The exploration of
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
began with the arrival of the ''
New Horizons'' probe in July 2015, though proposals for such a mission had been studied for many decades.
There are no plans as yet for a follow-up mission, though follow-up concepts have been studied.
Early mission proposals
Pluto, discovered by
Clyde Tombaugh
Clyde William Tombaugh (February 4, 1906 January 17, 1997) was an American astronomer. He discovered Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper belt. At the time of discovery, Pluto was cons ...
in 1930, is an interesting target for planetary exploration, but Pluto presents significant challenges for exploration because of its small mass and great distance from Earth.
In 1964,
Gary Flandro of the
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 ...
proposed a mission called
Grand Tour
The Grand Tour was the principally 17th- to early 19th-century custom of a traditional trip through Europe, with Italy as a key destination, undertaken by upper-class young European men of sufficient means and rank (typically accompanied by a tut ...
, taking advantage of the fact that an alignment of planets in the late 1970s would enable a single spacecraft to visit all of the outer planets, including Pluto, by using
gravity assists. Due to the cost, the mission was not funded, although the planetary alignment was used by the two probes of the
Voyager program
The Voyager program is an American scientific program that employs two robotic interstellar probes, ''Voyager 1'' and ''Voyager 2''. They were launched in 1977 to take advantage of a favorable alignment of Jupiter and Saturn, to fly near t ...
, launched in 1977.
One of many possibilities for the ''
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 ...
'' spacecraft after its
flyby of Saturn in 1980 was to use Saturn as a
slingshot
A slingshot is a small hand-powered projectile weapon. The classic form consists of a Y-shaped frame, with two natural rubber strips or tubes attached to the upper two ends. The other ends of the strips lead back to a pocket that holds the pro ...
towards Pluto for a flyby as early as March 1986.
However, scientists decided that a flyby of
Titan during the Saturn encounter would be a more important scientific objective, making a subsequent flyby of Pluto impossible, because the close approach to Titan put it on a trajectory that slingshotted it out of the
plane of the solar system. Because no mission to Pluto was planned by any space agency at the time, it would be left unexplored by interplanetary spacecraft for years to come.
After the success of the Voyager program, planetary scientists looked to Pluto as the destination for a subsequent mission. In the 1980s,
Robert Farquhar proposed that a Pluto mission could be added to the proposed
Solar Probe mission, which at the time was proposed to fly by Jupiter on the was to its solar encounter. The mission was tagged "Fire and Ice." However, the proposal was not accepted.
The Pluto Underground, Pluto 350 and Mariner Mark II
![CRAF](https://upload.wikimedia.org/wikipedia/commons/9/99/CRAF.jpg)
In May 1989, a group of scientists and engineers, including
Alan Stern
Sol Alan Stern (born November 22, 1957) is an American engineer and planetary scientist. He is the principal investigator of the ''New Horizons'' mission to Pluto and the Chief Scientist at Moon Express.
Stern has been involved in 24 suborbita ...
and Fran Bagenal, formed an alliance called the "Pluto Underground". It was named in homage of the Mars Underground, another group of scientists that successfully lobbied for the restart of missions to Mars, following the lack of such since the
Viking
Vikings ; non, víkingr is the modern name given to seafaring people originally from Scandinavia (present-day Denmark, Norway and Sweden),
who from the late 8th to the late 11th centuries raided, pirated, traded and se ...
program. The group started a letter writing campaign which aimed to bring to attention Pluto as a viable target for exploration. In 1990, because of pressure from the
scientific community
The scientific community is a diverse network of interacting scientists. It includes many " sub-communities" working on particular scientific fields, and within particular institutions; interdisciplinary and cross-institutional activities are als ...
