Let’s get it out of the way now. Any comments that might come to mind when you hear the name of the seventh planet from the sun pronounced a certain way—keep them to yourself. No giggles. Absolutely no wisecracks, damn it. No jokes! This is an earnest story about Uranus.
Because this is a big day for Uranus. Once every decade, NASA asks the National Academies of Science, Engineering, and Medicine to bring a bunch of planetary scientists together to decide on the country’s priorities for future missions in the solar system. And the latest report, released today, has put this outer planet front and center. According to the powers that be, the top priority for the next decade of space exploration is spending several billion dollars on a shiny, new flagship mission to Uranus.
Uranus deserves a closer look. Only one spacecraft, Voyager 2, has ever visited the planet, and that was in 1986, with technology developed in the ’70s. The visit was short; Uranus was one stop on the mission’s grand tour of the solar system, and the probe made its observations while on the move. What scientists are recommending now is a mission designed specifically to study everything about the Uranian system, including the planet, its moons, and its rings—yes, Uranus has rings!
Given our history of lowbrow humor with Uranus—which is pronounced Yoor-uh-nus, by the way—one might assume that we’re pretty familiar with our cosmic neighbor. But even with the best space telescopes, it’s difficult to study a planet that is twice as far from Earth as is Saturn, and much smaller than the ringed planet. Although we know more about the universe than ever before, we still don’t understand one of the planets in our very own solar system.
Name any aspect of the Uranian system, and planetary scientists can give you a list of unanswered questions about it. Scientists suspect that Uranus, like Neptune, is made of different kinds of ice, gases, and rock, but they can’t say anything for sure about the composition of its hazy atmosphere or the structure of its interior. Unlike the other planets, Uranus spins on its side, which means summers of constant sunlight and winters of complete darkness; it was probably knocked over by a giant impact many eons ago, but what kind? Planetary scientists have no idea how its magnetic field works, and they really love knowing how magnetic fields work. Those rings—what are they made of? And Uranus’s moons—could some of them have subsurface oceans? The imagery from Voyager showed the sideways planet as a mostly featureless, bluish orb, but telescope observations in the past two decades have captured intriguing bursts of cloud activity in its atmosphere. What’s that all about?
The Uranus mission—if NASA decides to take the space-science community’s suggestion—would mean putting a spacecraft in orbit around the planet, and even dropping a probe into its atmosphere. With Voyager, “it’s almost like if you drove by a person and had a few seconds to look at them,” Jonathan Fortney, an astronomer at UC Santa Cruz, told me. “Do you feel like you’d have a good understanding of who they were?” A spacecraft in residence, by contrast, is akin to “spending years talking to them and getting to know their idiosyncrasies,” he said.
By getting to know Uranus, planetary scientists can learn about a class of planets called ice giants. The space community is already quite familiar with the workings of the terrestrial planets in our solar system, such as Earth and Mars, and NASA is already lining up new missions to Venus. Scientists also have a good handle on Saturn and Jupiter, our gas giants, thanks to dedicated missions that spent years orbiting those planets. But Uranus and Neptune, the ice giants? Those pages in our cosmic understanding are nearly blank.
Which is particularly frustrating for scientists, because ice giants might be one of the most common planets in the galaxy. Research on exoplanets—planets beyond our solar system—has shown that gas giants like Saturn and Jupiter are rare, but worlds about the size of Uranus and Neptune are everywhere. So before anyone can understand the ice giants out there, scientists should examine more closely one of our own. Planetary scientists picked Uranus over Neptune for practical reasons; Neptune, which since 2006 has had the distinction of being the outermost planet in our solar system (well, depending on whom you ask), is farther. “They’re both absolutely compelling, and they’re not identical,” Amy Simon, a planetary scientist at NASA who studies the ice giants, told me. For example, Uranus, even though it’s closer to the sun, is actually colder than Neptune—and, you guessed it, scientists don’t know why that is, either. But with the rocket systems currently in operation, Uranus is easier to reach.
Any mission to the outer planets still takes quite a bit of time. If a Uranus mission were to launch in 2031, the earliest opportunity suggested by the National Academies report, it wouldn’t reach the planet until the end of that decade. Any later, and the spacecraft would arrive in the 2040s. But planetary scientists are used to playing the long game, Heidi Hammel, an astronomer at the Association of Universities for Research in Astronomy, told me. Hammel worked on the Voyager mission during the Uranus flyby, and by the time this new mission reaches its target, “I’ll be watching from the front porch of the retired-planetary-scientist old-age home,” she joked. Maybe NASA would let her totter into mission control, she said, to watch a new generation of researchers bask in the experience. She knows how wonderful it is to see a world come into view in real time. And she and her colleagues caught only a glimpse of Uranus 36 years ago. This time, if NASA indeed makes a dedicated Uranus mission a priority, humanity can stick around for a while. “I guarantee that when we get there,” Hammel said, “the young scientists working on this are going to see things that they never even imagined.”