Late in 2021, the astronomy community released its decadal survey, a road map of scientific priorities for the next 10 years, which describes the hardware we need to build in order to achieve them. That survey was focused on distant objects and recommended projects like large, broad-spectrum space telescopes.
This week sees the release of a second decadal survey, this one focused on the needs of astronomers and planetary scientists who focus on the objects in our Solar System. This survey's big-ticket recommendations are orbiters for Uranus and Enceladus, while smaller missions include preparations for sample returns from Mars, the Moon, and Ceres. As always, what we get done will depend on whether the planetary science budgets do better than keeping pace with inflation.
The survey lays out the overall scientific themes behind the priorities, but they're broad enough that they pretty much cover everything. As listed, they include a look at the materials present in small bodies within the Solar System to infer the details of planet formation from the protoplanetary disk, and observations of the planets to track their evolution since then. Also a priority: moon formation; studying the interiors and atmospheres of the planets; and the role of impacts in shaping planet evolution. Finally, there's the possibility of life existing at present or in the past on a body other than Earth.
That seems to cover just about everything in the Solar System, which means these research priorities could justify just about any mission. So what hardware has the scientific community chosen to pursue?
The big-ticket item is the Uranus Orbiter and Probe, or UOP, which will undoubtedly get a better name prior to launch. Much like earlier Galileo and Cassini missions, UOP will consist of an orbiter that stays in place to study the system, and an atmospheric probe that will make a one-way trip into the planet (or, in Cassini's case, the atmosphere of the moon Titan). Ideally, UOP will be constructed within the next decade in order to use a gravity assist from Jupiter that will be available if it's launched within a window that ends in 2032.
Why Uranus? We've already done extensive study of the gas giants Jupiter and Saturn, but the two ice giants of the outer Solar System, Neptune and Uranus, have only been visited by Voyager 2 decades ago. Exoplanet surveys have revealed that Neptune-size planets are quite common elsewhere in our galaxy, so their study will be generally informative. Uranus in particular is interesting because it seems to have been struck violently early in its history, causing its axis of rotation to shift by nearly 90 degrees. It also has moons that seem to have been geologically active and may harbor oceans. Aside from all that, it happens to be considerably closer than Neptune.
Should budget increases outpace inflation, the survey recommends a second flagship mission, this one to Enceladus, one of Saturn's moons. Enceladus appears to have a sub-ice ocean and geysers that release some of its contents to space. The "Enceladus Orbilander" will fly through the plumes of these geysers to analyze their content and then land for two years of operation on the moon's surface. The goal would be to have it launched in time to reach the moon by the 2050s, when orbital variations will provide more sunlight on the southern hemisphere of Enceladus, where the geysers are located.
Sample all the things
Bringing samples back to Earth allows us to study them with a wider variety of scientific instruments and repeat the analysis with better instruments as technology evolves. Inspired by a few recent successful returns, the next decade will see a variety of sample-return missions.
Mars is top on the list of missions. "The highest scientific priority of NASA’s robotic exploration efforts this decade should be completion of Mars Sample Return as soon as is practicably possible," the survey concludes. This will involve sending hardware to the red planet to gather the samples currently being obtained by the Perseverance rover. Details of the project are still being worked out, and the survey notes worries that its budget will explode over time, so the authors argue for a cap of $5.3 billion, after which separate funding should be obtained.
Beyond the existing rovers exploring Mars, the survey recommends funding for the Mars Life Explorer, which would land hardware near an ice deposit away from the Martian polar regions. A number of ice deposits have already been identified.
A similar rover/return program is recommended for the Moon. This would see a rover called Endurance-A that would wander the large impact basin at the Moon's south pole, collecting as much as 100 kilograms of samples. These would be returned by a crewed mission to the Moon planned for NASA's Artemis program. Notably, the rover itself is expected to be delivered to the Moon through NASA's planned support of a Commercial Lunar Payload Service.
For medium New Frontiers-class missions, the committee expects the budget can handle at least one and possibly two missions. While it doesn't make specific recommendations, it lists a number of possibilities, several of which include sample returns. The potential targets of this sampling include a comet, the dwarf planet Ceres, and a trojan asteroid.
Other options in this category include dropping a probe into Saturn, an orbiter for Saturn's moon Titan, and a multiple-flyby mission to Enceladus in advance of the Orbilander mentioned above. Beyond Saturn, potential options include installing a geophysical monitoring network on the Moon or a lander for the surface of Venus.
No specific missions for the smaller Discovery Class were detailed; only budgetary concerns were discussed.
Beyond the science
Budget concerns are interspersed throughout the survey in ways that go beyond describing what might have to be cut if budgets can't outpace inflation. In some cases, it's a matter of priorities. NASA's Planetary Science Division, for example, has let funding for research and analysis activities fall below 10 percent of its budget. This, the authors of the survey argue, risks a failure to get a full return on the data obtained by the hardware NASA has deployed.
As in the survey released at the end of last year, the authors of the new one note that the astronomy community isn't very diverse and has had issues with minorities and women advancing in the field. Ensuring that the field benefits from the highest talent, regardless of ethnicity or gender, is in everyone's interests, and it calls on the astronomy community to do better in this regard.
Finally, because the survey includes objects circulating in the Solar System, planetary protection from impacts also ends up within this category. In that regard, the decadal survey recommends launching a survey mission to identify near-Earth objects (NEOs) that might pose a threat. It would also like to see the development of a rapid response mission to a difficult-to-reach NEO to ensure we can develop the tech to intervene if a real threat is identified.