, including those of the Pluto Underground, engineers at NASA decided to look into concepts for a mission to Pluto. At the time, it was thought that the
atmosphere of Pluto
The atmosphere of Pluto is the tenuous layer of gases surrounding Pluto. It consists mainly of nitrogen (N2), with minor amounts of methane (CH4) and carbon monoxide (CO), all of which are vaporized from their ices on Pluto's surface. It contain ...
would freeze and fall to the surface during winter, and so a lightweight spacecraft was desirable, as it would be able to reach Pluto before such an event would occur. One of the earliest concepts was for a 40-kilogram spacecraft that would reach Pluto in five to six years. The idea was shortly scrapped, however, because of the infeasibility of miniaturizing scientific instruments aboard such a spacecraft to that size.
![20150626 pluto350-right](https://upload.wikimedia.org/wikipedia/commons/0/07/20150626_pluto350-right.jpg)
Another mission concept, known as Pluto 350, was developed by
Robert Farquhar of the
Goddard Space Flight Center, with Alan Stern and Fran Bagenal of the Pluto Underground, who both served as study scientists for the project. Pluto 350 aimed to send a spacecraft, weighing 350 kilograms, to Pluto.
[ Stern, Alan; ] The spacecraft's minimalistic design was to allow it to travel faster and be more cost-effective, in contrast to most other big-budget projects NASA were developing at the time, such as ''
Galileo'' and ''
Cassini''. Pluto 350, however, would later become controversial among mission planners at NASA, who considered the project to be too small and too high-risk. An alternative plan which was considered at one point was to send to Pluto a configuration of the
Mariner Mark II
Mariner Mark II was NASA's planned family of unmanned spacecraft for the exploration of the outer Solar System that were to be developed and operated by JPL between 1980 through the year 2010.
Summary
After the "flagship" multibillion-dollar ...
spacecraft, which would weigh 2,000 kilograms and cost
US$
The United States dollar (symbol: $; code: USD; also abbreviated US$ or U.S. Dollar, to distinguish it from other dollar-denominated currencies; referred to as the dollar, U.S. dollar, American dollar, or colloquially buck) is the official ...
3.2 billion, in sharp contrast to Pluto 350's $543 million cost.
While both projects competed for approval, Pluto 350 was more favored by NASA mission planners, who were starting to adopt smaller missions such as ''
Mars Pathfinder
''Mars Pathfinder'' (''MESUR Pathfinder'') is an American robotic spacecraft that landed a base station with a roving probe on Mars in 1997. It consisted of a lander, renamed the Carl Sagan Memorial Station, and a lightweight, wheeled robot ...
'' and
NEAR Shoemaker
''Near Earth Asteroid Rendezvous – Shoemaker'' (''NEAR Shoemaker''), renamed after its 1996 launch in honor of planetary scientist Eugene Shoemaker, was a robotic space probe designed by the Johns Hopkins University Applied Physics Labora ...
.
Pluto postage stamp
In October 1991, the
United States Postal Service
The United States Postal Service (USPS), also known as the Post Office, U.S. Mail, or Postal Service, is an independent agency of the executive branch of the United States federal government responsible for providing postal service in the U ...
released a series of stamps commemorating
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
's exploration of the Solar System.
[Smithsonian National Postal Museum (date unknown). Space Exploration Issue - 29c Pluto single - Scott Catalogue USA: 2577. Retrieved from http://arago.si.edu/index.asp?con=2&cmd=1&id=192129&img=1&pg=1.] The series featured a stamp for all planets, displaying an image of the planet and highlighting an associated spacecraft which was sent to it. The stamp for Pluto, however, depicted a featureless sphere, presented with the phrase "not yet explored" in place of the name of a spacecraft. The stamps were unveiled in a ceremony at the
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 ...
.
Two scientists who attended the event, World Space Foundation president Robert Staehle] and JPL scientist Stacy Weinstein, were inspired by Pluto's status on the stamp, such that they started to inquire about the feasibility of sending a spacecraft to Pluto.
Engineers at the Jet Propulsion Laboratory, inspired by the "Not Yet Explored" status of Pluto, also started to put forward ideas about a mission to Pluto.
In August 1992, Staehle telephoned Pluto's discoverer,
Clyde Tombaugh
Clyde William Tombaugh (February 4, 1906 January 17, 1997) was an American astronomer. He discovered Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper belt. At the time of discovery, Pluto was cons ...
