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''Ulysses'' ( , ) was a
robotic Robotics is an interdisciplinary branch of computer science and engineering. Robotics involves design, construction, operation, and use of robots. The goal of robotics is to design machines that can help and assist humans. Robotics integrate ...
space probe A space probe is an artificial satellite that travels through space to collect scientific data. A space probe may orbit Earth; approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; or ...
whose primary mission was to orbit the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
and study it at all latitudes. It was launched in 1990 and made three "fast latitude scans" of the Sun in 1994/1995, 2000/2001, and 2007/2008. In addition, the probe studied several comets. ''Ulysses'' was a joint venture of the
European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (120 ...
(ESA) and the United States'
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 ...
(NASA), under leadership of ESA with participation from Canada's National Research Council. The last day for mission operations on ''Ulysses'' was 30 June 2009. To study the Sun at all latitudes, the probe needed to change its
orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...
and leave the plane of the Solar System. To change the orbital inclination of a spacecraft to about 80° requires a large change in heliocentric velocity, the energy to achieve which far exceeded the capabilities of any
launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload (spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pads, supported by a launch control center and system ...
. To reach the desired orbit around the Sun, the mission's planners chose a
gravity assist 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 p ...
maneuver around
Jupiter Jupiter is the fifth planet from the Sun and the 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 slightly less than one-thousandth th ...
, but this Jupiter encounter meant that ''Ulysses'' could not be powered by solar cells. The probe was powered instead by a
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 ...
(RTG). The
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, ...
was originally named ''Odysseus'', because of its lengthy and indirect trajectory to study the solar poles. It was renamed ''Ulysses'', the
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
translation Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
of "
Odysseus Odysseus ( ; grc-gre, Ὀδυσσεύς, Ὀδυσεύς, OdysseúsOdyseús, ), also known by the Latin variant Ulysses ( , ; lat, UlyssesUlixes), is a legendary Greek king of Ithaca and the hero of Homer's epic poem the ''Odyssey''. Odyss ...
", at ESA's request in honor not only of
Homer Homer (; grc, Ὅμηρος , ''Hómēros'') (born ) was a Greek poet who is credited as the author of the '' Iliad'' and the '' Odyssey'', two epic poems that are foundational works of ancient Greek literature. Homer is considered one of ...
's mythological hero but also of
Dante Dante Alighieri (; – 14 September 1321), probably baptized Durante di Alighiero degli Alighieri and often referred to as Dante (, ), was an Italian people, Italian Italian poetry, poet, writer and philosopher. His ''Divine Comedy'', origin ...
's character in the ''Inferno''. ''Ulysses'' was originally scheduled for launch in May 1986 aboard the
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 nam ...
''Challenger'' on
STS-61-F STS-61-F was a NASA Space Shuttle mission planned to launch on 15 May 1986 using ''Challenger''. It was canceled after ''Challenger'' was destroyed earlier that year. Crew Mission objectives The main objective of STS-61-F was to deploy ...
. Due to the 28 January 1986 loss of ''Challenger'', the launch of ''Ulysses'' was delayed until 6 October 1990 aboard ''Discovery'' (mission STS-41).


