Ice giants and icy moons: The planetary science decadal survey looks beyond Mars to the outer solar system

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The last time the planetary science community came together to conduct a decadal survey, Mars was ascendant. NASA had flown a series of Mars missions and was working on its most ambitious rover yet, Curiosity. The decadal survey endorsed continuing that Mars exploration strategy by backing a mission to collect samples as the first step to returning those samples to Earth.

The situation is somewhat different in the latest planetary science decadal survey, published April 19 by a National Academies committee. While Mars is still at the center of much of present-day planetary science at NASA, the planetary decadal makes the case that the future is further out in the solar system, among distant planets as well as icy moons that could harbor life.

URANUS OVER NEPTUNE, ENCELADUS OVER EUROPA

While the decadal survey offers a broad survey of planetary science, from an overview of current knowledge of the field to the state of the profession itself, the section that gets the most scrutiny is its recommendations for the next flagship missions NASA should pursue. Those recommendations drive decisions on missions costing billions of dollars. The two flagship missions from the previous decadal survey in 2011, a Mars rover to cache samples and a Europa orbiter, took shape as the Perseverance rover currently on Mars and Europa Clipper, set to launch to that icy moon of Jupiter in 2024.

The leaders of the decadal survey said that, in their deliberations, it became clear that the focus of the next major flagship mission should be two of the solar system’s least-studied planets, Uranus and Neptune. Both have been visited by just a single spacecraft: Voyager 2, which flew by Uranus in 1986 and Neptune in 1989. Both are called “ice giants” by scientists, as they’re smaller than the gas giant worlds of Jupiter and Saturn, and may have some mix of rock and ice in their interiors.

Uranus and Neptune are the solar system’s least-studied planets. Both have been visited by just a single spacecraft: Voyager 2, which flew by Uranus in 1986 and Neptune in 1989. Credit: NASA/JPL-CALTECH

“This is the only class of planet in the solar system that hasn’t had a dedicated orbital tour,” said Robin Canup of the Southwest Research Institute, one of the co-chairs of the steering committee for the survey. “Understanding the composition and the properties of either one would revolutionize our understanding of ice giant systems and solar system origins.”

Another factor is that studying Uranus or Neptune could provide insights into exoplanets, given the large number of ice giants discovered around other stars. “This may, we think, be the most common class of planet in the universe,” she said.

The key question then became whether to send a mission to Uranus or Neptune. Here, technical readiness tipped the scales in favor of a Uranus mission. “For the Uranus Orbiter and Probe, we have a viable end-to-end mission concept right now on currently available launch vehicles,” Canup said. “There are no new technologies required for this mission.”

The $4.2 billion mission, launching as soon as 2031 on a Falcon Heavy or similar large launch vehicle, would place a large spacecraft in orbit around Uranus to study the planet and its moons and rings, and also deploy a probe into the planet’s atmosphere, as Galileo did at Jupiter in the 1990s. A launch in 2031 or 2032 could take advantage of a gravity assist by Jupiter to reach Uranus in about 13 years, while a launch later in the 2030s would require gravity assists in the inner solar system, reaching Uranus about 15 years after launch.

The decadal also looked at a Neptune orbiter, but a key issue was uncertainty about the launch vehicle: it would require the upgraded Block 2 version of the Space Launch System with an additional Centaur upper stage. It would also cost about $1 billion more than the Uranus mission.

The second-ranked flagship mission is one to Enceladus, the icy moon of Saturn that has a subsurface ocean, with plumes of material from that ocean erupting into space. “This addresses the fundamental question: is Enceladus inhabited?” Canup said. It would do so first from orbit, sampling plume materials as they’re ejected into space, and then from the surface.

The Enceladus Orbilander mission, costing between $4.2 billion and $4.9 billion, would launch in the late 2030s on either an SLS or Falcon Heavy. That would allow the spacecraft to land in the south polar regions of Enceladus, the site of many of those plumes, in the early 2050s, when lighting conditions are favorable.

NASA has already been studying a mission to land on an icy moon, but not Enceladus. The agency did initial studies of a Europa Lander mission several years ago at the behest of John Culberson, at the time the chairman of the House appropriations subcommittee that funds NASA and a staunch advocate of exploring Europa. A new Europa lander mission was among the flagships considered by the decadal survey, but it didn’t make the cut.

Philip Christensen of Arizona State University, the other co-chair, said both the prominent plumes on Enceladus as well as a more benign environment there helped that mission win out over a Europa lander. The plumes at Europa are more sporadic, and the harsh radiation environment means a lander could operate for only weeks versus years at Enceladus. “We just felt that, if we have one opportunity to explore an ocean world with a flagship mission, Enceladus provided the best opportunity,” he said.

“Enceladus is just the right opportunity for this time,” Canup added. “Hopefully, we’ll land on Europa some time, too.”

MARS AFTER SAMPLE RETURN

None of the other flagship mission concepts studied in detail by the decadal survey included Mars. The report, though, endorsed NASA’s ongoing Mars Sample Return campaign, which includes Perseverance and now two landers to retrieve the samples that rover collected and a European-led orbiter to bring the samples back to Earth.

