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It’s time to explore Uranus and Neptune again — and here's how NASA could do it

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A new report details four missions that could visit the ice giants in the mid-2040s

Uranus (L) and Neptune (R) as seen from Voyager 2.

A group of researchers from NASA and various US universities have come up with plans to explore two of the least visited planets in our Solar System: Uranus and Neptune. That’s because compared to the other worlds in our cosmic neighborhood, these ice giants have been sorely neglected.

To fix that, researchers released a report this week detailing four different types of missions that could be sent to Uranus and Neptune sometime in the next decade or so. The concepts include vehicles that could orbit the planets for 10 to 15 years and even carry probes to dive into the worlds’ atmospheres. The main focus of each mission would be to figure out what the planets are made of — and how their interiors are structured.

“The curious thing about Uranus and Neptune is that, although they look very similar, something about their interiors is actually quite a bit different,” Jonathan Fortney, a professor at UC Santa Cruz and one of the authors on the report, tells The Verge. “And we don’t really know why that is.”

Up until now, the only vehicle that has ever visited Uranus and Neptune was NASA’s Voyager 2. Launched in 1977, the spacecraft did flybys of all four gas giants in our Solar System — Jupiter, Saturn, Uranus, and Neptune — before heading out to interstellar space. And we learned a lot: Voyager 2 discovered new rings and moons at Uranus, and that an ocean of boiling water may lurk underneath the planet’s surface. New rings and moons were also found at Neptune, as well as a huge spinning storm called the “Great Dark Spot” that has since disappeared.

An artist concept of the Voyager 2 spacecraft.
Image: NASA

During these flybys, Voyager 2 came within 50,000 miles of Uranus and 3,000 miles of Neptune — but that’s as close as we’ve ever been. The probe made some measurements and took pictures of the moons and planets. “But that was it; we just had one pass,” says Fortney.

The flybys raised some questions that still need to be answered: we don’t really know what the planets are made of, for instance. Uranus and Neptune are referred to as the ice giants, since they’re thought to be made of heavier elements than Jupiter and Saturn. But ultimately, it’s unclear exactly what elements are in them, since we only have data from Voyager 2 and telescopes here on Earth. The planets may also seem similar, but Neptune — the farthest of the two — puts off 10 times more energy than Uranus does, says Fortney. Researchers are curious to know what the interior of each planet is like to better understand this extreme difference in heat.

That’s why there has been a big push in recent years to send a new mission to either Uranus and Neptune, preferably one that will hang around a little bit longer than Voyager 2. In 2011, the planetary science community — which meets every 10 years to propose needed space missions — said an Uranus mission was one of the top three priorities. The others were a new Mars rover and a mission to Jupiter’s moon Europa. While NASA is currently working on the rover and a Europa spacecraft, there is no mission to Uranus currently in the works.

But in 2015, NASA asked a group of scientists — including Fortney — to come up with concepts for missions that could be sent to Uranus, as well as Neptune. The team studied 20 different mission designs, ultimately settling on four different concepts: three Uranus missions and one to Neptune. The focus on Uranus is mostly a matter of distance, says Fortney. “Neptune’s further away and it takes longer to get there, so missions to Neptune are always going to be more expensive,” he says. All four mission plans would run about $2 billion each — a little more than half the cost of the Cassini mission at Saturn.

The four different missions that the recent NASA report came up with: three to Uranus and one to Neptune.

The three missions to Uranus include two orbiters and a flyby, while the mission to Neptune would also be an orbiter. The vehicles also have the option of carrying a probe that could descend into the planets to measure the composition of the gases in the atmosphere. Meanwhile, the orbiters could better study the interiors of Neptune and Uranus by measuring their gravity fields, which tells a lot about a planet’s density. Another option is to observe how Uranus or Neptune oscillate when hit with light from the Sun. That’s another solid method for determining a body’s interior structure.

Though four missions are proposed, realistically only one could be picked. “There’s no way there’d be money for more than one,” says Fortney. Plus, it would be quite some time before any such spacecraft would see Uranus or Neptune. The best launch windows would be sometime between 2029 and 2034, the report notes. And then it would be another 10 to 13 years before the spacecraft actually reached its destination — meaning we probably won’t get there until the mid-2040s.

Sill Fortney says now is the time to think about getting started on one of these missions. The year 2029 may seem far off, but spacecraft take years to develop. “You can’t just wait around till the mid 2020s,” he says. “Too much time would have passed.” But soon the planetary science community will meet again to determine the top mission priorities for the years ahead. This most recent report will factor into those decisions, and Fortney thinks there may be a strong case to go to Uranus and Neptune.

“I think those are going to be some of the main science targets of the next decade,” he says.