NASA is set to handle some unfinished business across the solar system, as evidenced by four possible missions it announced yesterday. They are the finalists in the space agency’s Discovery Program competition, a Shark Tank–like face-off in which labs and universities have proposed small, focused spacecraft to explore disparate worlds. More than a dozen ideas, each designed to cost less than $500 million, were submitted to NASA for consideration. Of the four now remaining, only two or perhaps just one will be chosen next year for eventual flight.

“It’s an interesting mix,” says Margaret Kivelson, chair of the Space Studies Board at the National Academies of Sciences, Engineering, and Medicine. “They are well thought-out missions—and to fascinating objects.” Under consideration are spacecraft that would travel to the Jovian moon Io, the most active volcanic body in the solar system; Venus, considered Earth’s “evil twin” because its environmental conditions are hellish, despite the planet being nearly identical to our own in size, mass and composition; and Triton, a moon of Neptune thought to harbor a subsurface ocean.

The proposed Io Volcano Observer, Kivelson says, fits nicely in a broad, ongoing science campaign at Jupiter. NASA intends to launch Europa Clipper, a multibillion-dollar “flagship” craft, as early as 2023 to investigate the mission’s namesake: the icy Jovian moon Europa, which also possesses a subsurface ocean. On arrival, Europa Clipper’s studies will seek to determine whether that lightless ocean could sustain life as we know it. Meanwhile the European Space Agency will launch JUICE (Jupiter Icy Moon Explorer), an orbiter bound for the planet’s largest moon, Ganymede, in 2022. Both craft could conceivably arrive within the operational lifetime of NASA’s Juno mission, which is presently studying Jupiter’s interior. “The idea that you might get an Io observer at Jupiter at the same time that Europa Clipper might be up there and Juno might be working and JUICE at Ganymede—can you imagine the understanding of Jupiter we would have after that?”

Two of the mission finalists—DAVINCI+ and VERITAS—target Venus for further study. If either was used, it would mark a belated return by NASA to Earth’s troubled next-door neighbor. The agency has not launched a dedicated Venus spacecraft since 1989—a painful delay for a generation of scientists keen to explore the world as methodically as we have investigated Mars. With the discovery of extrasolar planets across the galaxy, “comparative planetology” has become an urgent field of study: How is it that two extremely similar planets can evolve so very differently?

To answer that question, DAVINCI+—or Deep Atmosphere Venus Investigation of Noble Gases, Chemistry, and Imaging Plus)—would drift to the Venusian surface and take precise measurements of its atmospheric composition, with an onboard camera taking images during the descent. VERITAS—short for Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy—would orbit Venus and map the entire planet, building three-dimensional topography and seeking to learn more about why this broadly Earth-like planet lacks water and possesses a surface temperature hot enough to melt lead.

Trident, a “flyby” mission that would not enter orbit around its destination, would confirm or cast doubt on the presence of a subsurface ocean on Triton, Neptune’s largest moon, which is thought to be a captured object from farther out in the solar hinterland. The question of Triton’s putative ocean traces back to at least 1989, when NASA’s Voyager 2 spacecraft partially mapped the moon’s surface and witnessed plumes of vapor blasting into space from the interior during a flyby. If those plumes originated from an inner ocean, that finding could extend the so-called habitable zone—the region around a star where liquid water exists and life is possible—to encompass the entire solar system.

Kivelson, who helped to discover the ocean of Europa, says that the Trident mission and the possibility of it revealing liquid water on Triton would fit a broader pattern of planetary science. “We now know there are oceans beneath the surface of [Saturn’s moons] Enceladus and Titan, and it’s really remarkable—a little bit like the discovery of extrasolar planets,” she says. “It wasn’t long ago that we didn’t know that any stars other than ours had solar systems. And now almost every one we look at does. It fits in that same mold: the more we look, the more we find. It’s a big change in our thoughts about outer-planet moons.”

Trident would arrive at the Neptune system a half-century after Voyager 2. Historically, missions to the outer solar system have price tags of billions of dollars. Louise Prockter, principal investigator of Trident, says that her team’s project would thus fundamentally change what is possible with NASA’s small planetary mission class. “Before now, no Discovery mission has ever been implemented to fly beyond the Jupiter system, five [astronomical units (AU) from the sun],” she says (one AU is the Earth-sun distance). “We would go to 30 AU. No one ever imagined we could do this kind of fundamental science in the deep outer solar system on this kind of budget.”

Trident’s path to the final round of Discovery was a hard-fought one. NASA initially forbade the use of nuclear power sources for the competition. Prockter, her team and the outer-planets community lobbied hard against this decision, arguing that without nuclear power sources, the deep outer solar system would effectively be closed off to exploration. (Solar panels are useless at such vast distances from the sun.) Their argument prevailed.

“We are thrilled to be one step closer to exploring one of the solar system’s weirdest icy worlds,” Prockter says. “We will answer fundamental questions, such as whether a captured world can be an ocean world, how icy worlds evolve and why Triton might be active today. We will also image the largest remaining unmapped surface of any world in the solar system.”

Trident is the only mission to make the final round that has not been a previous entrant in past Discovery competitions. Versions of VERITAS and DAVINCI+ were finalists in the preceding contest, and this one includes the Io Volcano Observer’s (IVO’s) third shot at selection. Each iteration of IVO proposals, says Alfred McEwen, its principal investigator, has seen engineering breakthroughs and enhanced science potential. Among the questions the craft seeks to answer is whether Io has a magma ocean. Such interior bodies of molten rock are fundamental for understanding the evolution of terrestrial planets. Moreover, McEwen says, IVO will allow planetary scientists to complete their study of Jupiter’s four largest moons—which form an interrelated system with unique orbital resonances and other complex interactions. “You really can’t understand one of these worlds in isolation,” he says. “We need Io data to complete the science.”

Perhaps even more so than inhospitable Venus, the Jupiter system may be the harshest environment in the solar system, with belts of blistering radiation surrounding the planet and bathing Io and other nearby moons. IVO, developed at the University of Arizona and the Johns Hopkins University Applied Physics Laboratory, has benefitted greatly from the Europa Clipper mission, which must survive in the most punishing parts of that field. “A lot of money is being spent on Europa Clipper instrument development and figuring out problems of the Jovian environment,” McEwen says. IVO would use near copies of those radiation-hardened instruments, saving on development costs. Moreover the craft would be powered by high-efficiency solar panels, a technology proved by Juno.

The four candidate mission teams will spend the next nine months developing detailed concept study reports, due in November. From there, NASA will review the reports and run them through full, independent “technical, management and cost” reviews, as well as complete evaluations of the mission science. Additionally, the prospective missions will be subjected to one-day “site visits,” in which reviewers will meet face-to-face with the proposal teams, who will give presentations of their projects and then undergo question-and-answer sessions, described by one NASA insider as “very fun but very tense.”

The missions chosen for flight will be announced in or around June 2021 and could launch as early as January 2025.