An image of Jupiter’s swirling south polar region captured by NASA’s Juno spacecraft, December 2017. Image: NASA.
In February, NASA will land a nuclear-powered car on Mars. In 2022, the agency will be well into the construction of a Jupiter orbiter the length of a basketball court. Both missions—called “flagships” because of their multibillion-dollar budgets and what NASA describes as “civilisation-scale science” for their potential impact—are part of the agency’s search for life elsewhere in the solar system. But neither mission is likely to find life on its own, unless an animal scurries up to Perseverance rover’s camera or a geyser on Jupiter’s ocean moon blasts a fish into space right in front of Europa Clipper spacecraft.
To find life, follow-up missions will be needed for both. Perseverance will bottle up samples of Martian soil, but a subsequent spacecraft will need to collect the canisters and launch them back to Earth. Europa Clipper will characterise where life might most likely be found on its namesake moon, but a lander will need to saw its way into Europa’s ice shell and start sniffing for signs of things that wiggle. Due to these missions’ complexity, both Mars and Europa will again need flagships. And that means billions of dollars and several years of development.
Because the two follow-up missions take humankind so tantalisingly close to answering perhaps the most consequential question in all of science, religion, and philosophy—not to mention that they would complete work that’s already been started—it stands to reason that NASA would pursue one or both to build and fly next. But that’s not how NASA works. It’s a big solar system out there, and there are quite a few compelling destinations beyond Mars and Europa for NASA’s Large Strategic Science Missions, as flagships are officially called.
NASA does not make its decisions in a vacuum. Rather, the planetary science community itself makes exploration recommendations though a formal process and resultant document called the decadal survey. Every 10 years, the National Research Council of the National Academy of Sciences invites some of the top minds in the field to meet and read and synthesise hundreds of white papers written by planetary scientists before coming to consensus on the state of knowledge in the field, where next to explore, and how. This process and its resultant document is called the Planetary Science Decadal Survey.
The next decadal survey, which will recommend missions for flight in the 2023–32 time frame, is presently underway. If NASA follows custom, the agency will use those recommendations as a framework for mission development. And of the hundreds of thousands of bodies in the solar system, the next Decadal seems likely to choose among four key targets for flagship missions and rank them by priority. The places are Mars, Europa, Venus, and the ice giants Uranus and Neptune.
Historically, whenever there is an option to go to Mars or someplace else, NASA goes to Mars. It’s not merely a matter of proximity; NASA hasn’t launched a mission to Venus, which is closer, since 1989. The agency hasn’t landed anything on the moon since 1972, with Apollo 17. Mars, though, has Mars Odyssey, the Mars Reconnaissance Orbiter, MAVEN, InSight, and Curiosity all operating today. (Other countries have spacecraft orbiting Mars as well.) In a matter of weeks, they’ll be joined by NASA’s Perseverance and with it a helicopter named Ingenuity.
“The Mars program has been very successful because they’ve done a good job of establishing a sequence of missions to fill in the gaps in Mars science,” says Louise Prockter, chief scientist in the Space Exploration Sector at the Johns Hopkins University Applied Physics Laboratory. “They are able to build on preexisting missions and coordinate and produce science from multiple assets they have at Mars right now.”
The community of scientists pushing for evermore Mars exploration has been disciplined and organised in its priorities. Each answer they find on Mars never seems to close the door but instead opens a dozen more. Bruce Jakosky—the chair of the Mars Architecture Strategy Working Group, a group developing overarching approaches to future Mars exploration, and principal investigator of NASA’s MAVEN spacecraft—has been involved in Mars missions since the Viking program in the 1970s. “What stands out to me,” he says, “is that each Mars mission has seen a completely different planet than what we understood before. Each one has made fundamental discoveries because we’re making new measurements, and that gives us new insights.”
His working group is already looking beyond the sample launcher, whose necessity is self-evident for Mars science to continue apace. Getting a bottle of Martian soil into Earth-based labs will allow for analytical techniques and equipment impossible to put on a rover—and because the samples, once here, will be here forever, they will be available for state-of-the-art analyses a century from now as well. Still, says Jakosky, there are fundamental science goals that will not be addressed by sample return, such as investigating subsurface habitability and certain atmospheric processes: “Mars exploration doesn’t stop there. And there are plausible mission scenarios that could address those goals while being carried out affordably.”
