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Header of the Discovery Program website (January 2016)
Depictions of the Lucy and Psyche missions
Asteroid Eros regolith, as viewed by Discovery's NEAR Shoemaker mission
The Discovery Program was founded in 1992[contradictory] to implement then-NASA Administrator Daniel S. Goldin's policy of "faster, better, cheaper"[This quote needs a citation]planetary science missions. Existing NASA programs had specified mission targets and objectives in advance, then sought bidders to construct and operate them. In contrast, Discovery missions are solicited through a call for proposals on any science topic, and assessed through peer review. Selected missions are led by a scientist called the Principal Investigator (PI) and may include contributions from industry, universities, or government laboratories.
The Discovery Program also includes Missions of Opportunity, which fund US participation in spacecraft operated by other space agencies (e.g. by contributing a single scientific instrument). It can also be used to re-purpose an existing NASA spacecraft for a new mission.
As of 2017[update], the most recently-selected Discovery missions are Lucy and Psyche, the thirteenth and fourteenth missions in the program.
In 1989, NASA's Solar System Exploration Division began to define a new strategy for Solar System exploration up to the year 2000. This included a Small Mission Program Group that investigated missions that would be low cost and allow focused scientific questions to be addressed in shorter time than existing programs. The result was a request for rapid studies of potential missions and NASA committed funding in 1990. The new program was called 'Discovery'. The panel assessed several concepts that could be implemented as low-cost programs, selecting 'Near Earth Asteroid Rendezvous' (NEAR) as the first mission.
NEAR became the first launch in the Discovery Program on February 17, 1996. The second mission, Mars Pathfinder, launched on December 4, 1996, carrying the Sojourner rover to Mars.
Near Earth Asteroid Rendezvous - Shoemaker (named after Eugene Shoemaker) performed a flyby of 253 Mathilde on June 27, 1997, and an Earth flyby in 1998. It flew by 433 Eros once in 1998, before a second approach allowed it to enter orbit around Eros of February 14, 2000. After a year of orbital observations, the spacecraft was landed on the asteroid on February 2001, and surprisingly continued to function successfully touched down softly at under 2 m/s, becoming the first probe to soft-land on an asteroid. The probe continued to emit signals until February 28, 2001, and the final attempt to communicate with the spacecraft was on December 10, 2002.
It was launched a month after the Mars Global Surveyor. After entering the Martian atmosphere, the hypersonic capsule deployed complex landing system including a parachute and an airbag to hit the surface at 14 m/s. The lander deployed Sojourner rover (10.5 kg) on the surface on July 4, 1997 on Mars's Ares Vallis thus becoming the first rover to operate outside the Earth-Moon system. It carried a series of scientific instruments to analyze the Martian atmosphere, climate, geology and the composition of its rocks and soil. It completed its primary and extended mission and after over 80 days, the last signal was sent on September 27, 1997. Mission was terminated on March 10, 1998.
A Moon orbiter to characterize the lunar mineralogy, including polar ice deposits, measurements of magnetic and gravity fields, and study of lunar outgassing events. After preliminary mappings, it achieved the targeted primary Lunar orbit on January 16. Primary mission in this orbit lasted one year until January 28, 1999, followed up by a half-year extended mission in a lower orbit for higher resolution. On July 31, 1999 it deliberately impacted into the Shoemaker crater near the Lunar South pole in hope to produce water vapor plumes that would be observable from Earth.
Mission to collect interstellar dust and dust particles from the nucleus of comet 81P/Wild for study on Earth. After a flyby of Earth and then of asteroid 5535 Annefrank in November 2002. After a flyby of comet Wild 2 in January 2004, during which the Sample Collection plate collected dust grain samples from the coma. Later, it separated from its sample-return capsule, which returned to Earth on January 15, 2006. This capsule is on display at the National Air and Space Museum in Washington D.C. Scientists worldwide are studying the comet dust samples while citizen scientists are finding interstellar dust bits through the Stardust@home project, and in 2014, scientists announced the identification of possible interstellar dust particles. Meanwhile, the spacecraft averted reentry, and was diverted for a flyby of Tempel 1 comet, as part of Stardust-NExT extension, to observe the crater left by Deep Impact. 'Stardust did a final burn to deplete its remaining fuel on March 24, 2011.