Spacecraft

The spacecraft was designed by ESA and built by Dornier Systems, a German aircraft manufacturer. The body was roughly a box, approximately in size. The box mounted the
dish antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or pa ...
and the GPHS-RTG
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 ...
(RTG) power source. The box was divided into noisy and quiet sections. The noisy section abutted the RTG; the quiet section housed the instrument electronics. Particularly "loud" components, such as the preamps for the radio dipole, were mounted outside the structure entirely, and the box acted as a
Faraday cage A Faraday cage or Faraday shield is an enclosure used to block electromagnetic fields. A Faraday shield may be formed by a continuous covering of conductive material, or in the case of a Faraday cage, by a mesh of such materials. Faraday cages ...
. ''Ulysses'' was spin-stabilised about its z-axis which roughly coincides with the axis of the dish antenna. The RTG,
whip antenna A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so ...
s, and instrument boom were placed to stabilize this axis, with the spin rate nominally at 5
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 dimensi ...
. Inside the body was a
hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine h ...
fuel tank. Hydrazine
monopropellant Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with biprop ...
was used for course corrections inbound to Jupiter, and later used exclusively to repoint the spin axis (and thus, the antenna) at Earth. The spacecraft was controlled by eight thrusters in two blocks. Thrusters were pulsed in the time domain to perform rotation or translation. Four sun sensors detected orientation. For fine attitude control, the S-band antenna feed was mounted slightly off-axis. This offset feed combined with the spacecraft spin introduced an apparent oscillation to a radio signal transmitted from Earth when received on board the spacecraft. The amplitude and phase of this oscillation were proportional to the orientation of the spin axis relative to the Earth direction. This method of determining the relative orientation is called
conical scanning Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used ...
and was used by early radars for automated tracking of targets and was also very common in early infrared guided missiles. The spacecraft used S-band for uplinked commands and downlinked telemetry, through dual redundant 5-watt transceivers. The spacecraft used
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 ...
for science return (downlink only), using dual 20 watts
TWTA A traveling-wave tube (TWT, pronounced "twit") or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range. The TWT belongs t ...
s until the failure of the last remaining TWTA in January 2008. Both bands used the dish antenna with prime-focus feeds, unlike the
Cassegrain Cassegrain may refer to * Cassegrain reflector, a design used in telescopes * Cassegrain antenna, a type of parabolic antenna * Cassegrain (crater), on the Moon The Moon is Earth's only natural satellite. It is the fifth largest sate ...
feeds of most other spacecraft dishes. Dual tape recorders, each of approximately 45-megabit capacity, stored science data between the nominal eight-hour communications sessions during the prime and extended mission phases. The spacecraft was designed to withstand both the heat of the inner Solar System and the cold at Jupiter's distance. Extensive blanketing and electric heaters protected the probe against the cold temperatures of the outer Solar System. Multiple computer systems (CPUs/microprocessors/Data Processing Units) are used in several of the scientific instruments, including several radiation-hardened RCA CDP1802 microprocessors. Documented 1802 usage includes dual-redundant 1802s in the COSPIN, and at least one 1802 each in the GRB, HI-SCALE, SWICS, SWOOPS and URAP instruments, with other possible microprocessors incorporated elsewhere. Total mass at launch was , of which 33.5 kg was
hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine h ...
propellant used for attitude control and orbit correction.


