The strength of Discovery missions lies in their frequency. By flying more frequently than larger missions, Discovery projects allow scientists greater opportunities and shorter waiting periods to fly their instruments (a particularly important proposition for early-career scientists). And since Discovery missions are competitively selected, they provide increased opportunities for scientists outside of NASA centers to design missions.

Discovery is a fiercely competitive program. In this latest round NASA received twenty-eight proposals, which is the same number the agency received in 2010 during the twelfth Discovery mission competition. This round includes a number of new competitors, while some from the last round are not competing this time. The competition is particularly strong this round, in part due to the long wait for NASA’s solicitation for proposals. Discovery competitions are supposed to occur every two to three years.

NASA will now conduct reviews of all twenty-eight proposals, evaluating them in terms of science quality, technical maturity, and cost. Each of these factors is important. The proposed mission must answer scientific questions that the planetary science community is interested in; the spacecraft must not rely on technology that is insufficiently mature; and the mission must have a good chance of staying within the cost cap, as determined by an independent group hired by NASA to perform an evaluation.

NASA does not disclose which proposals it receives for Discovery competitions and mission proposers do not have to make any aspect of their participation public. Some principal investigators choose to keep their proposals secret so as not to tip off their competitors. Considering that it is common for teams to lose once or more before they eventually get selected, there is logic to this secrecy. Unusually (and somewhat surprisingly), two-thirds of the proposing teams during this thirteenth round have publicly revealed basic information about their proposals.

Because the principal investigators for so many of these missions have gone public, it is possible to discuss them, although generally not in much detail. There is considerable information available for some of them, such as IVO, but little more than a discussion of science goals for others. The list below is categorized by target or mission type and includes nineteen missions. In addition, the author is aware of another Venus mission and another comet mission, although their principal investigators have not publicly revealed them.

Moon Mission Proposals

Following the Apollo program, two decades passed before the United States returned a spacecraft to the Moon. The Department of Defense’s Clementine spacecraft orbited in the early 1990s, later followed by the third Discovery mission, Lunar Prospector, and the eleventh Discovery mission, GRAIL. NASA has also launched LADEE, LCROSS, and the Lunar Reconnaissance Orbiter (LRO) as part of other programs. LRO still operates around the Moon, although the president’s most recent budget proposes shutting it down. This would reduce NASA’s presence at the Moon to the ARTEMIS extension of the THEMIS mission, a heliophysics project from the Small Explorer program. The Moon still poses many scientific questions, but because previous missions have answered most of the basic questions, the remaining questions are more complicated and require more complex spacecraft to answer – and some of those spacecraft are too expensive to fit in the Discovery program. Nevertheless, some scientists continue to propose Discovery lunar missions. Two of them are publicly known for this Discovery round.

NanoSWARM is a CubeSat Discovery mission to study space weathering, lunar magnetism, lunar water, and small-scale magnetospheres. It would consist of several CubeSats in lunar orbit. NASA is currently undertaking a small CubeSat mission known as Lunar Flashlight, but so far no CubeSats have ventured beyond low Earth orbit so this would be a novel mission.

MARE (Moon Age and Regolith Explorer) is a proposed lunar lander mission that would provide more accurate dating of the lunar surface than can be achieved without bringing samples all the way back to Earth.

Phobos/Deimos Mission Proposals

Mars’ moons Phobos and Deimos have always been bridesmaids and never brides when it comes to Discovery mission proposals. Scientists have proposed Phobos and Deimos missions at least a handful of times before, such as the Aladdin mission proposal which came very close to selection in 1999. Russia launched the Phobos-Grunt mission in 2011 to bring a sample of Phobos back to Earth only to have the spacecraft fail as soon as it reached Earth orbit. Future Russian Phobos plans appear to be on hold due to the country’s financial situation. The Martian moons pose a number of scientific questions, such as their origin—are they captured asteroids or parts of Mars itself?—and the nature of unusual features on Phobos’ surface. One question of possible interest to future human missions to Mars is whether the moons contain any volatiles. There are three proposals for missions to these moons during this Discovery round, two orbiters and a lander.

PANDORA (Phobos ANd Deimos ORigin Assessment) is a JPL Phobos/Deimos mission that would use a Boeing spacecraft equipped with solar-electric propulsion.