12 through E19, were all dedicated to Europa, and ''Galileo'' paid it a final visit on E26 during the GMM.
Images of Europa also showed few impact craters. It seemed unlikely that it had escaped the meteor and comet impacts that scarred Ganymede and Callisto, so this indicated Europa has an active geology that renews the surface and obliterates craters. Clark Chapman
Clark R. Chapman is a senior scientist and astronomer at the Southwest Research Institute's Boulder, Colorado, Department of Space Studies His research generally focuses on astronomy including meteorology and comets. He earned an undergraduate ...
argued that if we assume that a crater occurs in Europa once every million years, and given that only about twenty have been spotted on Europa, the implication is that the surface must only be about 10 million years old. With more data on hand, in 2003 a team led Kevin Zahle at NASA's Ames Research Center
The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laborat ...
arrived at a figure of 30 to 70 million years. Tidal flexing of up to per day was the most likely the culprit. But not all scientists were convinced; Michael Carr, a planetologist from the US Geological Survey
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, an ...
, argued that, on the contrary, the surface of Europa was subjected to less impacts than Callisto or Ganymede.
Evidence of surface renewal hinted at the possibility of a viscous layer below the surface of warm ice or liquid water. NIMS observations by ''Galileo'' indicated that the surface of Europa appeared to contain magnesium and sodium salts. A likely source was brine below the ice crust. Further evidence was provided by the magnetometer, which reported that the magnetic field was induced by Jupiter. This could be explained by the existence of a spherical shell of conductive material like salt water. Since the surface temperature on Europa was a chilly , any water breaching the surface ice would instantly freeze over. Heat required to keep water in a liquid state could not come from the Sun, which had only 4 percent of the intensity of Earth, but ice is a good insulator, and the heat could be provided by the tidal flexing. ''Galileo'' also yielded evidence that the crust of Europa had slipped over time, moving south on the hemisphere facing Jupiter, and north on the far side.
There was acrimonious debate among scientists over the thickness of the ice crust, and those who presented results indicating that it might be thinner than the proposed by the accredited scientists on the ''Galileo'' Imaging Team faced intimidation, scorn, and reduced career opportunities. The ''Galileo'' Imaging Team was led by Michael J. Belton from the Kitt Peak National Observatory
The Kitt Peak National Observatory (KPNO) is a United States astronomy, astronomical observatory located on Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono Oʼodham Nation, west-southwest of Tucson, Arizona. With ...
. Scientists who planned imaging sequences had the exclusive right to the initial interpretation of the ''Galileo'' data, most which was performed by their research students. The scientific community did not want a repetition of the 1979 Morabito incident, when Linda A. Morabito
Linda A. Morabito (born November 21, 1953), also known as Linda Kelly, Linda Hyder, and Linda Morabito-Meyer, is the astronomer who discovered volcanic activity on Volcanism on Io#Discovery, Io, a moon of Jupiter. She made this finding on March 9, ...
, an engineer at the JPL working on ''Voyager 1'', discovered the first active extraterrestrial volcano on Io. The Imaging Team controlled the manner in which discoveries were presented to the scientific community and the public through press conferences, conference papers and publications.
Observations by the Hubble Space Telescope in 1995 reported that Europa had a thin oxygen atmosphere. This was confirmed by ''Galileo'' in six experiments on orbits E4 and E6 during occultations when Europa was between ''Galileo'' and the Earth. This allowed Canberra and Goldstone to investigate the ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
of Europa by measuring the degree to which the radio beam was diffracted by charged particles. This indicated the presence of water ions, which were most likely water molecules that had been dislodged from the surface ice and then ionized by the Sun or the Jovian magnetosphere. The presence of an ionosphere was sufficient to deduce the existence of a thin atmosphere on Europa. On December 11, 2013, NASA reported, based on results from the ''Galileo'' mission, the detection of " clay-like minerals" (specifically, phyllosilicates
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, silica (silicon dioxide, ) is usually consid ...
), often associated with organic materials
Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
, on the icy crust of Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
. The presence of the minerals may have been the result of a collision with an asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
or comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
.
