The decadal planetary science survey points beyond Mars to the outer solar system in search of ice giants and icy moons.

Mars was ascendant the last time the planetary scientific community convened for a decadal review. NASA had flown several Mars missions and was currently working on its most ambitious rover, Curiosity. The decadal survey supported continuing that Mars exploration approach by supporting a sample collection mission as the first step toward returning those samples to Earth.

The situation is slightly different in the most recent planetary science decadal assessment, which was released on April 19 by a National Academies committee. While Mars remains at the heart of much of NASA’s current planetary science, the planetary decadal makes the case that the future lies elsewhere in the solar system, among distant planets and icy moons that potentially sustain 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.

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.

“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.”

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 Europe. 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 Europe 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.

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.

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