Instruments

The twelve different Instruments came from ESA and NASA. The first design was based on two probes, one by NASA and one by ESA, but the probe of NASA was defunded and in the end the instruments of the cancelled probe were mounted on Ulyssses. * Radio/Plasma antennas: Two
beryllium copper Beryllium copper (BeCu), also known as copper beryllium (CuBe), beryllium bronze, and spring copper, is a copper alloy with 0.5–3% beryllium but can contain other elements as well. Beryllium copper combines high strength with non-magnetic a ...
antennas were unreeled outwards from the body, perpendicular to the RTG and spin axis. Together this
dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
spanned 72 meters (236.2 ft). A third antenna, of hollow beryllium copper, was deployed from the body, along the spin axis opposite the dish. It was a
monopole antenna A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or ...
, 7.5 meters (24.6 ft) long. These measured radio waves generated by plasma releases, or the plasma itself as it passed over the spacecraft. This receiver ensemble was sensitive from DC to 1 MHz. * Experiment Boom: A third type of boom, shorter and much more rigid, extended from the last side of the spacecraft, opposite the RTG. This was a hollow carbon-fiber tube, of 50 mm (2 in.) diameter. It can be seen in the photo as the silver rod stowed alongside the body. It carried four types of instruments: a solid-state
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10 nanometers, corresponding to frequencies in the range 30  ...
instrument, composed of two
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
detectors, to study X-rays from
solar flare A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar ph ...
s and Jupiter's aurorae; the Gamma-Ray Burst experiment, consisting of two CsI scintillator crystals with photomultipliers; two different
magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, on ...
s, a helium vector magnetometer and a fluxgate magnetometer; and a two-axis magnetic search coil antenna measured AC magnetic fields. * Body-Mounted Instruments: Detectors for
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kno ...
s,
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
s, neutral gas,
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...
, and cosmic rays were mounted on the spacecraft body around the quiet section. * Lastly, the radio communications link could be used to search for
gravitational waves Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
(through
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 ...
s) and to probe the Sun's atmosphere through
radio occultation Radio occultation (RO) is a remote sensing technique used for measuring the physical properties of a planetary atmosphere or ring system. Atmospheric radio occultation Atmospheric radio occultation relies on the detection of a change in a radio ...
. No gravitational waves were detected. * Total instrument mass was 55 kg. * Magnetometer (MAG): MAG measured the magnetic field in the heliosphere. Measurements of Jupiter's magnetic field were also performed. * Solar Wind Plasma Experiment (SWOOPS): detected the solar wind at all solar distances and latitudes and in three dimensions. It measured positive ions and electrons. * Solar Wind Ion Composition Instrument (SWICS): determined composition, temperature and speed of the atoms and ions that comprise the solar wind. * Unified Radio and Plasma Wave Instrument (URAP): picked up radio waves from the Sun and electromagnetic waves generated in the solar wind close to the spacecraft. * Energetic Particle Instrument (EPAC) and GAS: EPAC investigated the energy, fluxes and distribution of energetic particles in the heliosphere. GAS studied the uncharged gases (helium) of interstellar origin. * Low-Energy Ion and Electron Experiment (HI-SCALE): investigated the energy, fluxes and distribution of energetic particles in the heliosphere. * Cosmic Ray and Solar Particle Instrument (COSPIN): investigated the energy, fluxes and distribution of energetic particles and galactic cosmic rays in the heliosphere. * Solar X-ray and Cosmic Gamma-Ray Burst Instrument (GRB): studied cosmic gamma ray bursts and X-rays from solar flares. * Dust Experiment (DUST): Direct measurements of interplanetary and
interstellar dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...
grains to investigate their properties as functions of the distance from the Sun and solar latitude.


Mission


Planning

Until ''Ulysses'', the Sun had only been observed from low solar latitudes. The Earth's orbit defines the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth's orbit, Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the e ...
plane, which differs from the Sun's equatorial plane by only 7.25°. Even spacecraft directly orbiting the Sun do so in planes close to the ecliptic because a direct launch into a high-inclination solar orbit would require a prohibitively large launch vehicle. Several spacecraft (
Mariner 10 ''Mariner 10'' was an American robotic space probe launched by NASA on 3 November 1973, to fly by the planets Mercury and Venus. It was the first spacecraft to perform flybys of multiple planets. ''Mariner 10'' was launched approximatel ...
,
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 ...
, and ''Voyagers 1'' and ''2'') had performed
gravity assist 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 p ...
maneuvers in the 1970s. Those maneuvers were to reach other planets also orbiting close to the ecliptic, so they were mostly in-plane changes. However, gravity assists are not limited to in-plane maneuvers; a suitable flyby of
Jupiter Jupiter is the fifth planet from the Sun and the 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 slightly less than one-thousandth th ...
could produce a significant plane change. An Out-Of-The-Ecliptic mission (OOE) was thereby proposed. ''See article''
Pioneer H Pioneer H is an unlaunched unmanned space mission that was part of the US Pioneer program for a planned 1974 launch. Had this mission and spacecraft been launched, it would have been designated Pioneer 12; that designation was later applied to t ...
. Originally, two spacecraft were to be built by NASA and ESA, as the International Solar Polar Mission. One would be sent over Jupiter, then under the Sun. The other would fly under Jupiter, then over the Sun. This would provide simultaneous coverage. Due to cutbacks, the U.S. spacecraft was cancelled in 1981. One spacecraft was designed, and the project recast as ''Ulysses,'' due to the indirect and untried flight path. NASA would provide the Radioisotope Thermoelectric Generator (RTG) and launch services, ESA would build the spacecraft assigned to Astrium GmbH,
Friedrichshafen Friedrichshafen ( or ; Low Alemannic: ''Hafe'' or ''Fridrichshafe'') is a city on the northern shoreline of Lake Constance (the ''Bodensee'') in Southern Germany, near the borders of both Switzerland and Austria. It is the district capital (''Kre ...
, Germany (formerly Dornier Systems). The instruments would be split into teams from universities and research institutes in Europe and the United States. This process provided the 12 instruments on board. The changes delayed launch from February 1983 to May 1986 when it was to be deployed by the Space Shuttle ''Challenger'' (boosted by the proposed Centaur G Prime upper stage. However, the ''Challenger'' disaster forced a two-and-a-half year stand down of the shuttle fleet, mandated the cancellation of the Centaur-G upper stage, and pushed the launch date to October 1990.