Mission to collect solar wind charged particles for analysis on Earth. After reaching L1 orbit on November 16, 2001, it collected solar wind for 850 days between 2001-2004. It left Lissajous orbit and began its return to Earth on April 22, 2004, but on September 8, 2004, the sample-return capsule's parachute failed to deploy, and the capsule crashed into the Utah desert. However, solar wind samples were salvaged and are available for study. Despite the hard landing, Genesis has met or anticipates meeting all its baseline science objectives.
Comet Nucleus Tour was a failed mission to visit and study comets. On August 15, 2002, after a planned maneuver that was intended to propel it out of Earth orbit and into its comet-chasing solar orbit, the spacecraft was lost. The investigation board concluded the probable cause was structural failure of the spacecraft due to plume heating during the Star-30 solid-rocket motor burn. Subsequent investigation revealed that it broke into at least three pieces, the cause likely being structural failure during the rocket motor burn that was to push it from Earth orbit into a solar orbit.
Mercury Surface, Space Environment, Geochemistry and Ranging conducted the first orbital study of Mercury. Its science goals were to provide the first images of the entire planet and collect detailed information on the composition and structure of Mercury's crust, its geologic history, the nature of its thin atmosphere and active magnetosphere, and the makeup of its core and polar materials. It was only the second spacecraft to flyby Mercury, after Mariner 10 in 1975. After one Earth flyby, two of Venus and three of Mercury, it finally entered orbit around Mercury on March 18, 2011. The primary science mission began on April 4, 2011 and lasted until March 17, 2012. It achieved 100% mapping of Mercury on March 6, 2013, and completed its first year-long extended mission on March 17, 2013. After two extension missions, the spacecraft ran out of propellant and was deorbited on April 30, 2015.
The spacecraft released a 350 kg impactor into the path of comet Tempel 1 on July 3, 2005. The impact occurred on July 4, 2005 releasing an energy equivalent of 4.7 tons of TNT. Plume of the impact were observed with several space-based observatories. The 2007 Stardust spacecraft NExT mission determined the resulting crater's diameter to be 150 meters (490 ft). After the successful completion of its mission, it was put in hibernation and then reactivated for a new mission designated EPOXI. On November 4, 2010 it performed a flyby of comet Hartley 2. In 2012 it performed long-distance observations of comet Garradd C/2009 P1, and in 2013 of Comet ISON. Contact was lost in August 2013, later attributed to a Y2K-like error.
Dawn is the first spacecraft to orbit two extraterrestrial bodies, the two most massive objects of the asteroid belt: the protoplanetVesta and the dwarf planetCeres. This was made possible by employing highly efficient solar electricion thrusters, with just 425 kg of xenon for the entire mission after escaping Earth. After a 2009 Mars flyby, it entered orbit around Vesta on July 16, 2011. It entered its lowest Vesta orbit on December 8, 2011, and after a year-long Vesta mission, it left its orbit on September 5, 2012. It entered Ceres' orbit on March 6, 2015 thus becoming the first spacecraft to visit a dwarf planet. It began its lowest orbit on December 16, 2015, and in June 2016 it was approved for an extended mission. On October 19, 2017, NASA announced that the mission would be extended until its hydrazine fuel runs out, possibly in the second half of 2018.
A space observatory named after Johannes Kepler in an heliocentric, Earth-trailing orbit tasked to explore the structure and diversity of exoplanet systems, with a special emphasis on the detection of Earth-size planets in orbit around stars outside our Solar System. It announced its first exoplanet discoveries in January 2010. Initially planned for 3.5 years, it is still functional after 9 years, including a K2 "Second Light" extension mission. By 2015, the spacecraft had detected over 2,300 confirmed planets, including hot Jupiters, super-Earths, circumbinary planets, and planets located in the circumstellar habitable zones of their host stars. In addition, Kepler has detected over 3,600 unconfirmed planet candidates and over 2,000 eclipsing binary stars. The telescope retired on October 30, 2018 after finally running out of fuel.
