The planetary science community rippled with euphoria in June when Dragonfly, a bold mission to send a nuclear-powered dual quadcopter to Saturn’s largest moon, Titan, was given the green light by NASA. Yet even as the Dragonfly team erupted in celebration at the news, those working on its competitor, the Comet Astrobiology Exploration SAmple Return (CAESAR) mission, mourned. If CAESAR ever launches, it will be many years after Dragonfly, much later than its proposers had planned; more likely, in its current form, it will never be built at all.

Years of effort and staggering amounts of person-hours go into a typical planetary-science mission proposal, and for each one “you don’t know if you’re wasting your time, or even your career,” says Dante Lauretta, a member of the CAESAR team at the University of Arizona. “You gotta say, ‘we’re gonna win’… or else you can’t do the job. And when you lose, it’s crushing.”

Members of the CAESAR team are not alone in their loss: Dragonfly won out over 11 other proposals. In fact, every single time NASA holds a competition to select a new robotic interplanetary mission, the eventual winner only reaches space by leaving many more in the dust. The effects stretch beyond individuals and institutions to impact fundamental science itself: as objective as the process aims to be, it is undeniable that it favors missions to certain locations, such as ever-popular Mars, while other worthy interplanetary destinations—Venus, for instance—languish unvisited.

Impartial as it may be, the mission-selection process inevitably fosters an imbalance in our comprehension of various corners of the solar system. Could there be a better way?

An Interplanetary Balancing Act

Several of NASA’s family of robotic exploration programs rely on teams of scientists to propose missions and compete against one another for funding. One prolific example is the agency’s Discovery-class missions; these are meant to be narrowly focused and relatively low cost (as far as interplanetary spacecraft go), with cost caps of around $600 million. Winning examples include Psyche, set to launch to a bizarre all-metal asteroid in 2022. NASA’s larger New Frontiers-class missions, as another example, have a roughly $1 billion cap and more technical capabilities. New Horizons, which recently flew past Pluto, was a winner of this process, as was Dragonfly.

Both programs solicit submissions every few years, enlisting the aid of multiple review panels and deploying cross-examinations of each team’s scientists to intensely scrutinize each proffered proposal. When only a handful remain after ruthless rounds of winnowing, the final decision falls solely to one individual, the associate administrator for NASA’s Science Mission Directorate (SMD), whose appetite for risk may wildly vary from his or her predecessors and successors.

Michael New, the deputy associate administrator for research within NASA’s SMD, says the review process is an “incredible gauntlet,” but one that maintains a strict objectivity. Tom Wagner, program scientist for NASA’s Discovery-class missions, explains that the process is carefully structured and monitored to ensure there’s no bias in the hugely scientifically diverse reviewer pool.

Crucially, there is never a preselection preference for any mission types or their destinations, says Curt Niebur, program scientist for NASA’s New Frontiers–class mission group. In other words, missions to Mars, Venus or anywhere else in the solar system are meant to be treated on an equal basis right from the beginning. In all cases, reviewers favor technically proficient proposals that strike a balance between delivering compelling science and calling for costly research and development. Missions promising revolutionary discoveries can rely on equally revolutionary new technologies, raising the risks of devastating budgetary overruns (and usually lowering their odds of selection).

Consequently, plenty of phenomenal proposals inevitably lose out. Niebur explains, however, that officials always sit down with unsuccessful teams to review their proposal’s strengths and weaknesses. “We want them to learn from that experience, so when they come back the next time, when they show their persistence, they come back better, they come back stronger,” he says.

Winners and Losers

This process has produced a remarkable number of game-changing science missions that have dramatically expanded our view of the solar system, but it is not without its blind spots. Consider, for example, that—opportunistic, occasional flybys aside—the last NASA-led mission to Venus was Magellan, a spacecraft that spent four years mapping our sister world in the early 1990s. Venus is Earth’s planetary twin, albeit one where something went horribly awry—hence its crushing atmosphere, lead-melting temperatures and sulfuric-acid rains. This freakish environment demands to be investigated, says Larry Esposito, a planetary scientist at the University of Colorado Boulder who has worked on several Venusian proposals. By studying Venus, researchers could learn a lot about how a promising Earth-like planet—our own, or those orbiting other stars—can become uninhabitable.

Another chronic blind spot is the outer solar system, particularly the “ice giants” of Uranus and Neptune, which have each only been visited once, in passing, during the late-1980s flybys by the Voyager 2 probe. Mark Hofstadter, a planetary scientist at NASA’s Jet Propulsion Laboratory, says that models predicting how solar systems form suggest that ice giants should not exist—and yet, they can be found around plenty of stars including our own. Sending a spacecraft to study Uranus and Neptune up close would help unravel this enigma—if only NASA (or another space agency) would select one of the many existing proposals to do just that.

Casey Dreier, chief advocate and senior space policy adviser at the Planetary Society, says that this process will always have “an intractable problem”: when funding is limited, there will always be too many great destinations and mission proposals to go around.