One way or another, a Mars sample return launcher seems likely in the next decade, either through a partnership between NASA and other national space agencies or through congressional appropriations and direction. Informally, however, there seems to be a presumption that the next NASA flagship to launch will go to the ice giants—either Uranus or Neptune. It is a reasonable expectation. NASA sent Galileo to the Jupiter system in 1989, dispatched Cassini to the Saturnian system in 1997, and then passed over the ice giants in favor of Europa exploration and then Mars. It’s Neptune’s turn, goes the refrain.
If one were to ascribe to a fairness model of flagship-class exploration, the ice giants would be a no-brainer for decadal study endorsement. Humankind has had only fleeting glimpses of those worlds: flybys of Voyager 2 in 1986 (Uranus) and 1989 (Neptune). Some moons circling the planets, like several of those circling Jupiter and Saturn, likely have subsurface oceans, and thus the potential for life. The 2013 decadal study gave the ice giants a strong endorsement for flagship-class exploration, behind only Mars sample return and a Europa orbiter. Perseverance will land on Mars in February, and Europa Clipper is well into development and will launch within the next four years. Arguably the ice giants should be next, but a growing chorus of planetary scientists are questioning whether a multibillion-dollar, politically fraught flagship-class spacecraft is the best way to explore them.
Flagships are risky endeavours. It took 20 years for NASA to approve a Europa mission despite two decadal endorsements. When budgets fell, so too did Europa’s fortunes. Likewise, Cassini, Europa Clipper’s outer-planets predecessor, was nearly canceled in 1991 in favour of funding the International Space Station and had to endure painful engineering limitations to account for diminished funding. The Reagan administration sought to kill Galileo—what’s there to learn at Jupiter, really? Before that, the Nixon administration slashed the number of Voyager spacecraft from four to two.
There is no reason to think the next flagship, wherever it goes, will fare any better. NASA’s fiscal and political realities have not improved in the last 30 years. The agency still gets an imperceptible slice of the U.S. federal budget—0.5 percent. Technology and NASA engineering prowess, however, have improved dramatically—particularly since the inception of the agency’s New Frontiers program, a tightly managed, billion-dollar class of spacecraft. (Its smallest mission class, Discovery, flies pathfinder spacecraft in the half-billion-dollar range.)
Today, $1 billion will take you a lot farther into deep space than it would have in the past. The New Frontiers selection process is like Shark Tank for space exploration. Academic, government, and commercial institutions compete bitterly to win those contracts, submitting comprehensive mission designs to NASA, which, through multiple elimination rounds, chooses the soundest concept that best addresses science questions posed by the decadal survey. Such competitions have yielded inventive and unexpected new ways of exploring the cosmos, from the OSIRIS-Rex asteroid sample mission (a spacecraft the size of a UPS truck but with the navigational agility of a hummingbird) to Dragonfly, a spacecraft unlike any NASA has ever flown: a drone-like quadcopter that will soar across the skies of Titan, Saturn’s largest moon, and land to take in situ measurements of the only non-Earth world with stable liquid bodies on its surface. (Where Earth’s lakes and seas are filled with water, however, Titan’s seas are filled with liquid methane.)
All of which raises a question that the planetary science community, perhaps through the decadal process, must now answer: What is the role of a flagship in an age of nimble New Frontiers missions, and when should a flagship be flown? Among those asking is Scott Bolton, the principal investigator of the Juno mission at Jupiter, which captured the public’s imagination with the largest planet’s unexpected, stunning blue poles.
“There will always be a role for flagships,” says Bolton, but the rationale for when they should be flown is changing. Juno, he says, was conceived as a flagship of sorts on a New Frontiers budget. The mission was just extended, and with its new lease on life comes more ambitious science: “We’re actually going to get close flybys of the satellites using gravity assists, which makes us a full system explorer. And we have a very broad science payload to be able to do that. To me, it gets us closer and closer to the idea of a flagship. And with a demonstration like that, I think the science community should start to ask whether a flagship is necessary to accomplish most of the science of an ice giants mission, or could you do it instead by flying the next generation of Juno?”