, requesting permission to visit his planet. "I told him he was welcome to it", Tombaugh later remembered, "though he's got to go one long, cold trip".
Pluto Fast Flyby
That year, Staehle, with the help of JPL engineers and students from the
California Institute of Technology
The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
, formed the
Pluto Fast Flyby project. The mission heralded the same ideology as the Pluto 350 concept: small in size and cost-effective in scope, so that the spacecraft would be able to get to Pluto faster and be affordable to develop and launch. Described as a "radical" mission concept, the mission would see two spacecraft being sent to Pluto. Both spacecraft were to weigh only around 35-50 kilograms each (including 7 kg worth of scientific instruments), and the project would cost less than US$500 million to develop, excluding launch costs.
Described by Staehle as a "faster, better,
ndcheaper" approach than the Pluto 350 and Mariner Mark II projects, it caught the attention of then-NASA Administrator
Daniel S. Goldin, who ordered all work on both Pluto 350 and Mariner Mark II to cease and shift all resources to the new Pluto Fast Flyby project instead.
During the development of Pluto Fast Flyby, however, there were multiple concerns from both NASA, Administrator Goldin and the mission's development team. As research and development into the mission progressed, the project's size, scope, and budget all expanded. Additionally, morale among the team and personnel working on interplanetary missions was low following the loss of the
Mars Observer
The ''Mars Observer'' spacecraft, also known as the ''Mars Geoscience/Climatology Orbiter'', was a robotic space probe launched by NASA on September 25, 1992, to study the Martian surface, atmosphere, climate and magnetic field. During the int ...
spacecraft in August 1993. Alan Stern would later cite that event as a significant factor towards the low enthusiasm for the Pluto Fast Flyby project.
The spacecraft were intended to be launched using
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 Forc ...
rockets, which would have cost US$400 million each, thus raising the budget to over US$1 billion.
Because of growing budget constraints, the dual-spacecraft concept was scrapped in favor of sending a single spacecraft to Pluto. The project was still too expensive, however, in the eyes of Administrator Goldin.
Alan Stern, as a compromise, reached an agreement with
Russian Space Research Institute
The Russian Space Research Institute (russian: Институт космических исследований Российской академии наук, Space Research Institute of the Russian Academy of Sciences, SRI RAS, Russian abbrevia ...
scientists in Moscow, in which Pluto Fast Flyby would be launched atop a
Proton rocket
Proton (Russian: Протон) (formal designation: UR-500) is an expendable launch system used for both commercial and Russian government space launches. The first Proton rocket was launched in 1965. Modern versions of the launch system are sti ...
, saving NASA over US$400 million in launch costs. Alec Galeev, head of the Russian Space Research Institute, reached the agreement with Stern by stipulating that Russia would include an atmospheric probe that would impact Pluto after studying its atmosphere with a mass spectrometer. The proposal was forwarded to Administrator Goldin, but he vetoed the proposal, recommending instead that the JPL look into the feasibility of Pluto Fast Flyby being launched aboard a smaller rocket, such as the
Delta II
Delta II was an expendable launch system, originally designed and built by McDonnell Douglas. Delta II was part of the Delta rocket family and entered service in 1989. Delta II vehicles included the Delta 6000, and the two later Delta 7000 ...
.
Kuiper belt, Pluto Kuiper Express and cancellation
![Pluto express](https://upload.wikimedia.org/wikipedia/commons/7/7d/Pluto_express.jpg)
During the course of the late 1990s, a number of
trans-Neptunian object
A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au).
Typically ...
s were discovered, confirming the existence of a
Kuiper belt. Interest in a mission to the Kuiper belt arose such that NASA instructed the JPL to re-purpose the mission as not only a Pluto flyby, but also a Kuiper belt object (KBO) flyby. The mission was thus re-branded as the
Pluto Kuiper Express
''Pluto Kuiper Express'' was an interplanetary space probe that was proposed by Jet Propulsion Laboratory (JPL) scientists and engineers and under development by NASA. The spacecraft was intended to be launched to study Pluto and its moon Charo ...