Ganymede
The largest of the Galilean moons with a diameter of , Ganymede is larger than Earth's moon, the dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
or the planet Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
. It is the largest of the moons in the Solar system that are characterized by large amounts of water ice, which also includes Saturn's moon Titan, and Neptune's moon Triton
Triton commonly refers to:
* Triton (mythology), a Greek god
* Triton (moon), a satellite of Neptune
Triton may also refer to:
Biology
* Triton cockatoo, a parrot
* Triton (gastropod), a group of sea snails
* ''Triton'', a synonym of ''Triturus' ...
. Ganymede has three times as much water for its mass as Earth has.
When ''Galileo'' entered Jovian orbit, it did so at an inclination to the Jovian equator, and therefore the orbital plane of the four Galilean moons. To transfer orbit while conserving propellant, two slingshot maneuvers were performed. On G1, the gravity of Ganymede was used to slow the spacecraft's orbital period from 21 to 72 days to allow for more encounters and to take ''Galileo'' out of the more intense regions of radiation. On G2, the gravity assist was employed to put it into a coplanar orbit to permit subsequent encounters with Io, Europa and Callisto. Although the primary purpose of G1 and G2 was navigational, the opportunity to make some observations was not missed. The plasma wave experiment and the magnetometer detected a magnetic field with a strength of about , more than strong enough to create a separate magnetosphere within that of Jupiter. This was the first time that a magnetic field had ever been detected on a moon contained within the magnetosphere of its host planet.
This discovery led naturally to questions about its origin. The evidence pointed to an iron or iron sulphide core and mantle below the surface, encased in ice. Margaret Kivelson, the scientist in charge of the magnetometer experiment felt that the induced magnetic field required an iron core, and speculated that an electrically conductive layer was required, possibly a brine ocean below the surface. ''Galileo'' returned to Ganymede on orbits G7 and G9 in April and May 1997, and on G28 and G29 in May and December 2000 on the GMM. Images of the surface revealed two types of terrain: highly cratered dark regions, and grooved terrain sulcus. Images of the Arbela Sulcus taken on G28 made Ganymede look more like Europa, but tidal flexing could not provide sufficient heat to keep water in liquid form on Ganymede, although it may have made a contribution. One possibility was radioactivity, which might provide sufficient heat for liquid water to exist below the surface. Another possibility was volcanism. Slushy water or ice reaching the surface would quickly freeze over, creating areas of a relatively smooth surface.
Callisto
Callisto is the outermost of the Galilean moons, and the most pockmarked, indeed the most of any body in the Solar system. So many craters must have taken billions of years to accumulate, which gave scientists the idea that its surface was as much as four billion years old, and provided a record of meteor activity in the Solar system. ''Galileo'' visited Callisto on orbits C3, C9 and C100 during the prime mission, and then on C20, C21, C22 and C23 during the GEM. When the cameras observed Callisto close up, there was a puzzling absence of small craters. The surface features appeared to have been eroded, indicating that they had been subject to active geological processes.
''Galileo'' flyby of Callisto on C3 marked the first time that the Deep Space Network operated a link between its antennae in Canberra and Goldstone that allowed them to operate as a gigantic array, thereby enabling a higher bit rate despite the spacecraft's long distance from Earth. With the assistance of the antenna at Parkes, this raised the effective bandwidth to as much as 1,000 bits per second.
Data accumulated on C3 indicated that Callisto had a homogeneous composition, with heavy and light elements intermixed. This was estimated to be composed of 60 percent rock and 40 percent ice. This was overturned by further radio Doppler observations on C9 and C10, which indicated that rock had settled towards the core, and therefore that Callisto indeed has a differentiated internal structure, although not as much so as the other Galilean moons.
Observations made with ''Galileo'' magnetometer indicated that Callisto had no magnetic field of its own, and therefore lacked an iron core like Ganymede's, but that it did have an induced field from Jupiter's magnetosphere. Because ice is too poor a conductor to generate this effect, it pointed to the possibility that Callisto, like Europa and Ganymede, might have a subsurface ocean of brine. ''Galileo'' made its closest encounter with Callisto on C30, when it made a pass over the surface, during which it photographed the Asgard
In Nordic mythology, Asgard (Old Norse: ''Ásgarðr'' ; "enclosure of the Æsir") is a location associated with the gods. It appears in a multitude of Old Norse sagas and mythological texts. It is described as the fortified home of the Æsir ...