Selling a worthwhile voyage to Venus, however, may be especially difficult. A surface mission could conduct pioneering science, but the planet’s hellish environment limits most landers’ lifetime to just an hour or two. Such a short duration would be unappealing compared with a mission to a different destination that would last for a year or more on the same budget and gather far more data overall, says David Grinspoon, a senior scientist at the Planetary Science Institute in Tucson, and a prominent advocate for Venusian exploration.

At the same time, argues Lauretta, some places are inherently easier sells. Everyone agrees that Mars is a very high priority target, if for no other reason than that past or present life might conceivably be found there. Scientists may want to study Mercury too, but with no obvious prospects for alien or human life there, it is all too easy—all too rational, even—to pass over the closest planet to the Sun in favor of another visit to Mars or some other more promising place.

There are, of course, other concerns beyond the quest to discover extraterrestrial life. Our solar system’s lonely ice giants, Hofstadter says, are so distant that any voyage capable of reaching them within the careers and lifetimes of present-day researchers requires spacecraft to have an extremely diverse and advance array of instrumentation in order to conduct a justifiably thorough scientific investigation. Fitting such missions into the budgets presently allowed for Discovery- or New Frontiers–class missions may be a fool’s errand, Dreier says.

Caught in a Feedback Loop

Arguably, the most pernicious problem facing interplanetary exploration is the fact that it easily becomes a victim of its own victories. “There’s kind of a dynamic in our field where success breeds success,” Grinspoon says.

Take NASA’s triumphant series of recent Mars missions—not only its multiple generations of landers and rovers, but also the orbiters surveying Mars and acting as communications relays to Earth. The spectacular science those missions have conducted also elicits significant public excitement and an understandable propensity towards additional follow-up Mars missions, which in turn attracts more funding—funding often used to support early-career researchers. In this way—naturally, almost inevitably—an entire generation of planetary scientists could become focused on Mars, sustaining the cycle with yet more research into and support of future missions to the Red Planet.

“I really think that that feedback loop has been affecting our mission selections,” Grinspoon says, suggesting this is partly why places like Venus have been so long neglected.

Clearly, greater government investment in planetary science would alleviate some of these problems, and such a request would hardly be impertinent; $1 billion for a single mission stretched across a decade or more may sound like a lot, but it is a figurative drop in the ocean compared with the amount spent annually on national security—a figure that reached $716 billion in 2019 alone. “The defense budget has increased by roughly a NASA-and-a-half in the last couple of years with almost no debate,” Dreier says.

Absent some improbable boost in budgetary bottom lines from the federal government, however, plummeting launch costs due to innovative rockets from companies such as SpaceX and Blue Origin are having an outsized effect on plans for future interplanetary exploration. Additionally, today’s rockets are more likely to save costs by carrying multiple payloads—a development that Jon Morse, CEO of the BoldlyGo Institute and a former director of NASA’s astrophysics programs, says the agency is beginning to leverage. NASA’s recent InSight lander, for example, was accompanied to Mars by CubeSats.

If NASA continues to take advantage of both these trends, Lauretta says, it could gain more freedom in its interplanetary planning, keeping the same price-point for missions but spending more on upping the payload and the mission’s technical capabilities rather than rocket launches. Similarly, freed-up funds from lower-cost launches could instead be used to support more proof-of-concept technology demonstrations, Morse says —something that might help those technically troublesome Venusian missions prove their worth to wary mission planners.

NASA’s selection process itself might also benefit from a bit more flexibility. Although it does not have to be religiously adhered to, New Frontiers somewhat restricts its exploration targets and types based on the advice of the planetary science Decadal Survey, a ranked listing of future research priorities produced every 10 years by U.S. researchers. The latest planetary Decadal Survey, published in 2011, did not list Titan as an eligible target, but thanks to subsequent eye-opening studies using the Cassini orbiter, Titan and another Saturnian moon, Enceladus, were added in 2016 to the New Frontiers competition under the “Ocean Worlds” mission banner, paving the way for Dragonfly.

Discovery-class missions already benefit from always being open to (almost) any destination. “I don’t see why that shouldn’t be true for those mid-class missions,” Dreier says.

Ready to Fly

Those that have been part of unselected mission teams all agree that it is an extremely grueling process. Looking back at CAESAR, Lauretta reckons that “there was nothing we could have done better,” adding that “it was absolutely ready to fly.”

They nevertheless remain sanguine. Their sentiments about the selection process are largely echoed by Hofstadter, who says that “I don’t feel in any way that it’s been unfair. You’ve just gotta keep working at it.”

As the principal investigator for asteroid sample-return mission OSIRIS-REx, a proposal that won an earlier New Frontiers competition, Lauretta knows both sides of the coin. “It’s a rollercoaster, there’s no doubt about it. But you can’t be bitter, because Dragonfly is amazing,” he says. “A quadcopter on Titan? Hell yeah! If they can pull it off, they absolutely deserve to win.”

Dragonfly team member Sarah Hörst, a planetary scientist at Johns Hopkins University, has been on several unsuccessful proposals in the past herself, for missions to Venus, Enceladus and Titan. “We’ve all been on the bad side of the decision,” she says. That’s why, she adds, the team agreed that it is now their job to honor the teams that did not get selected, by doing the very best they can with the opportunity they have been given.