Gravity Recovery and Interior Laboratory provided higher-quality gravitational field mapping of the Moon to determine its interior structure. The two small spacecrafts GRAIL A (Ebb) and GRAIL B (Flow) separated soon after the launch and entered Lunar orbits on December 31, 2011 and January 1, 2012, respectively. The primary scientific phase was achieved in May 2012. After the extended mission phase, the two spacecrafts impacted the Moon on December 17, 2012. MoonKAM (Moon Knowledge Acquired by Middle school students) was an education related sub-program and instrument of this mission.
Interior Exploration using Seismic Investigations, Geodesy and Heat Transport is a 358 kg lander reusing technology from the Mars Phoenix lander. It is intended to study the interior structure and composition of Mars and advance understanding of the formation and evolution of terrestrial planets. Its launch was delayed from 2016 to May 2018. The lander touched down successfully on November 26, 2018, at a site about 600 km (370 mi) from the Curiosity rover.
Psyche orbiter will travel to and study the asteroid 16 Psyche, the most massive metallic asteroid in the asteroid belt, thought to be the exposed iron core of a protoplanet. The launch is planned for 2022. It will carry an imager, a magnetometer, and a gamma-ray spectrometer.
Missions of opportunity
These provide opportunities to participate in non-NASA missions by providing funding for a science instrument or hardware components of an instrument, or for an extended mission for a spacecraft that may different from its original purpose.
Moon Mineralogy Mapper (M3) is a NASA-designed instrument placed on board the ISRO's Chandrayaan orbiter selected in February 2005. Launched in 2008, it was designed to explore the Moon's mineral composition at high resolution. M3's detection of water on the Moon was announced in Sept. 2009, one month after the mission ended. The Principal Investigator was Carle Pieters of Brown University.
The Extrasolar Planet Observations and Characterization (EPOCh) mission used the Deep Impact high-resolution camera in 2008 to better characterize known giant extrasolar planets orbiting other stars and to search for additional planets in the same system. The Principal Investigator was L. Drake Deming of NASA's Goddard Space Flight Center.
Nucleus of Comet Hartley 2
The Deep Impact eXtended Investigation of Comets (DIXI) mission used the Deep Impact mission's spacecraft for a flyby mission to a second comet, Hartley 2. The goal was to take pictures of its nucleus to increase our understanding of the diversity of comets. The flyby of Hartley 2 was successful with closest approach occurring on November 4, 2010. Michael A'Hearn of the University of Maryland was the Principal Investigator.
New Exploration of Tempel 1 (NExT) was selected in July 2007 together with the EPOXI extension. It was a new mission for the Stardust spacecraft to fly by comet Tempel 1 in 2011 and observe changes since the Deep Impact mission visited it in July 2005. Later in 2005, Tempel 1 made its closest approach to the Sun, possibly changing the surface of the comet. The flyby was completed successfully on February 15, 2011. Joseph Veverka of Cornell University is the Principal Investigator.
Strofio is a unique mass spectrometer that is part of the SERENA instrument package on board the European Space Agency's BepiColombo/Mercury Planetary Orbiter spacecraft. Strofio will study the atoms and molecules that compose Mercury's atmosphere to reveal the composition of the planet's surface. Stefano Livi of Southwest Research Institute is the Principal Investigator.
Possible configuration of a lunar sample return spacecraft
Mercury by Discovery's MESSENGER
However often the funding comes in, there is a selection process with perhaps two dozen concepts. These sometimes get further matured and re-proposed in another selection or program. An example of this is Suess-Urey Mission, which was passed over in favor of the successful Stardust mission, but was eventually flown as Genesis, while a more extensive mission similar to INSIDE was flown as Juno in the New Frontiers program. Some of these concepts went on to become actual missions, or similar concepts were eventually realized in another mission class. This list is a mix of previous and current proposals.
Additional examples of Discovery-class mission proposals include:
Whipple, space-observatory to detect objects in the Oort cloud by transit method.