, Valhalla
In Norse mythology Valhalla (;) is the anglicised name for non, Valhǫll ("hall of the slain").Orchard (1997:171–172) It is described as a majestic hall located in Asgard and presided over by the god Odin. Half of those who die in combat e ...
and Bran craters. This was used for slingshot maneuvers to set up the final encounters with Io on I31 and I32.
Amalthea
Although ''Galileo'' main mission was to explore the Galilean moons, it also captured images of four of the inner moons, Thebe
Thebe may refer to:
* Any of several female characters in Greek mythology - see List of mythological figures named Thebe
* Thebe (moon), a moon of Jupiter
* Thebe (currency), 1/100 of a Botswana pula
* Thebe, an Amazons, Amazon
* Thebe, alternate ...
, Adrastea, Amalthea, and Metis
Metis or Métis may refer to:
Ethnic groups
* Métis, recognized Indigenous communities in Canada and America whose distinct culture and language emerged after early intermarriage between First Nations peoples and early European settlers, prima ...
. Such images were only possible from a spacecraft; to Earth-based telescopes they were merely specks of light. Two years of Jupiter's intense radiation took its toll on the spacecraft's systems, and its fuel supply was running low in the early 2000s. ''Galileo'' cameras were deactivated on January 17, 2002, after they had sustained irreparable radiation damage.
NASA engineers were able to recover the damaged tape recorder electronics, and ''Galileo'' continued to return scientific data until it was deorbited in 2003, performing one last scientific experiment: a measurement of Amalthea's mass as the spacecraft swung by it. This was tricky to arrange; to be useful, ''Galileo'' had to fly within of Amalthea, but not so close as to crash into it. This was complicated by its irregular potato-like shape. It was tidally locked, pointing its long axis towards Jupiter at all times. A successful flyby meant knowing which direction the asteroid was pointed in relative to ''Galileo'' at all times.
''Galileo'' flew by Amalthea on November 5, 2002, during its 34th orbit, allowing a measurement of the moon's mass as it passed within of its surface. The results startled the scientific team; they revealed that Amalthea weighed , and with a volume of , it therefore had a density of 857 ± 99 kilograms per cubic meter, less than that of water.
A final discovery occurred during the last two orbits of the mission. When the spacecraft passed the orbit of Amalthea, the star scanner detected unexpected flashes of light that were reflections from seven to nine moonlets. None of the individual moonlets were reliably sighted twice, so no orbits were determined. It is believed that they were most likely debris ejected from Amalthea and form a tenuous, and perhaps temporary, ring.
Star scanner
''Galileo'' star scanner was a small optical telescope that provided an absolute attitude reference. It also made several scientific discoveries serendipitously. In the prime mission, it was found that the star scanner was able to detect high-energy particles as a noise signal. This data was eventually calibrated to show the particles were predominantly > electrons that were trapped in the Jovian magnetic belts, and released to the Planetary Data System.
A second discovery occurred in 2000. The star scanner was observing a set of stars which included the second magnitude star Delta Velorum
Delta Velorum (δ Velorum, abbreviated Delta Vel, δ Vel) is a triple star system in the southern constellation of Vela, near the border with Carina, and is part of the False Cross. Based on parallax measurements, it is approxima ...
. At one point, this star dimmed for 8 hours below the star scanner's detection threshold. Subsequent analysis of ''Galileo'' data and work by amateur and professional astronomers showed that Delta Velorum is the brightest known eclipsing binary
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...
, brighter at maximum than even Algol
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
. It has a primary period of 45 days and the dimming is just visible with the naked eye.
Mission extension
After the primary mission concluded on December 7, 1997, most of the mission staff departed, including O'Neil, but about a fifth of them remained. The ''Galileo'' orbiter commenced an extended mission known as the ''Galileo'' Europa Mission (GEM), which ran until December 31, 1999. This was a low-cost mission, with a budget of $30 million (equivalent to $ million in ). The reason for calling it as the "Europa" mission rather than the "Extended" mission was political; although it might seem wasteful to scrap a spacecraft that was still functional and capable of performing a continuing mission, Congress took a dim view of requests for more money for projects it thought had already been fully funded. This was avoided through rebranding.