“Our recommendation is that sample return is the highest scientific priority of NASA’s robotic mission, and Mars sample return should be completed as soon as practically possible with no changes in its current design,” Christensen said.

However, he said NASA should closely watch the mission’s cost. The report stated that Mars Sample Return would cost $5.3 billion over the next decade, a figure NASA had not previously disclosed and is even more expensive than other flagship mission concepts studied by the decadal.

That raises worries that cost increases would affect other planetary missions. “Looking back over the last 20 to 30 years, Mars exploration has clearly figured very prominently in NASA’s planetary exploration program,” he said, accounting for 25–35% of the overall planetary budget. Mars Sample Return accounts for 20% of the projected planetary budget for the next decade, he said, so there is some room for cost growth. However, the report recommended that NASA seek a “budget augmentation” if Mars Sample Return overruns its projected cost by 20% or more.

The Enceladus Orbilander is shown in orbit and in its landed configuration in this Applied Physics Laboratory’s rendering of the proposed mission to Saturn’s sixth largest moon. Credit: Johns Hopkins University Applied Physics Laboratory

That leaves very little room in the budget for other Mars missions, even as existing missions are projected to end over the next decade. The only new Mars mission the decadal endorsed was a lander called Mars Life Explorer, which would search for evidence of present-day life near the surface. That $2.1 billion mission would not launch until the mid-2030s, in part because work could not start until after Mars Sample Return got past its peak spending levels later this decade.

By then, NASA will be shifting its attention to human missions to Mars, with agency officials today projecting the first crewed Mars missions could launch by the late 2030s. In the coming decade, there will be an overlap of human and robotic exploration of the moon, as NASA sends both robotic landers and Artemis crewed missions to the lunar surface.

The decadal survey pushed NASA to incorporate planetary science into its human exploration plans. “NASA’s moon-to-Mars plans hold real promise for tremendous benefit to the nation and to the world,” Christensen said. “However, we feel strongly that a robust science program is the key that provides the motivating rationale for a truly sustained human program.”

This fed into another mission the decadal recommended, a lunar rover called Endurance-A. The rover would be delivered on a robotic lander to the south polar regions of the moon, traveling more than 1,000 kilometers and collecting 100 kilograms of samples along the way. The rover would deliver the samples to astronauts on an Artemis mission to return to Earth on their lander.

“It would truly revolutionize our understanding of not only the moon but of the early solar system,” he said. “It would begin to really get humans and robots all working to accomplish a truly remarkable goal.”

DOING MORE WITH MORE

An ambitious program of missions does not come cheap. The “recommended program” of missions included in the report assumes NASA’s planetary science budgets grow by 17.5% over the decade. An alternative “level program” that keeps pace with inflation (or, at least, expectations of 2% inflation when the report was developed) would delay the start of the Uranus mission to the late 2020s and defer the Enceladus Orbilander and Mars Life Explorer missions entirely.

Another reason for the higher budgets is to reflect the true cost of smaller missions, including the Discovery and New Frontiers programs. Christensen said that while the Discovery program has an official $500 million cost cap, excluding launch and operations, the most recent missions have total costs twice that.

“That total cost is totally commensurate with their expected scientific return,” he said. “However, the large difference between the cost cap and the true lifecycle costs undermines budget planning and creates a potential mismatch between the expectations and the budget reality.” The report instead recommends a revised cost cap of $800 million, including operations but not launch.

The same is true for the larger New Frontiers program. The most recent competition had a cost cap of $850 million, excluding launch and operations. The winner, the Dragonfly mission to Saturn’s moon Titan, will have a projected lifecycle cost more than twice as high.

“This is the kind of mission we want to see done at New Frontiers,” Canup said of Dragonfly, but said it demonstrated the need for a better cost cap. The report recommended a revised cost cap of $1.65 billion for New Frontiers, a figure that includes operations but not launch. There would also be $30 million a year for “quiet cruise” phases of the mission, en route to its destination, to avoid penalizing missions that require long travel times. “You’re competing on a level playing field in terms of the scientific instruments and your spacecraft, and the science you do once you reach your target.”

NEXT STEPS

The report is now in the hands of NASA. “We’re all really excited,” said Lori Glaze, director of NASA’s planetary science division, calling the report “incredibly compelling and exciting and inspiration.”

During that inspiration and excitement into implementation plans will take time. “Our plan is to take 90 days to absorb this really comprehensive document,” she said at a NASA science town hall meeting the same day as the report’s release. By mid-July, she said, NASA plans to offer a preliminary response through town halls other meetings, with a more detailed plan later this year.

So far, the report has received a positive reaction, with more concerns about funding than the choice of missions. “This particular survey is an ambitious, inspirational, and pragmatic plan for NASA that The Planetary Society looks forward to working to help realize,” said Bethany Ehlmann, a planetary scientist at Caltech and president of The Planetary Society, said of the decadal.

The report’s authors believe the plan sets forth a strategy to answer some of the central questions in planetary science and beyond. “There’s a true desire this decade,” said Canup, “to make progress not just in studying habitability but also in trying to detect whether life exists elsewhere in our solar system.”

This article originally appeared in the May 2022 issue of SpaceNews magazine.