Uranus and Neptune are a distinct class of planets in the solar system. The gas giants Jupiter and Saturn are primarily composed of hydrogen and helium. The ice giants Uranus and Neptune, by contrast, are believed to have formed further from the sun where ices would have been more common. As a result, these are water worlds (although the water is under tremendous pressure and believed to be in an ionic state and mixed with ammonia and methane). Above the water are atmospheres of hydrogen and helium and below rocky cores. However, the report notes that with our current knowledge, Uranus and Neptune, “challenge our understanding of planetary formation, evolution and physics.” We also have learned that planets of their size are a common class of worlds orbiting other stars.

We need to return to these worlds to both understand our own and other planetary systems.

In one way, exploring the solar system is like building medieval cathedrals—it is a process that takes generations. If the plans laid out in the report come to fruition, almost six decades will pass between the Voyager 2 flybys of these worlds and their next visit. Approximately a quarter century will pass from today to arrival. This is not unusual. Nearly a quarter century will also pass between the first serious discussions I’m aware of for a dedicated mission to Europa and the expected arrival of the Europa Clipper spacecraft. Now is the time to begin serious planning to take advantage of the good flight opportunities around 2030.

The goal of the report is to provide the planetary community a range of options it can use when they consider goals for planetary exploration in the 2020s and early 2030s. That process, called the Decadal Survey, looks across the solar system and recommends a balance of missions to address top scientific questions within the expected budget. The last Survey, covering 2013 to 2022, ranked a mission to Uranus as the third priority after a rover to cache Martian samples and a mission to explore the habitability of Europa. Available funding allowed development of those latter two missions to begin this decade. The possible start of work on a mission to Uranus was deferred to the 2020s. With that delay, the changing alignment of the planets opens up the alternative to explore Neptune instead of Uranus.

The first use of the report will be by a committee conducting a mid-term assessment of the current Decadal Survey. According to Dwayne Day, the study director of the National Academies' mid-term assessment, the review committee has already been briefed about the ice giants study, and has also heard from the Outer Planets Assessment Group about future outer planets missions. Dr. Day was involved in running the planetary decadal. He notes that one of the challenges that group faced was that few planetary mission studies had been done prior to the last Decadal Survey, which limited the options the survey members had when they started. That created a crush during the study to develop new mission concepts and conduct mission evaluations while the survey was underway. Having the ice giants study in hand prior to the development of the next Decadal Survey is a real asset.

The report is a menu of options: perform a flyby only (not recommended), orbit Uranus or Neptune or both (each have unique characteristics), possibly deliver a probe that would enter the atmosphere (recommended), and carry 3, 7, or 13 (recommended) orbiter instruments. There’s also a choice of launch vehicles.

With all these options, it can be difficult to answer what was my basic question in reading the report: When we return, how will we explore whichever world is prioritized in the next Decadal Survey? In this post, I take one set of options and look at that question. For anyone reading the report, this generally follows Option 5 for a Uranus orbiter but with seven instead of three orbiter instruments listed for this option. I also look at how the goals would change if Neptune were selected instead.

I recommend Jason Davis' post on the Planetary Society's blog for additional background on the scientific reasons for returning to these worlds.