Planetary scientists could get a mission through the New Frontiers program faster, he says, and for the price of single flagship could get one to Uranus and one to Neptune—and still save money.
Kevin Hand, the pre-project scientist for the Europa Lander mission concept at NASA’s Jet Propulsion Laboratory, agrees. “Flagship missions should be the mission class of last resort,” he says. “As a planetary scientist, you should be screaming and running away from flagship missions if you can fit your science into a Discovery or New Frontiers class. You should be excited by the prospect of getting that science done without having to go down the path of a flagship.”
Flagships have historically been defined by cost and epic, systemwide science: multibillion-dollar-spacecraft to explore the entire solar system, as with the Voyagers, or the Jovian system, as with Galileo. What should define the flagships of tomorrow, Hand argues, goes beyond celestial objects or one’s preferred discipline of study. “We have not just a scientific responsibility to pursue civilisation-scale questions,” says Hand, “but also a responsibility to pursue technological innovations that enable new kinds of exploration, be that in aeronautics, space exploration, or other kinds of technologies that, to quote the NASA mission, transform our understanding of the universe, unlock new opportunities, and inspire the world.”
That is something, he says, the Europa Lander will do. Space is hard, but landing on Europa will be really hard. The ocean moon exists in a pulsing, punishing radiation belt with conditions like those that follow the detonation of an atomic bomb. (Europa Clipper will orbit Jupiter rather than Europa specifically to avoid marinating in that radiation; the lander will have no such luxury.) Where Europa Clipper will study habitability, Europa Lander would be looking for life. And it will take massive technical feats to do so. The nuclear-powered spacecraft would have to touch down in some safe spot on the surface (the nature of which is unknown), lower a saw, and dig like hell. It will reach 10 centimetres into the moon’s granite-hard ice shell, beneath which any evidence of life would be protected from the pitiless radiation. Hand says engineers can work with those parameters—indeed, such technical challenges are precisely why engineers join NASA in the first place.
“Engineers come to Jet Propulsion Laboratory to do things that are hard—to dare mighty things and push the frontier in ways that have never been done before,” Hand says, whether it’s a new kind of landing system to touch down on Mars or a Europa system derived from that Mars tech to alight gently on an ice-covered ocean for the first time. More than $100 million, appropriated and directed in recent years by the US Congress, has already been spent on Europa Lander development, which gives the prospective flagship a developmental advantage over other missions of that class. Working against the lander previously was its total absence in the 2013 decadal survey. Though Congress was keen to see the mission fly, NASA, without a recommendation from the scientific community, lent no support to the project beyond what the law required. But the next decadal survey will likely include the lander in its flagship priority queue.
Venus, meanwhile, is the dark horse of the lot. As a matter of comparative planetology, Venus is an intriguing target for intensive study. Why is Venus, which is about the same size as Earth, is made of the same stuff, and has the same gravity and density, so different from Earth? What caused the runaway greenhouse effect that leaves Venus hotter even than Mercury, the closest planet to the sun? With astrobiology driving most NASA exploration, Venus has generally been at a disadvantage: Nothing we would want to meet is alive on its 471 degree C surface. Last year, however, astronomers reported the discovery of phosphine in the Venusian clouds, which would suggest the presence of microbial life. Since then, the finding has been questioned, but that doesn’t mean phosphine isn’t there; it means that much more study is needed. If answers can be found with smaller, inexpensive precursor spacecraft, they might strengthen the case for a flagship there—though not in this decade, as NASA has no such preliminary Venus probes in its portfolio of approved missions.
The members of the decadal survey now underway will make the recommendation, ultimately. Perhaps Venus will surprise everyone. Or perhaps the decadal will find the ice giants are better served in New Frontiers, giving its flagship endorsement instead to either Mars or a Europa Lander, or both. Whatever the decadal decides, the flagship it chooses will be with us for a long time. If history is any indication, because of the delays every flagship seems to incur, a very, very long time indeed.