Launch

''Ulysses'' was deployed into low Earth orbit from the Space Shuttle ''Discovery''. From there, it was propelled on a trajectory to Jupiter by a combination of solid rocket motors. This upper stage consisted of a two-stage
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 produc ...
IUS (Inertial Upper Stage), plus a
McDonnell Douglas McDonnell Douglas was a major American aerospace manufacturing corporation and defense contractor, formed by the merger of McDonnell Aircraft and the Douglas Aircraft Company in 1967. Between then and its own merger with Boeing in 1997, it pro ...
PAM-S (
Payload Assist Module The Payload Assist Module (PAM) is a modular upper stage designed and built by McDonnell Douglas (now Boeing), using Thiokol Star-series solid propellant rocket motors. The PAM was used with the Space Shuttle, Delta, and Titan launchers and carri ...
-Special). The IUS was inertially stabilised and actively guided during its burn. The PAM-S was unguided and it and ''Ulysses'' were spun up to 80 rpm for stability at the start of its burn. On burnout of the PAM-S, the motor and spacecraft stack was yo-yo de-spun (weights deployed at the end of cables) to below 8 rpm prior to separation of the spacecraft. On leaving Earth, the spacecraft became the fastest ever artificially-accelerated spacecraft, and held that title until the ''
New Horizons ''New Horizons'' is an 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 Institute (SwRI), with a ...
'' probe was launched. On its way to Jupiter, the spacecraft was in an elliptical non-
Hohmann transfer orbit In astronautics, the Hohmann transfer orbit () is an orbital maneuver used to transfer a spacecraft between two orbits of different altitudes around a central body. Examples would be used for travel between low Earth orbit and the Moon, or ano ...
. At this time, ''Ulysses'' had a low orbital inclination to the ecliptic.


Jupiter swing-by

It arrived at Jupiter on 8 February 1992 for a swing-by maneuver that increased its inclination to the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth's orbit, Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the e ...
by 80.2°. The giant planet's gravity bent the spacecraft's flight path southward and away from the ecliptic plane. This put it into a final orbit around the Sun that would take it past the Sun's north and south poles. The size and shape of the orbit were adjusted to a much smaller degree so that aphelion remained at approximately 5 AU, Jupiter's distance from the Sun, and perihelion was somewhat greater than 1 AU, the Earth's distance from the Sun. The orbital period is approximately six years.


Polar regions of the Sun

Between 1994 and 1995 it explored both the southern and
north North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north'' is ...
ern polar regions of the Sun, respectively.


Comet C/1996 B2 (Hyakutake)

On 1 May 1996, the spacecraft unexpectedly crossed the ion tail of
Comet Hyakutake Comet Hyakutake (, formally designated C/1996 B2) is a comet, discovered on 31 January 1996, that passed very close to Earth in March of that year. It was dubbed the Great Comet of 1996; its passage near the Earth was one of the closest com ...
(C/1996 B2), revealing the tail to be at least 3.8 AU in length.