The smaller GEM team did not have the resources to deal with problems, but when they arose it was able to temporarily recall former team members for intensive efforts to solve them. The spacecraft performed several flybys of Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
, Callisto Callisto most commonly refers to:
*Callisto (mythology), a nymph
*Callisto (moon), a moon of Jupiter
Callisto may also refer to:
Art and entertainment
*''Callisto series'', a sequence of novels by Lin Carter
*''Callisto'', a novel by Torsten Kro ...
and Io. On each one the spacecraft collected only two days' worth of data instead of the seven it had collected during the prime mission. The radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
environment near Io, which ''Galileo'' approached to within on November 26, 1999, on orbit I25, was very unhealthy for ''Galileo'' systems, and so these flybys were saved for the extended mission when loss of the spacecraft would be more acceptable.
By the time GEM ended, most of the spacecraft was operating well beyond its original design specifications, having absorbed three times the radiation exposure that it had been built to withstand. Many of the instruments were no longer operating at peak performance, but were still functional, so a second extension, the ''Galileo'' Millennium Mission (GMM) was authorized. This was intended to run until March 2001, but it was subsequently extended until January 2003. GMM included return visits to Europa, Io, Ganymede and Callisto, and for the first time to Amalthea. The total cost of the original ''Galileo'' mission was about (equivalent to $ billion in ). Of this, (equivalent to $ million in ) was spent on spacecraft development. Another $110 million (equivalent to $ million in ) was contributed by international agencies.
Radiation-related anomalies
Jupiter's uniquely harsh radiation environment caused over 20 anomalies over the course of ''Galileo'' mission, in addition to the incidents expanded upon below. Despite having exceeded its radiation design limit by at least a factor of three, the spacecraft survived all these anomalies. Work-arounds were found eventually for all of these problems, and ''Galileo'' was never rendered entirely non-functional by Jupiter's radiation. The radiation limits for ''Galileo'' computers were based on data returned from '' Pioneers 10'' and '' 11'', since much of the design work was underway before the two ''Voyagers'' arrived at Jupiter in 1979.
A typical effect of the radiation was that several of the science instruments suffered increased noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
while within about of Jupiter. The SSI camera began producing totally white images when the spacecraft was hit by the exceptional 'Bastille Day' coronal mass ejection in 2000, and did so again on subsequent close approaches to Jupiter. The quartz crystal used as the frequency reference for the radio suffered permanent frequency shifts with each Jupiter approach. A spin detector failed, and the spacecraft gyro output was biased by the radiation environment.
The most severe effects of the radiation were current leakages somewhere in the spacecraft's power bus, most likely across brushes
A brush is a common tool with bristles, wire or other filaments. It generally consists of a handle or block to which filaments are affixed in either a parallel or perpendicular orientation, depending on the way the brush is to be gripped durin ...
at a spin bearing connecting rotor and stator sections of the orbiter. These current leakages triggered a reset of the onboard computer and caused it to go into safe mode. The resets occurred when the spacecraft was either close to Jupiter or in the region of space magnetically downstream of Jupiter. A change to the software was made in April 1999 that allowed the onboard computer to detect these resets and autonomously recover, so as to avoid safe mode.
Tape recorder problems
Routine maintenance of the tape recorder involved winding the tape halfway down its length and back again to prevent it sticking. In November 2002, after the completion of the mission's only encounter with Jupiter's moon Amalthea, problems with playback of the tape recorder again plagued ''Galileo''. About 10 minutes after the closest approach of the Amalthea flyby, ''Galileo'' stopped collecting data, shut down all of its instruments, and went into safe mode, apparently as a result of exposure to Jupiter's intense radiation environment. Though most of the Amalthea data was already written to tape, it was found that the recorder refused to respond to commands telling it to play back data.
After weeks of troubleshooting of an identical flight spare of the recorder on the ground, it was determined that the cause of the malfunction was a reduction of light output in three infrared Optek OP133 light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
s (LEDs) located in the drive electronics of the recorder's motor encoder wheel. The gallium arsenide
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
LEDs had been particularly sensitive to proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
-irradiation-induced atomic lattice displacement defects, which greatly decreased their effective light output and caused the drive motor's electronics to falsely believe the motor encoder wheel was incorrectly positioned.
