Three of the downselected missions aim for asteroids, but the missions are quite different:

Psyche, led by Lindy Elkins-Tanton of Arizona State University, would rendezvous with a single asteroid, (16) Psyche, a 213-kilometer-diameter object located in the outer part of the main asteroid belt. Astronomers think that Psyche is a metal asteroid based on its high radar reflectivity and high density. Did Psyche form with its metal surface, or did an ancient hit-and-run collision strip it of its mantle? If the former, we'd see a completely different kind of puzzle piece from the formation of the solar system; if the latter, we'd get to peer into the core of a planetary body. Either way, it'd be a new kind of world.

NEOCam, led by Amy Mainzer of the Jet Propulsion Laboratory, would hunt for undiscovered asteroids in the space close to Earth. Building on the success of Mainzer's NeoWISE asteroid survey mission, it would survey the sky in infrared wavelengths from a position between the Sun and Earth (the Sun-Earth L1 point). Earth-based surveys have successfully reduced our risk from near-Earth asteroids by discovering most of the larger potentially hazardous asteroids, but there is still a fair amount of space that's difficult to survey, where asteroids lurk in sky positions close to the Sun as seen from Earth. If NEOCam lasts long enough, it could find almost all of the remaining potentially hazardous objects. Not to mention that its surveys would catch approximately one million main-belt asteroids and discover potential future targets for in-situ exploration -- maybe even human exploration. NEOCam would be popular in the amateur community, promising extremely rapid archival data release, within two weeks of receipt on Earth, with no proprietary period (p. 16 of this presentation).

Lucy, led by Hal Levison of the Southwest Research Institute, would perform a reconnaissance mission to three Trojan asteroids, at least one of which is binary, using versions of instruments developed for New Horizons. The likely asteroids would be (3548) Eurybates, (21900) 1999 VQ10, (11351) 1997 TS25, and the binary (617) Patroclus | Menoetius. It would also visit a main-belt asteroid (1981 EQ5) on the way. The Trojan asteroids are a very large population -- there may be as many Trojans as there are main-belt asteroids! -- which swarm along Jupiter's orbit in the leading and trailing Lagrangian points. Models of the solar system's formation suggest that many Trojans originated in the Kuiper belt; their very dark surfaces hint at compositions rich in the organic materials that coat Kuiper belt worlds. So a mission to Trojans could tell us about a much more distant region of the solar system.

The other two missions would travel to Venus, a planet that has not seen a NASA spacecraft since Magellan entered its atmosphere in 1994. Venus has been a difficult planet to study; its opaque clouds and corrosive, hot, high-pressure atmosphere act like a force field to keep spacecraft away from its intriguing surface. Magellan gave us a radar image map of the globe at 75-meter resolution and topography at 4-kilometer resolution, but that data set left us with more questions than we had when Magellan arrived.

VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy), led by Sue Smrekar of the Jet Propulsion Laboratory, would fly a modern, shorter-wavelength radar instrument to map Venus again at much higher resolution over the course of three Venus years. VERITAS would produce a global digital elevation model for Venus, with spatial resolution of 250 meters and elevation accuracy of 5 meters.

DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging), led by Lori Glaze of the Goddard Spaceflight Center, is a Venus descent craft about which there is very little information on the Internet. According to the NASA release, "DAVINCI would study the chemical composition of Venus’ atmosphere during a 63-minute descent. It would answer scientific questions that have been considered high priorities for many years, such as whether there are volcanoes active today on the surface of Venus and how the surface interacts with the atmosphere of the planet."

With so many great missions to choose from, the question naturally arises: can’t NASA build more than one? The answer, as with many questions about NASA, comes down to money. There are no physical restrictions preventing NASA from building two Discovery missions at the same time, but that effectively doubles the money needed during the late 2010s—a time when the Mars 2020 rover project will peak in its funding needs and when the Europa Clipper will be ramping up its development. Assuming NASA wants to start another New Frontiers mission in the meantime (a beefier version of the Discovery mission program), it’s hard to see any realistic scenario that could encompass the addition of an extra Discovery mission.

That said, you never know what can happen with a supportive Congress and a new Presidential Administration. It never hurts to keep reminding them that their constituents love exploring the solar system.