Comet C/1999 T1 (McNaught–Hartley)

An encounter with a comet tail happened again in 2004 when ''Ulysses'' flew through the ion tailings of C/1999 T1 (McNaught-Hartley). A
coronal mass ejection A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted ...
carried the cometary material to ''Ulysses''.Ulysses Catches Another Comet by the Tail
/ref>
/ref>


Second Jupiter encounter

''Ulysses'' approached
aphelion 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 ...
in 2003/2004 and made further distant observations of Jupiter.


Comet C/2006 P1 (McNaught)

In 2007, ''Ulysses'' passed through the tail of comet
C/2006 P1 Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic comet discovered on 7 August 2006 by British-Australian astronomer Robert H. McNaught using the Uppsala Southern Schmidt Telescope ...
(McNaught). The results were surprisingly different from its pass through Hyakutake's tail, with the measured solar wind velocity dropping from approximately 700 kilometers per second (1,566,000 mph) to less than 400 kilometers per second (895,000 mph).


Extended mission

ESA's Science Program Committee approved the fourth extension of the ''Ulysses'' mission to March 2004 thereby allowing it to operate over the Sun's poles for the third time in 2007 and 2008. After it became clear that the power output from the spacecraft's RTG would be insufficient to operate science instruments and keep the
attitude control Attitude control is the process of controlling the orientation of an aerospace vehicle with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc. Controlling vehicle ...
fuel,
hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine h ...
, from freezing, instrument power sharing was initiated. Up until then, the most important instruments had been kept online constantly, whilst others were deactivated. When the probe neared the Sun, its power-hungry heaters were turned off and all instruments were turned on. On 22 February 2008, 17 years and 4 months after the launch of the spacecraft,
ESA , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (120 ...
and NASA announced that the mission operations for ''Ulysses'' would likely cease within a few months. On 12 April 2008, NASA announced that the end date will be 1 July 2008. The spacecraft operated successfully for over four times its
design life The design life of a component or product is the period of time during which the item is expected by its designers to work within its specified parameters; in other words, the life expectancy of the item. It is not always the actual length of tim ...
. A component within the last remaining working chain of
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 ...
downlink subsystem failed on 15 January 2008. The other chain in the X-band subsystem had previously failed in 2003.
Downlink In a telecommunications network, a link is a communication channel that connects two or more devices for the purpose of data transmission. The link may be a dedicated physical link or a virtual circuit that uses one or more physical links or sha ...
to
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 surface ...
resumed on
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 ...
, but the
beamwidth The beam diameter or beam width of an electromagnetic beam is the diameter along any specified line that is perpendicular to the beam axis and intersects it. Since beams typically do not have sharp edges, the diameter can be defined in many differ ...
of the
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 performanc ...
in the S-band was not as narrow as in the X–band, so that the received downlink signal was much weaker, hence reducing the achievable data rate. As the spacecraft traveled on its outbound
trajectory A trajectory or flight path is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete traje ...
to the orbit of Jupiter, the downlink signal would have eventually fallen below the receiving capability of even the largest antennas (70 meters - 229.7 feet - in diameter) of the
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 ...
. Even before the downlink signal was lost due to distance, the
hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine h ...
attitude control fuel on board the spacecraft was considered likely to freeze, as the radioisotope thermal generators (RTGs) failed to generate enough power for the heaters to overcome radiative heat loss into space. Once the hydrazine froze, the spacecraft would no longer be able to maneuver to keep its high gain antenna pointing towards Earth, and the downlink signal would then be lost in a matter of days. The failure of the X-band communications subsystem hastened this, because the coldest part of the fuel pipework was routed over the X-band traveling-wave tube amplifiers, because they generated enough heat during operation to keep the propellant plumbing warm. The previously announced mission end date of 1 July 2008, came and went but mission operations continued albeit in a reduced capacity. The availability of science data gathering was limited to only when ''Ulysses'' was in contact with a ground station due to the deteriorating S-band downlink margin no longer being able to support simultaneous real-time data and tape recorder playback. When the spacecraft was out of contact with a ground station, the S-band transmitter was switched off and the power was diverted to the internal heaters to add to the warming of the hydrazine. On 30 June 2009, ground controllers sent commands to switch to the low gain antennas. This stopped communications with the spacecraft, in combination with previous commands to shut down its transmitter entirely.