''Galileo'' flight team then began a series of " annealing" sessions, where current was passed through the LEDs for hours at a time to heat them to a point where some of the crystalline lattice defects would be shifted back into place, thus increasing the LED's light output. After about 100 hours of annealing and playback cycles, the recorder was able to operate for up to an hour at a time. After many subsequent playback and cooling cycles, the complete transmission back to Earth of all recorded Amalthea flyby data was successful.
End of mission and deorbit
When the exploration of Mars was being considered in the early 1960s, Carl Sagan and Sidney Coleman
Sidney Richard Coleman (7 March 1937 – 18 November 2007) was an American theoretical physics, theoretical physicist noted for his research in high-energy theoretical physics.
Life and work
Sidney Coleman grew up on the Far North Side o ...
produced a paper concerning contamination of the red planet. In order that scientists could determine whether or not native life forms existed before the planet became contaminated by micro-organisms from Earth, they proposed that space missions should aim at a 99.9 percent chance that contamination should not occur. This figure was adopted by the Committee on Space Research
The Committee on Space Research (COSPAR) was established on October 3, 1958 by the International Council for Scientific Unions (ICSU). Among COSPAR's objectives are the promotion of scientific research in space on an international level, wi ...
(COSPAR) of the International Council of Scientific Unions
The International Council for Science (ICSU, after its former name, International Council of Scientific Unions) was an international non-governmental organization devoted to international cooperation in the advancement of science. Its members ...
in 1964, and was subsequently applied to all planetary probes. The danger was highlighted in 1969 when the Apollo 12
Apollo 12 (November 14–24, 1969) was the sixth crewed flight in the United States Apollo program and the second to land on the Moon. It was launched on November 14, 1969, by NASA from the Kennedy Space Center, Florida. Commander Pete Conra ...
astronauts returned components of the Surveyor 3 spacecraft that had landed on the Moon three years before, and it was found that microbes were still viable even after three years in that harsh climate. An alternative was the Prime Directive
In the fictional universe of '' Star Trek'', the Prime Directive (also known as "Starfleet General Order 1", "General Order 1", and the "non-interference directive") is a guiding principle of Starfleet that prohibits its members from interferin ...
, a philosophy of non-interference with alien life forms enunciated by the original ''Star Trek'' television series that prioritized the interests of the life forms over those of scientists. Given the (admittedly slim) prospect of life on Europa, scientists Richard Greenberg and Randall Tufts proposed that a new standard be set of no greater chance of contamination than that which might occur naturally by meteorites.
''Galileo'' had not been sterilized prior to launch and could have carried bacteria from Earth. Therefore, a plan was formulated to send the probe directly into Jupiter, in an intentional crash to eliminate the possibility of an impact with Jupiter's moons, particularly Europa, and prevent a forward contamination. On April 14, 2003, ''Galileo'' reached its greatest orbital distance from Jupiter for the entire mission since orbital insertion, , before plunging back towards the gas giant for its final impact. At the completion of J35, its final orbit around the Jovian system, ''Galileo'' impacted Jupiter in darkness just south of the equator on September 21, 2003, at 18:57 UTC. Its impact speed was approximately .
Major findings
# The composition of Jupiter differs from that of the Sun, indicating that Jupiter has evolved since the formation of the Solar System.
# ''Galileo'' made the first observation of ammonia clouds in another planet's atmosphere. The atmosphere creates ammonia ice particles from material coming up from lower depths.
# Io was confirmed to have extensive volcanic activity that is 100 times greater than that found on Earth. The heat and frequency of eruptions are reminiscent of early Earth.
# Complex plasma interactions in Io's atmosphere create immense electrical currents which couple to Jupiter's atmosphere.
# Several lines of evidence from ''Galileo'' support the theory that liquid oceans exist under Europa's icy surface.
# Ganymede possesses its own, substantial magnetic field – the first satellite known to have one.
# ''Galileo'' magnetic data provided evidence that Europa, Ganymede and Callisto have a liquid salt water layer under the visible surface.
# Evidence exists that Europa, Ganymede, and Callisto all have a thin atmospheric layer known as a "surface-bound exosphere
The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the densit ...
".