, after briefly being billed as Pluto Express prior to the revision. The weight of the spacecraft was raised again, this time to 175 kilograms, and NASA allowed further liberty with the project's budget.
However, Goldin later decided that Pluto Kuiper Express was of low importance, and thus cut funding to the project drastically. Eventually, despite official selection of scientific instruments and the appointment of several investigators, then-
Science Mission Directorate
The Science Mission Directorate (SMD) of the National Aeronautics and Space Administration (NASA) engages the United States’ science community, sponsors scientific research, and develops and deploys satellites and probes in collaboration with NA ...
Edward J. Weiler ordered the cancellation of the entire Pluto and Kuiper belt mission in 2000, citing growing budget constraints, which had plagued the project since its inception in 1992. At the time of cancellation, the projected costs surpassed $1 billion.
The cancellation of Pluto Kuiper Express angered some of the space-exploration scientific community, which led to groups, such as
The Planetary Society
The Planetary Society is an American internationally-active non-governmental nonprofit organization. It is involved in research, public outreach, and political space advocacy for engineering projects related to astronomy, planetary science, a ...
, lobbying NASA for either a reboot of the Pluto Kuiper Express or a restart of a mission to Pluto. Internal divisions within NASA, including its Scientific Advisory Council, also voiced support for a Pluto mission.
In response to the backlash caused by the cancellation of Pluto Kuiper Express, it was decided to inaugurate a new class of missions that would fit between the big-budget
Flagship Program
NASA's large strategic science missions or large strategic missions, formerly known as Flagship missions or Flagship-class missions, are the costliest and most capable NASA science spacecraft. Flagship missions exist within all four divisions ...
and the low-budget
Discovery Program, creating a compromise for missions such as the former Pluto Kuiper Express, which proved to be too expensive for the Discovery Program. A competition was held, in which NASA would select a mission concept to fund as part of the first mission of the
New Frontiers program
The New Frontiers program is a series of space exploration missions being conducted by NASA with the purpose of furthering the understanding of the Solar System. The program selects medium-class missions which can provide high science returns.
...
.
Proposed exploration (2003)
A Pluto orbiter/lander/sample return mission was proposed in 2003. The plan included a twelve-year trip from Earth to Pluto, mapping from orbit, multiple landings, a warm water probe, and possible ''in situ'' propellant production for another twelve-year trip back to Earth with samples. Power and propulsion would come from the bimodal MITEE nuclear reactor system.
''New Horizons''
After an intense political battle, a revised mission to Pluto called ''
New Horizons'' was granted funding from the US government in 2003.
''New Horizons'' was launched successfully on 19 January 2006. The mission leader,
S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.
![First Pluto sighting from New Horizons](https://upload.wikimedia.org/wikipedia/commons/1/1c/First_Pluto_sighting_from_New_Horizons.gif)
''New Horizons'' captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager.
The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of 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 ...
from
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 t ...
.
On 4 February 2015, NASA released new images of Pluto (taken on 25 and 27 January) from the approaching probe. ''New Horizons'' was more than away from Pluto when it began taking the photos, which showed Pluto and its largest moon, Charon. On 20 March 2015, NASA invited the general public to suggest names for surface features that will be discovered on Pluto and Charon. On 15 April 2015, Pluto was imaged showing a possible polar cap.
Between April and June 2015, ''New Horizons'' began returning images of Pluto that exceeded the quality that the Hubble Space Telescope could produce.
Pluto's small moons, discovered shortly before and after the probe's launch, were considered to be potentially hazardous, as debris from collisions between them and other Kuiper belt objects could have produced a tenuous dusty ring. If ''New Horizons'' had travelled through such a ring system, there would have been an increased risk of potentially disabling damage.