Results

During cruise phases, ''Ulysses'' provided unique data. As the only spacecraft out of the ecliptic with a
gamma-ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically sh ...
instrument, ''Ulysses'' was an important part of the
InterPlanetary Network The InterPlanetary Network (IPN) is a group of spacecraft equipped with gamma ray burst (GRB) detectors. By timing the arrival of a burst at several spacecraft, its precise location can be found. The precision for determining the direction of a G ...
(IPN). The IPN detects
gamma ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten millise ...
s (GRBs); since gamma rays cannot be focused with mirrors, it was very difficult to locate GRBs with enough accuracy to study them further. Instead, several spacecraft can locate the burst through
multilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multilateration, for emp ...
. Each spacecraft has a gamma-ray detector, with readouts noted in tiny fractions of a second. By comparing the arrival times of gamma showers with the separations of the spacecraft, a location can be determined, for follow-up with other telescopes. Because gamma rays travel at the speed of light, wide separations are needed. Typically, a determination came from comparing: one of several spacecraft orbiting the Earth, an inner-Solar-system probe (to
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmosp ...
,
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 ...
, or 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. ...
), and ''Ulysses''. When ''Ulysses'' crossed the ecliptic twice per orbit, many GRB determinations lost accuracy. Additional discoveries: * Data provided by ''Ulysses'' led to the discovery that the Sun's magnetic field interacts with the Solar System in a more complex fashion than previously assumed. * Data provided by ''Ulysses'' led to the discovery that dust coming into the Solar System from deep space was 30 times more abundant than previously expected. * In 2007–2008 data provided by ''Ulysses'' led to the determination that the magnetic field emanating from the Sun's poles is much weaker than previously observed. * That 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 sol ...
has "grown progressively weaker during the mission and is currently at its weakest since the start of the Space Age".


Fate

Ulysses will most likely continue in heliocentric orbit around the Sun indefinitely. However, there is a chance that in one of its re-encounters with Jupiter, that a close fly-by with one of the Jovian moons would be enough to alter its course and so the probe would enter a
hyperbolic trajectory In astrodynamics or celestial mechanics, a hyperbolic trajectory or hyperbolic orbit is the trajectory of any object around a central body with more than enough speed to escape the central object's gravitational pull. The name derives from the fa ...
around the Sun and leave the Solar System.


See also

* * * * * * * * *


References


External links


ESA ''Ulysses'' website

ESA ''Ulysses'' mission operations website

ESA ''Ulysses'' Home page

NASA/JPL ''Ulysses'' website

''Ulysses'' Measuring Mission Profile
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NASA's Solar System Exploration


* ttps://web.archive.org/web/20060928042857/http://ulysses-ops.jpl.esa.int/ulsfct/rgpCafe/solsys/solsys.html Where is ''Ulysses' now!
Max Planck Institute ''Ulysses'' website

Interview with ''Ulysses'' Mission Operations Manager Nigel Angold on Planetary Radio

Interactive 3D visualisation of Ulysses Jupiter gravity assist and polar orbit around the Sun
{{Authority control European Space Agency space probes NASA space probes Missions to the Sun Missions to Jupiter Derelict satellites in heliocentric orbit Missions to comets Spacecraft launched by the Space Shuttle Derelict space probes Spacecraft launched in 1990 Spacecraft decommissioned in 2009 Solar space observatories