# Jupiter's ring system
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as ...
is formed by dust kicked up as interplanetary meteoroid
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as micr ...
s smash into the planet's four small inner moons. The outermost ring is actually two rings, one embedded with the other. There is probably a separate ring along Amalthea's orbit as well.
# The ''Galileo'' spacecraft identified the global structure and dynamics of a giant planet's magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
.
Follow-on missions
There was a spare ''Galileo'' spacecraft that was considered by the NASA-ESA Outer Planets Study Team in 1983 for a mission to Saturn, but it was passed over in favor of a newer design, which became ''Cassini–Huygens
''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space research, space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, i ...
''. While ''Galileo'' was operating, ''Ulysses
Ulysses is one form of the Roman name for Odysseus, a hero in ancient Greek literature.
Ulysses may also refer to:
People
* Ulysses (given name), including a list of people with this name
Places in the United States
* Ulysses, Kansas
* Ulysse ...
'' passed by Jupiter in 1992 on its mission to study the Sun's polar regions, and ''Cassini–Huygens
''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space research, space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, i ...
'' coasted by the planet in 2000 and 2001 en route to Saturn. ''New Horizons
''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...
'' passed close by Jupiter in 2007 for a gravity assist en route to Pluto, and it too collected data on the planet. The next mission to orbit Jupiter was the ''Juno
Juno commonly refers to:
*Juno (mythology), the Roman goddess of marriage and queen of the gods
*Juno (film), ''Juno'' (film), 2007
Juno may also refer to:
Arts, entertainment and media Fictional characters
*Juno, in the film ''Jenny, Juno''
*Ju ...
'' spacecraft, which entered Jovian orbit in July 2016.
''Juno''
NASA's ''Juno
Juno commonly refers to:
*Juno (mythology), the Roman goddess of marriage and queen of the gods
*Juno (film), ''Juno'' (film), 2007
Juno may also refer to:
Arts, entertainment and media Fictional characters
*Juno, in the film ''Jenny, Juno''
*Ju ...
'' spacecraft, launched in 2011 and planned for a two-year tour of the Jovian system, successfully completed Jupiter orbital insertion on July 4, 2016.
Jupiter Icy Moons Explorer
The European Space Agency is planning to return to the Jovian system with the Jupiter Icy Moons Explorer
The Jupiter Icy Moons Explorer (JUICE) is an interplanetary spacecraft in development by the European Space Agency (ESA) with Airbus Defence and Space as the main contractor. The mission will study three of Jupiter's Galilean moons: Ganymede ...
(JUICE), which is designed to orbit Ganymede in the 2030s.
''Europa Clipper''
Even before ''Galileo'' concluded, NASA considered the Europa Orbiter
The Europa Orbiter was a planned NASA mission to Jupiter's Moon Europa, that was cancelled in 2002. Its main objectives included determining the presence or absence of a subsurface ocean and identifying candidate sites for future lander mis ...
, which was a mission to Jupiter's moon Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
, but it was canceled in 2002. After its cancellation, a lower-cost version was studied. This led to the ''Europa Clipper
Europa Clipper (previously known as Europa Multiple Flyby Mission) is an interplanetary mission in development by NASA comprising an orbiter. Planned for launch in October 2024, the spacecraft is being developed to study the Galilean moon Europ ...
'' being approved in 2015; it is currently planned for launch in the mid-2020s.
Europa Lander
A lander concept, simply called Europa Lander
The Europa Lander is a proposed astrobiology mission concept by NASA to send a Lander (spacecraft), lander to Europa (moon), Europa, an icy moon of Jupiter. If funded and developed as a large strategic science mission, it would be launched ...
is being assessed by the Jet Propulsion Laboratory. , this lander mission to Europa remains a concept, although some funds have been released for instrument development and maturation.
Footnotes
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External links
''Galileo'' mission site
by NASA's Solar System Exploration
''Galileo'' legacy site
by NASA's Solar System Exploration
''Galileo'' Satellite Image Mosaics
by Arizona State University
Galileo image album
by Kevin M. Gill
{{DEFAULTSORT:Galileo (Spacecraft)
Galileo program
Missions to Jupiter
Missions to main-belt asteroids
Galileo Galilei
Articles containing video clips