THE NEXT 10 YEARS

During the next decade, NASA will send 2 missions to the outer solar system: Europa Clipper and Dragonfly. Europa Clipper will make repeated flybys of Jupiter’s moon Europa, while Dragonfly will land on and then fly around the surface of Titan. The European Space Agency’s JUICE ( JUpiter ICy moons Explorer) mission will investigate Ganymede, Callisto, and Europa. All 3 of these missions have a focus on determining whether life could have arisen beyond Earth.

Other missions to the outer planets have also been proposed. The main job of the decadal survey will be to prioritize specific science goals and evaluate which missions will best address these goals. The future mission concepts fall into 4 broad themes.

Surveys of Uranus and Neptune: Each of the ice giants has only been visited once—by Voyager 2. A spacecraft in a polar orbit of either world would allow us to examine the planet’s interior, and diverse moons and complex ring systems promise new discoveries like those made by the Cassini mission at Saturn. The current decadal survey recommended starting work on a Uranus mission after a Europa mission was greenlit.

Astrobiology: Following the Europa Clipper mission, a Europa lander would examine the surface for signs of life. A dedicated Enceladus mission could directly sample plume material spewing into space.

Atmospheric probes: Probes at Saturn, Uranus, and Neptune are needed to complete the noble-gas and volatile-species survey started by the Galileo Probe at Jupiter in 1995. A second Jupiter probe would also be helpful since the Galileo Probe entered an anomalously dry location, its measurements of volatiles were inconclusive, and Juno’s measurements can benefit from an in-situ validation.

Long-duration surveys of active worlds: We need long-term data on the active phenomena that shape worlds in the outer solar system, including up-close looks at the surfaces of Enceladus, Triton, and Io, the dynamic atmospheres of the giant planets and Titan, and the rings of each of the 4 giant planets. In addition to missions that visit these destinations, a space telescope dedicated to solar system targets would ensure that active worlds like these are observed regularly without large gaps.

The decadal survey is a truly major undertaking and a good occasion to remind ourselves that even with a diversity of destinations with many mission possibilities, we are all unified under common long-standing goals, and the heart of the survey’s task is to recommend the best strategy to answer those fundamental goals. Some of the outer-planet missions will last decades and span multiple generations of scientists, so such strategic planning is also important in shaping the careers of the next generation of scientists. No matter what the recommendations become, the outer planets offer something for everyone, and the decadal survey is sure to illuminate a path toward our next big discoveries.

—KUNIO M. SAYANAGI is an associate professor at the Hampton University Department of Atmospheric and Planetary Sciences.