In late 2021, the astronomical community released its Decade Survey, a roadmap of science priorities for the next 10 years, outlining the hardware we need to build to achieve them. This survey focused on distant objects and recommended projects such as large, wide-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 objects in our solar system. The largest recommendations from this survey are orbiters for Uranus and Enceladus, while smaller missions include preparing sample returns from Mars, the Moon, and Ceres. As always, what we do will depend on whether planetary science budgets can do more than keep pace with inflation.
The survey presents the general scientific themes behind the priorities, but they are broad enough to cover just about everything. As noted, they include an overview of materials in small solar system bodies to infer details of planet formation from the protoplanetary disk, and observations of planets to track their evolution since then. Also a priority: the formation of the moon; study the interiors and atmospheres of planets; and the role of impacts in the evolution of the planet. Finally, there is the possibility that life exists now or in the past on a body other than Earth.
This seems to cover just about everything in the solar system, meaning these research priorities could justify just about any mission. So what material has the scientific community chosen to pursue?
The most expensive item is the Uranus Orbiter and Probe, or UOP, which will undoubtedly get a better name before launch. Much like previous 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 moon’s atmosphere. Titan). Ideally, UOP will be built over the next decade to use gravity assist from Jupiter that will be available if launched in a window that ends in 2032.
Why Uranus? We have already made an extensive study of the gas giants Jupiter and Saturn, but the two outer solar system ice giants, Neptune and Uranus, were only visited by Voyager 2 decades ago. Exoplanet surveys have revealed that Neptune-sized planets are quite common elsewhere in our galaxy, so studying them will generally be informative. Uranus in particular is interesting because it appears to have been hit hard early in its history, causing its axis of rotation to shift nearly 90 degrees. It also has moons that appear to have been geologically active and may harbor oceans. Apart from all that, it happens to be considerably closer than Neptune.
If 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 subglacial ocean and geysers that release some of its contents into space. The “Enceladus Orbilander” will fly over the plumes of these geysers to analyze their content and then will land on the surface of the Moon for two years. The goal would be for it to be launched in time to reach the moon by the 2050s, when orbital variations provide more sunlight over Enceladus’ southern hemisphere, where the geysers are located.