Hermes, a Mercury orbiter. (compare to MESSENGER Mercury orbiter)
INSIDE Jupiter, an orbiter that would map Jupiter's magnetic and gravity fields in an effort to study the giant planet's interior structure. The concept was further matured and implemented as Juno in the New Frontiers program.
The Dust Telescope is a space observatory that would measure various properties of incoming cosmic dust. The dust telescope would combine a trajectory sensor and a mass spectrometer, to allow the elemental and even isotopic composition to be analyzed.
OSIRIS (Origins Spectral Interpretation, Resource Identification and Security) was an asteroid observation and sample return mission concept selected in 2006 for further concept studies. It was further matured and launched September 8, 2016 as OSIRIS-REx in the New Frontiers Program.
Comet Coma Rendezvous Sample Return, a spacecraft designed to rendezvous with a comet, make extended observations within the cometary coma (but not land on the comet), gently collect multiple coma samples, and return them to Earth for study. (See also Stardust (spacecraft))
Micro Exo Explorer would use a new form of micro-electric propulsion, called 'Micro Electro-fluidic-spray Propulsion' to travel to a near Earth object and gather important data.
Mars Geyser Hopper would investigate 'spider' features on Mars, as imaged by an orbiter. Image size: 1 km (0.62 mi) across.
Merlin mission would place a lander on Mars' moon Deimos.
Mars Moons Multiple Landings Mission (M4), would conduct multiple landings on Phobos and Deimos.
Hall is a Phobos and Deimos sample return mission.
Aladdin was a Discovery-class Phobos and Deimos sample return mission. It was a finalist in the 1999 Discovery selection, with a planned launch in 2001 and return of the samples by 2006. Sample collection was intended to work by sending projectiles into the moons, then collecting the ejecta by means of a collector spacecraft flyby.
Twin Lunar Lander, a geophysics mission to the Moon.
The Venus Multiprobe Mission involved sending 16 atmospheric probes into Venus in 1999.
Venus Multiprobe, proposed for a 1999 launch, would have dropped 16 atmospheric probes into Venus, and fall slowly to the surface, making pressure and temperature measurements.
Vesper was a concept for a Venus orbiter focused on studying that planet's atmosphere. It was one of three concepts to receive funds for further study in the 2006 Discovery selection. Osiris and GRAIL were the other two, and eventually GRAIL was chosen and went on to be launched.
V-STAR (Venus Sample Targeting, Attainment and Return) is a Venus atmosphere sample return mission. While returning samples from the surface of Venus has noted difficulties, a Discovery-class sample return from the upper atmosphere is being proposed. Something along the lines of Stardust mission but using a free-return trajectory (it would not go into Venusian orbit).
VEVA (Venus Exploration of Volcanoes and Atmosphere) is an in atmosphere probe for Venus. The centerpiece is a 7-day balloon flight through the atmosphere accompanied by various tiny probes dropped deeper into the planet's thick gases.
Venus Pathfinder, a long-duration Venus lander.
VALOR, a Venus mission to study its atmosphere with a balloon. Twin balloons would circumnavigate the planet over 8 Earth-days.
Venus Aircraft, a robotic atmospheric flight on Venus' atmosphere using a long-duration solar-powered aircraft system. It would carry 1.5 kg of scientific payload and it must contend with violent wind, heat and a corrosive atmosphere.
Venus Landsailing Rover, a rover concept that would be propelled by the wind force on its vertical wingsail. Conceived in 2012, the project has since made progress in developing electronic components that would allow the vehicle to operate for 50 days on the surface of Venus without a cooling system.
Discovery 1 and 2
Mars Pathfinder's Sojourner mini-rover taking its Alpha Particle X-ray Spectrometer measurement of the Yogi Rock (1997)
The first two Discovery missions were Near Earth Asteroid Rendezvous (NEAR) (later called Shoemaker NEAR) and Mars Pathfinder. These initial missions did not follow the same selection process that started once the program was under-way.Mars Pathfinder was salvaged from the idea for a technology and EDL demonstrator from the Mars Environmental Survey program. Also, one of the goals of Pathfinder was to support the Mars Surveyor program. Later missions would be selected by a more sequential process involving Announcements of Opportunity.
