Making the case for an ice giants mission

The Voyager 2 team crowds around images of Neptune’s moon Triton following the flyby on August 25, 1989. Half a century will have passed before the next mission.

NASA/JPL

An armada of spacecraft keep a constant watch on the Red Planet. But the ice giants — Uranus and Neptune — were explored close up for a matter of days in the 1980s. Mark Hofstadter of NASA’s Jet Propulsion Laboratory hopes to change that. He’s been tasked with studying the merits and engineering requirements for a major mission to study these outer solar system worlds. The space agency’s head of planetary sciences, James Green, announced the potential mission in 2015 and said its final cost should be less than $2 billion. Past flagship missions include Cassini, Galileo, and Voyager.

By the end of this year, Hofstadter’s team will create a list of science goals for a mission to Uranus and/or Neptune, and provide the space agency with an initial game plan for what such a mission would look like. The mission will compete to be the next in NASA’s flagship class, the biggest and most expensive kind, and — if it wins — the spacecraft will fly sometime between 2023 and 2035.

Just 30 light-years from Earth, the Hubble Space Telescope recently found a Neptune-sized planet with an enormous tail of hydrogen. While the phenomenon has never been seen before, astronomers now accept that Neptune-sized planets are the most common in our galaxy.

NASA/ESA/G. Bacon (STScI)

The ice giants are an unusual place,” Hofstadter says. “They are dynamic. They are changing on human timescales. They have a lot to tell us about our solar system and our galaxy because this type of planet is the most common.”

In the decades since Voyager 2 made humanity’s only visit to each ice giant, these outer worlds have taken on increased significance. NASA’s Kepler space telescope has now shown Neptune-size worlds are abundant in the Milky Way. The exoplanet hunter has found roughly twice as many of these ice giants as it has Earth-sized worlds.

NASA’s Kepler space telescope has found far more Neptune-size planets than Earth-size ones.

NASA Ames/W Stenzel

And yet, mysteries abound. Astronomers still don’t fully understand the structure of ice giants in our own solar system. Jupiter and Saturn are mostly gas. Earth and the terrestrial planets are mostly rock. But Neptune and Uranus are entirely different. They seem to consist of about one-third rock, one-third ice, and one-third gas. And that material doesn’t seem to be fully segregated.

“One way to think of these planets would be you can take a big, rocky planet several times the size of Earth, and then put 10 Earth masses of ocean around it, and then a little bit of hydrogen and helium on top,” Hofstadter says.

Jupiter and Saturn were comparatively easy to figure out, but the data astronomers have on the ice giants is confusing and there’s not enough of it. Most information is leftover from Voyager 2.

Spacecraft silence

Seven spacecraft watch Mars’ every move, but there are few exploration plans in the outer solar system. Chances are good that NASA will send Europa Clipper to explore Jupiter’s moon sometime in the 2020s. However, scientists predict that 50 years will have passed before astronomers get a second look at Neptune and its ice-volcano-covered moon Triton, once suspected to be Pluto’s twin.

Astronomy: Roen Kelly and Eric Betz

“It’s certainly possible that when we get more data, we’ll have a eureka moment,” he says. “But the more interesting and exciting thing is that maybe as we get more data, we realize there’s something totally new going on here.”

And there are other properties that make the case for an ice giants mission. Both planets have rings and moons that are unique. For example, scientists aren’t certain what makes up Uranus’ ring particles. They’re too dark to be pure water ice like Saturn’s rings.

And Neptune’s moon Triton is the only large satellite in the solar system with a retrograde orbit — one that runs opposite from the way the planet spins. This and other evidence indicates that Triton is a captured Kuiper Belt object like Pluto. Its strange cantaloupe terrain and smokestack plumes also hint at an active world and possibly a subterranean ocean.

Currently, Hofstadter’s team is looking at a broad set of potential mission architectures. And they’ve already learned a few things even at this early stage.

“You cannot (currently) fly a single vehicle from Uranus and then to Neptune,” he says. “The planets are just not aligned properly to do that.”

Instead, his team is studying a single spacecraft that would go to one planet or the other. And they’re also exploring how they might use one rocket to send two spacecraft — one for each ice giant — something that’s never been tried before.

The mission is not dependent on the enormous Space Launch System rockets NASA is currently developing, however, the launch vehicle’s capabilities are something the team is considering.

In the coming weeks, the team will identify the top science goals for their spacecraft. And in the months after that, they’ll identify roughly 10 different potential mission architectures to meet those science goals. By fall, Hofstadter expects to report back to NASA with an initial strategy.

Whatever the final architecture, NASA’s plans to visit an ice giant should crystallize by year’s end.

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