NASA's Discovery Program (as compared to New Frontiers, Explorers, or Flagship Programs) is a series of lower-cost, highly-focused American scientific space missions that are exploring the Solar System. It was founded in 1992 to implement then-NASA Administrator Daniel S. Goldin's vision of "faster, better, cheaper" planetary missions. Discovery missions differ from traditional NASA missions where targets and objectives are pre-specified. Instead, these cost-capped missions are proposed and led by a scientist called the Principal Investigator (PI). Proposing teams may include people from industry, small businesses, government laboratories, and universities. Proposals are selected through a competitive peer review process. All of the completed Discovery missions are accomplishing ground-breaking science and adding significantly to the body of knowledge about the Solar System.
NASA also accepts proposals for competitively selected Discovery Program Missions of Opportunity. This provides opportunities to participate in non-NASA missions by providing funding for a science instrument or hardware components of a science instrument or to re-purpose an existing NASA spacecraft. These opportunities are currently offered through NASA's Stand Alone Mission of Opportunity program.
Successfully completed missions 
Standalone missions 
- NEAR Shoemaker, a mission to study asteroid 433 Eros. Launched in 1995, the spacecraft entered into orbit around Eros in 2000 and successfully touched down on its surface one year later. It has succeeded its primary and extended mission and is now complete. The Project Scientist was Andrew Chang of The Johns Hopkins University Applied Physics Laboratory.
- Mars Pathfinder, a Mars lander to deploy a miniature rover on the surface. Launched in 1996, it landed on Mars on July 4, 1997. It has completed its primary and extended mission. The Principal Investigator was Matthew Golombek of NASA’s Jet Propulsion Laboratory.
- Lunar Prospector, a Moon orbiter to characterize the lunar mineralogy. Launched in 1998, it spent 1½ years in lunar orbit. It has completed its primary and extended mission and deliberately impacted onto the Moon's surface. The Principal Investigator was Alan Binder of the Lunar Research Institute.
- Deep Impact, a mission in which a spacecraft released an impactor into the path of comet Tempel 1. Launched in January 2005, the impact occurred on July 4, 2005. After the successful completion of its mission, it was put in hibernation and then reactivated for a new mission designated EPOXI. The Principal Investigator was Michael A’Hearn of the University of Maryland.
- Stardust, a mission to collect interstellar dust and dust particles from the nucleus of comet 81P/Wild for study on Earth. Launched in 1999, it successfully collected samples between 2000–2004, then the sample return capsule returned to Earth on Jan. 15, 2006. The 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. The spacecraft has been assigned a new task, called Stardust-NExT. The Principal Investigator was Donald Brownlee of the University of Washington.
- Genesis, a mission to collect solar wind charged particles for analysis on Earth. Launched in 2001, it collected solar wind between 2002–2003. In Sept. 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. The Principal Investigator was Donald Burnett of the California Institute of Technology.
- Gravity Recovery and Interior Laboratory, (GRAIL) provided higher-quality gravity field mapping of the Moon to determine its interior structure; launched in September 2011. The Principal Investigator is Maria Zuber of the Massachusetts Institute of Technology. GRAIL spacecraft impacted the Moon on December 17, 2012.
Missions of opportunity 
- Moon Mineralogy Mapper is a NASA-designed instrument placed on board the ISRO's Chandrayaan orbiter. 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 Dr. Carle Pieters of Brown University.
- Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) was selected in 2007. It was a series of two new missions for the existing Deep Impact probe following its success at Tempel 1:
- 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 Dr. L. Drake Deming of NASA's Goddard Space Flight Center.
- The Deep Impact eXtended Investigation of Comets (DIXI) mission used the 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 Nov. 4, 2010. Dr. Michael A'Hearn of the University of Maryland was the Principal Investigator.
- New Exploration of Tempel 1 (NExT) 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 Feb. 15, 2011. Dr. Joseph Veverka of Cornell University is the Principal Investigator.
Failed mission 
- Comet Nucleus Tour (CONTOUR), a mission to visit and study comets Encke and Schwassmann-Wachmann-3. It was launched from Cape Canaveral, Florida on July 3, 2002. On August 15, contact with the craft was lost. 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.
Missions in progress 
Standalone missions 
- Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER), a mission to study and map the planet Mercury from orbit; launched in August 2004, has completed a series of flybys of Earth, Venus and Mercury, and following a successful orbital insertion maneuver in March 2011 has begun its yearlong primary mission. The Principal Investigator is Sean Solomon of the Carnegie Institution of Washington.
- Dawn, a mission to study the dwarf planet Ceres and large asteroid Vesta; launched in September 2007. It reached Vesta in July 2011. Dawn uses ion propulsion, allowing it to be the first spacecraft to orbit two bodies (excluding Earth and the Sun) in one mission. The Principal Investigator is Chris Russell of the University of California, Los Angeles.
- Kepler, a space telescope mission to continuously observe 100,000 stars in a fixed field of view in order to detect transits by exoplanets orbiting those stars; launched in March 2009 and announced its first exoplanet discoveries in January 2010. Kepler is the first spacecraft capable of finding Earth-size planets around other stars. The Principal Investigator is William Borucki of NASA’s Ames Research Center.
Missions of opportunity 
Future missions 
Standalone missions 
- InSight – An Announcement of Opportunity for the next Discovery mission was released by NASA on June 7, 2010. Twenty-eight proposals were submitted, and on May 5, 2011, three were chosen for further study. Following these one-year preliminary design studies, the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission was selected August 2012. InSight (initially named Geophysical Monitoring Station or GEMS) will study the structure and composition of the interior of Mars and advance understanding of the formation and evolution of terrestrial planets.
Missions of opportunity 
- Strofio is a unique mass spectrometer that is part of the SERENA instrument package that will fly 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.
- Lander Radio-Science on ExoMars, or LaRa, will use NASA's Deep Space Network of radio telescopes to track part of ESA's ExoMars mission. Scheduled to launch in 2016, the mission consists of a fixed lander and a rover that will roam Mars collecting soil samples for detailed analysis. William Folkner of NASA's Jet Propulsion Laboratory is the Principal Investigator.
- Titan Mare Explorer (TiME) would have provided the first direct exploration of an ocean environment beyond Earth by landing in, and floating on, a large methane-ethane sea on Saturn's moon Titan.
- Comet Hopper (CHopper) would have studied cometary evolution by landing on a comet multiple times and observing its changes as it interacts with the Sun.
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