In the case of NEAR, a working group for the program recommended that the first mission should be to a near-Earth asteroid. A series of proposals limited to missions to a near-Earth asteroid missions was reviewed in 1991. What would be the NEAR spacecraft mission was formally selected in December 1993, after which a 2-year development period would follow prior to launch. NEAR was launched on February 15, 1996, and arrived to orbit asteroid Eros on February 14, 2000.Mars Pathfinder launched on December 4, 1996, and landed on Mars on July 4, 1997, bringing along with it the first NASA Mars rover, Pathfinder.
Discovery 3 and 4
Thorium concentrations on the Moon, as mapped by Lunar Prospector
In August 1994, NASA made an Announcement of Opportunity for the next proposed Discovery missions. There were 28 proposals submitted to NASA in October 1994:
ASTER- Asteroid Earth Return
Comet Nucleus Penetrator
Comet Nucleus Tour (CONTOUR)
Cometary Coma Chemical Composition (C4)
Diana (Lunar and Cometary Mission)
FRESIP-A mission to Find the Frequency of Earth-sized Inner Planets
In February 1995, Lunar Prospector, a lunar orbiter mission, was selected for launch. Three other missions were left to undergo a further selection later in 1995 for the fourth Discovery mission: Stardust, Suess-Urey, and Venus Multiprobe.Stardust, a comet sample-return mission, was selected in November 1995 over the two other finalists.
Discovery 5 and 6
In October 1997, NASA selected Genesis and CONTOUR as the next Discovery missions, out of 34 proposals that were submitted in December 1996.
Mercury Surface, Space Environment, Geochemistry and Ranging mission (MESSENGER)
Venus Environmental Satellite (VESAT)
Discovery 7 and 8
Deep Impact impacted a comet nucleus
In July 1999, NASA selected MESSENGER and Deep Impact as the next Discovery Program missions.MESSENGER was the first Mercury orbiter and mission to that planet since Mariner 10. Both missions targeted a launch in late 2004 and the cost was constrained at about US$300 million each.
In 1998 five finalists had been selected to receive US$375,000 to further mature their design concept. The five proposals were selected out of about 30 with the goal of achieving the best science. Those missions were:
Aladdin and MESSENGER were also finalists in the 1997 selection.
Discovery 9 and 10
Scale comparison of Vesta, Ceres, and the Moon
Kepler spacecraft art
26 proposals were submitted to the 2000 Discovery solicitation, with budget initially targeted at US$300 million. Three candidates were shortlisted in January 2001 for a phase-A design study: Dawn, Kepler space telescope, and INSIDE Jupiter.INSIDE Jupiter was similar to a later New Frontiers mission called Juno; Dawn was a mission to asteroids Vesta and Ceres, and Kepler was a space telescope mission aimed to discover extrasolar planets. The three finalists received US$450,000 to further mature the mission concept.
In December 2001, Kepler and Dawn were selected for flight. At this time, only 80 exoplanets had been detected, and that was part of the mission of Kepler, to look for more exoplanets, especially Earth-sized. Both Kepler and Dawn were initially projected for launch in 2006.
The Discovery Program fell on hard-times after this, with several missions experiencing cost overruns, and the CONTOUR mission experiencing an engine failure in orbit. Although both Dawn and Kepler would become widely praised success stories, they missed their somewhat ambitious 2006 launch target, launching in 2007 and 2009 respectively. Kepler would go on to receive several mission extensions, and Dawn successfully orbited both Vesta and Ceres. Nevertheless, the next selection would take longer than previous as the program selection of new missions slowed down. As the successes of the new missions enhanced the image of the Discovery Program, the difficulties began to fade from the limelight. Also, the number of active missions in development or active began to increase as the program ramped up.
The Announcement of Opportunity for this Discovery mission was released in April 2006. There were three finalists for this Discovery selection including GRAIL (the eventual winner), OSIRIS, and VESPER. OSIRIS was very similar to the later OSIRIS-REx mission, an asteroid sample-return mission to 101955 Bennu, and Vesper, a Venus orbiter mission. A previous proposal of Vesper had also been a finalist in the 1998 round of selection. The three finalists were announced in October 2006 and awarded US$1.2 million to further develop their proposals for the final round.
