A team proposing the use of a flying rover to explore Saturn’s moon Titan, and another that wants to send a sample-collecting mission to Comet 67P/Churyumov-Gerasimenko, are the finalists in NASA’s search for its next interplanetary destination, officials announced Wednesday. The competition began in earnest in late April, when 12 teams submitted proposals to fly spacecraft to a wide variety of targets in our solar system. Each finalist team will receive $4 million to firm up its concept by mid-2019, when NASA will choose one to fully develop as the fourth member of the space agency’s “New Frontiers” program. Previous New Frontiers missions include the New Horizons probe sent to Pluto, the Juno orbiter at Jupiter and the OSIRIS-REx spacecraft, which is now en route to collect samples from the asteroid Bennu in 2018. The fourth New Frontiers mission would launch before the end of 2025.

“This is a giant leap forward in developing our next bold mission of science discovery,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington, D.C. “These are tantalizing investigations that seek to answer some of the biggest questions in our solar system today.”

Titan is Saturn’s largest moon, a cryogenically cold world with a surface shaped by hydrocarbon rivers and seas fed by methane, ethane and complex organic compounds raining out of its thick, smoggy atmosphere. Beneath its frosty exterior, Titan also hides a deep, liquid-water ocean. The New Frontiers finalist, called Dragonfly, would send a nuclear-powered helicopter-like drone to hop from place to place upon Titan’s surface to study its geology and evaluate its prospects for life.

With Dragonfly, “we can evaluate how far prebiotic chemistry has progressed in an environment that we know has ingredients for life,” says the mission’s lead investigator Elizabeth Turtle, a planetary scientist from Johns Hopkins University Applied Physics Laboratory. “By being a rotorcraft, we’re able to fly to multiple sites tens to hundreds of kilometers apart to make these measurements in different geological settings.” If chosen for flight, Dragonfly would reach Titan in 2034 and operate for a few years, Turtle says.

NASA’s Cassini orbiter mapped Titan in detail during its 13-year mission at Saturn. It also deployed a European-built lander, Huygens, that briefly transmitted data from the surface after touching down in 2005. Comet 67P/Churyumov-Gerasimenko was also previously visited by the European Space Agency’s Rosetta spacecraft, which deployed a lander, Philae, that malfunctioned and was lost on the surface. Rosetta’s mapping of the comet, as well as the lessons learned from Philae’s unsuccessful landing attempt, make it an attractive target for scientists and engineers eager to try again. Comets are the frozen leftovers of the primordial material from which planets coalesced, and also played a role in delivering water and organic molecules to the ancient Earth billions of years ago.

“We can design for specific conditions we know to exist, and that increases the chances of success for a very difficult activity, which is grabbing a sample from the surface of a comet,” says Steven Squyres, lead investigator of the second finalist, the Comet Astrobiology Exploration Sample Return (CAESAR). Squyres, a planetary scientist at Cornell University in Ithaca, New York, is also the lead investigator for NASA’s long-lived Opportunity Mars rover  (although he says he plans to step down from that role if CAESAR is selected for flight). CAESAR’s goal will be to gather at least 100 grams of ice and rock from the comet’s surface, then to return the sample to Earth in a sealed, thermally stabilized capsule in 2038. That, Squyres says, would be “really just the beginning of the mission,” as scientists would spend decades analyzing what would be the first ever returned from a comet’s surface.

Besides CAESAR and Dragonfly, NASA also chose two runner-up proposals to receive funding for further technological development. One targets Saturn’s Enceladus, a small, icy moon that is spouting plumes of briny water vapor from its south pole and is thought to harbor a potentially habitable ocean beneath its crust. Led by Chris McKay, a planetary scientist at NASA’s Ames Research Center in California, the Enceladus Life Signatures and Habitability (ELSAH) mission would fly through a plume to seek out signs of biology lurking within the tiny moon. The other runner-up, VICI (Venus In situ Composition Investigations), would send two landers to different locations on the surface of Earth’s sister world, to learn how this once-temperate planet transformed into its current hellish state.

According to Jim Green, head of NASA’s planetary science programs, while both ELSAH and VICI are very exciting missions, “some of the elements that are proposed aren’t quite at the maturity that would make the mission(s) a success if we would just start off and build things right away.” Once plans are more developed, Green suggests, the teams could then re-propose their missions for future slots in NASA’s portfolio of planetary-science programs.

With budgets capped at roughly $1 billion and a launch rate of about twice per decade, New Frontiers missions are arguably the best bets for breaking new ground in planetary science. Their balance of cost and capability occupies a highly prized sweet spot between NASA’s less-expensive, higher-cadence “Discovery” program, and its typically multibillion-dollar, once-per-decade “Flagship” missions. A New Frontiers mission can be big enough to address major scientific questions without breaking the bank, yet small enough to zoom through development and testing cycles that might delay larger and more ambitious missions for generations. Plus, unlike many other NASA space science missions that are conceived, developed, built and operated in-house, a New Frontiers concept can come from “principal investigators” outside the agency, offering a chance for more innovation to come with it.

“The New Frontiers program is really the premier program for our principal investigators, and indeed it is one of the most difficult programs to be selected for,” says Curt Neibur, NASA’s New Frontiers program scientist.

That lesson is not lost on the wider planetary science community, which sees the program as crucial for establishing both winners and losers in the high-stakes game of interplanetary exploration. Constrained by costly rockets, long development times, rare launch windows and relatively tight budgets, the chance to send a new mission to another world is a once-in-a-lifetime opportunity for most researchers.

But much more than individual careers hangs in the balance. Each time NASA winnows candidates for a new mission, it also helps dictate which subfields will soon benefit from a bumper crop of fresh data—and which are likely to languish in obscurity for lack of interest and resources. Studies of Mars, for instance, have boomed in recent decades thanks to the steady stream of missions sent there; meanwhile, NASA hasn’t sent a spacecraft to Venus in more than 20 years. Other worlds, namely Uranus and Neptune, are even more overlooked: Both have only been visited once apiece by NASA’s Voyager 2 spacecraft, which flew by them in the 1980s on its way out of the solar system.

With its latest selections that prefer previously (and recently) visited targets, NASA seems to be reinforcing the case that, at least in planetary science, the rich get richer. Out of the 12 submitted New Frontiers proposals, nearly half targeted Saturn or its moons, hoping to capitalize on the afterglow of the Cassini mission, which ended in a fiery plunge into Saturn’s atmosphere in September after a wildly successful 13 years around the ringed planet. Those that failed to progress included a proposal to send a spacecraft diving, as Cassini did, into Saturn’s atmosphere to study its composition and history, as well as a notional orbiter for Titan and a second plume-diving spacecraft for Enceladus independent of ELSAH. Similarly, besides CAESAR, another proposal, the Comet Nucleus Dust and Organics Return (CONDOR) mission, would have also targeted Comet 67P.

These also-rans, particularly ones targeting other less popular destinations—such as Venus, which had two additional proposals besides VICI—will have to wait until NASA or another space agency provides more opportunities for funding, a prospect that may be many years away. That outlook is decidedly grim, if history is any guide. Out of the latest batch of proposals, one—a mission called MoonRise aimed at retrieving samples from Earth’s moon—achieved coveted “finalist” status in the previous two New Frontiers selections, only to be passed over two and now three times.

“Patience is a virtue in this business,” Squyres quips, noting that like many of his competitors he had been feverishly working on his proposal for years. While the successful teams move forward with plans for exploring Titan and Comet 67P, the message to others waiting to explore other new frontiers remains stubbornly the same: Better luck next time.