Until NASA’s Voyager 2 spacecraft reached Uranus and Neptune in the 1980s, the outer planets were simply fuzzy blobs that could only be viewed through ground-based telescopes. We had no idea what they actually looked like.
But when Voyager 2 made its closest approach to Uranus in 1986, the space probe revealed a light teal planet with rings. Three years after the Uranus flyby, Voyager 2 introduced us to Neptune, a vibrant ultramarine blue planet with a dark oval storm that penetrated deep into the atmosphere. Suddenly these two ambiguous blobs became real worlds — as real as our own. They had clouds and storms as well as bands of winds and weather all their own. After hundreds of years of wondering, we finally glimpsed these distant worlds comparatively up close. So, why haven’t we been back for 30 years?
Funding is a perennial issue for NASA, and there simply isn’t enough money to reach every planet scientists want to visit. There’s also the sheer distance of Uranus and Neptune: Uranus is about two billion miles away from Earth, and Neptune is roughly another billion miles farther. Reaching these planets with the technology we have now can easily take a decade.
As it happens, the next ideal time to launch a mission to the outer planets is fast approaching. A spacecraft could use Jupiter as a gravity assist — a method of stealing some of Jupiter’s massive gravitational force to slingshot a spacecraft away even faster and therefore shaving years off the travel time. But to do that, a mission would need to be ready by the 2030s; otherwise our alignment with Jupiter will change, making a gravity assist impossible. NASA’s New Horizons spacecraft used Jupiter to gain speed, and while it became the fastest spacecraft headed to the outer solar system, it still took nine years to reach Pluto.
Scientists are urging NASA to take advantage of the timing and start planning a mission to Uranus and Neptune. Amy Simon, a senior scientist for planetary atmospheres research at NASA’s Goddard Space Flight Center, recently published a paper with her colleagues outlining an epic outer planets voyage. While both planets are categorized as ice giants, “they are quite different from each other,” says Simon. Simon’s proposed mission would visit both Uranus and Neptune as well as Neptune’s ocean-world moon Triton. “They are quite interesting, and we know so little about how anything works at those planets.”
“We’re going to go back to both Uranus and Neptune someday. And when we do go back, those planets are not going to look at all like what they were during Voyager.”
Because we’ve only visited the outer planets once, our knowledge of both is limited to what Voyager 2 discovered 30 years ago. “Uranus is tipped over on its side, so possibly a big collision happened,” says Simon. “But it seems like its moons formed in place [grew up in the same environment where Uranus formed]. On the other hand, Neptune has a normal tilt, and it seems to have captured its moons from the outer solar system. So there’s this mystery at both of those planetary systems.”
Past exploration of Uranus and Neptune raised questions for scientists about the planets, and many want the opportunity to answer them. Heidi Hammel, a planetary astronomer, Voyager team member, and the executive vice president of the Association of Universities for Research in Astronomy, recalls how little we knew about the outer planets before Voyager. “We knew that they had some kind of cloud features, and we could make guesses at rotation periods, but it was very mysterious,” she says. “We knew there were methane ice clouds. [That] was almost everything we knew.”
Voyager 2 revealed that Uranus and Neptune bore peculiar storms and wind patterns and wonky magnetic fields that scientists still don’t fully understand. Both planets have the highest wind speeds in the solar system: Neptune has clocked in at over 1,500 mph. (The fastest recorded wind speed on Earth, by comparison, has been 253 mph during a tropical cyclone near western Australia in 1996.) Scientists think leftover heat from when both planets formed continues to rise from the planets’ cores and fuels these winds. But while Neptune seems to have retained a lot of heat from its nascent days, Uranus has not. Their high-speed winds remain a mystery.
Neptune’s big dark spot — a surprise discovery — is another area of interest. “When we finally got the chance to turn Hubble to that scene five years [after the Voyager flyby], I was waiting to measure the great dark spot, and it was gone,” says Hammel. “How can that be? Why does this planet have these huge features that disappear in five years?”
Another reason scientists want the chance to properly study the outer planets is that the knowledge could lead to a better understanding of exoplanets — planets beyond our solar system that orbit around other stars. Most of the 2,500 exoplanets discovered to date are similar to Uranus and Neptune. They’re roughly the same mass and likely also mostly made of ice, rock, and gas. Traveling to other star systems to study these worlds is impossible right now, but studying Uranus and Neptune is achievable. We have analogs for exoplanets in our own solar system, and by understanding how they form, their chemical makeup, and whether or not they are habitable could give us insight into alien worlds we are unable to visit.
The hope for Hammel, Simon, and many others in the scientific community is that a mission to these mysterious blue-tinted worlds will launch in our lifetime.
“We’re going to go back to both Uranus and Neptune someday,” says Hammel. “And when we do go back, those planets are not going to look at all like what they were during Voyager. They are dynamic systems. The atmospheres are dynamic. The ring systems are dynamic.”
But the clock is ticking. Even if NASA gets on board with an outer planets mission, its spacecraft may not arrive until the late 2040s.
“Incredible things are waiting to be discovered there,” says Hammel. “We just need another ship.”