''New Horizons'' had its closest approach to Pluto on 14 July 2015—after a 3,462-day journey across the Solar System. Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter. ''New Horizons'' used a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyze Pluto's neutral atmosphere and its escape rate. ''New Horizons'' also photographed the surfaces of Pluto and Charon.
Photographs of Pluto taken on 14 July 2015 taken 15 minutes after ''New Horizon'' closest approach, from a distance of 18,000 kilometers and sent to Earth on 13 September 2015 show a near-sunset on Pluto with details of the surface and a haze in the atmosphere.
Future mission concepts
No follow-up missions to ''New Horizons'' have been formally planned, but at least two mission concepts have been studied. In April 2017, a workshop met in Houston, Texas to discuss ideas for a follow-up mission.
Possible objectives discussed by the group for a follow-up mission include mapping the surface at 30 feet per pixel, observations of Pluto's smaller satellites, observations of how Pluto changes as it rotates on its axis, and topographic mapping of Pluto's regions that are covered in long-term darkness due to its axial tilt. The last objective could be accomplished using infrared laser pulses. According to New Horizons principal investigator Alan Stern, “If we send an orbiter, we can map 100 percent of the planet, even terrains that are in total shadow."
Stern and David Grinspoon have also suggested that an orbiter mission could search for evidence of the subsurface ocean hinted at in ''New Horizons'' data.
![Fusion-Enabled Pluto Orbiter and Lander](https://upload.wikimedia.org/wikipedia/commons/0/0e/Fusion-Enabled_Pluto_Orbiter_and_Lander.png)
Shortly after the ''New Horizons'' flyby, Stern suggested a Charon lander as a follow-up that would observe Pluto from Charon's surface.
However, such a lander would only observe the Charon-facing hemisphere of Pluto, as Pluto and Charon are
tidally locked
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked bo ...
. Since the Houston workshop, Stern changed his mind to advocate instead for a ''Cassini''-style orbiter that would use Charon's gravity to adjust its orbit while studying Pluto and its moons.
The probe could use electric propulsion similar to NASA's ''
Dawn
Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight being scattered in Earth's atmosphere, when the centre of the Sun's disc has reached 18° below the observer's hori ...
'' mission. It would then have the option of using Charon's gravity to leave the Pluto system after all Pluto science objectives are completed and study new KBOs beyond Pluto. Stern envisaged the probe being launched in 2030, marking the 100th anniversary of Pluto's discovery, and spending 7–8 years traveling to the Pluto system.
Fusion-Enabled Pluto Orbiter and Lander
The Fusion-Enabled Pluto Orbiter and Lander was a 2017 phase I report funded by the NASA Innovative Advanced Concepts (
NIAC) program.
The report, written by principal investigator Stephanie Thomas of Princeton Satellite Systems, Inc., describes a
Direct Fusion Drive
Direct Fusion Drive (DFD) is a conceptual low radioactivity, nuclear- fusion rocket engine designed to produce both thrust and electric power for interplanetary spacecraft. The concept is based on the Princeton field-reversed configuration reacto ...
(DFD) mission to Pluto. A fusion reactor would be used to send a 1000 kg orbiter and lander to the Pluto system in only four years (more than twice as fast as ''New Horizons'').
Pluto Hop, Skip, and Jump
Global Aerospace Corporation presented a Pluto lander concept titled "Pluto Hop, Skip, and Jump" at the 2017 NIAC Symposium in Denver, Colorado.
The concept describes an entrycraft that would brake using the drag of Pluto's thin but highly spread-out atmosphere. Once on Pluto's surface, the vehicle would exploit Pluto's low gravity to hop between sites using propellant. This is similar to NASA's
Triton Hopper concept for exploring Neptune's largest 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' ...
.
Persephone
Another proposal submitted to NASA in 2020 is an orbital probe of the Pluto & Charon system, named "Persephone".
It would be powered by 5 RTGs, contain several high-resolution cameras, and orbit for 3 years. A key goal would be to determine whether there is a subsurface ocean on Pluto. Estimated cost would be $3 billion.
References
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Pluto
Spaceflight
Discovery and exploration of the Solar System