In November 2007 NASA selected the GRAIL mission as the next Discovery mission, with a goal of mapping lunar gravity and a 2011 launch. There were 23 other proposals that were also under consideration. The mission had a budget of US$375 million (then-year dollars) which included construction and launch.
Artist's impression of proposed TiME lake lander for Saturn's moon Titan
For this cycle, 28 proposals were received in 2010; 3 were for the Moon, 4 for Mars, 7 for Venus, 1 for Jupiter, 1 to a Jupiter Trojan, 2 to Saturn, 7 to asteroids, and 3 to comets. Out of the 28 proposals, three finalists received US$3 million in May 2011 to develop a detailed concept study:
Comet Hopper (CHopper) to study cometary evolution by landing on a comet multiple times and observing its changes as it interacts with the Sun.
In August 2012, InSight was selected for development and launch. The mission launched in May 2018 and successfully landed on Mars on November 26, 2018.
Discovery 13 and 14
NASA made ion thruster technology available for proposals for the thirteenth Discovery Program mission.
In February 2014, NASA released a Discovery Program 'Draft Announcement of Opportunity' for launch readiness date of December 31, 2021. On September 30, 2015, NASA selected five mission concepts as finalists, each received $3 million for one-year of further study and concept refinement.
On December 22, 2018, NASA released a draft of its Discovery 2019 Announcement of Opportunity, which outlined its intent to select up to two missions with launch readiness dates of July 1, 2025 - December 31, 2026 and/or July 1, 2028 - Dec. 31, 2029 as Discovery 15 and 16, respectively. The final Announcement of Opportunity was released on April 1, 2019, and proposal submissions were accepted between then and July 1, 2019. 
Finalists were announced on February 13, 2020. The final selections will be made in 2021. Finalists are:
DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus) is a Venus atmospheric probe.
Io Volcano Observer would be an orbiter to Jupiter to perform at least nine flybys of Jupiter's moon Io, which is volcanically active
FOSSIL (Fragments from the Origins of the Solar System and our Interstellar Locale) would place a spacecraft in an Earth-trailing orbit to determine the composition of the local and interplanetary dust cloud.
MANTIS (Main-belt Asteroid and NEO Tour with Imaging and Spectroscopy), is a mission concept that would flyby 14 asteroids covering a wide range of types and masses. 
HOVER (Hyperspectral Observer for Venus Reconnaissance), is a Venus orbiter concept that would perform spectral studies from the top of the atmosphere to the surface. Its main goal is understanding the mechanics of the Venus climate and atmospheric super-rotation.
Moon Diver would deploy a lunar rover to rappel down a deep pit to analyze the exposed geological layers and investigate if the pit connects to a lava tube.
Lunar Compass Rover would explore a nearside magnetic region and swirl, and would answer some outstanding questions in planetary science, including planetary magnetism, space plasma physics, space weathering, planetary geology, and the lunar water cycle.
NanoSWARM is a concept that proposes a lunar orbiter to investigate lunar swirls, space weathering, lunar water, lunar magnetism, and small-scale magnetospheres.
COMPASS (Climate Orbiter for Mars Polar Atmospheric and Subsurface Science) is a mission concept for a Mars orbiter to research the Martian climate record through the study of its ice deposits and their interaction with current climate. This mission is led by the Lunar and Planetary Laboratory at the University of Arizona and the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder.
Icebreaker Life is a mission concept led by Goddard Space Flight Center for a lander to search for direct signs of life on Mars via biomarker detection via a focus on sampling ice-cemented ground for its potential to preserve and protect biomolecules or biosignatures.
MAGIC (Magnetics, Altimetry, Gravity and Imaging of Callisto) is an orbiter reconnaissance concept to Jupiter's moon Callisto.
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^Zephyr: A Landsailing Rover For Venus. (PDF) Geoffrey A. Landis, Steven R. Oleson, David Grantier, and the COMPASS team. NASA John Glenn Research Center. 65th International Astronautical Congress, Toronto, Canada. February 24, 2015. Report: IAC-14,A3,P,31x26111