Neptune and Uranus have long been neglected when it comes to solar system exploration. Many astronomers think that is a shame, as these two planets are particularly weird outliers among the bodies that orbit the Sun. Uranus, for example, the seventh planet from our Sun, has a unique tilt that makes it appear to spin sideways, orbiting the Sun like a rolling ball while circled by nine, dense rings. Both Uranus and Neptune may also have diamond rain in their atmospheres.
Known as the Uranus Orbiter and Probe, the mission will most likely involve a cone-shaped probe that will dive down into Uranus's atmosphere, coupled with a kind of satellite that will get stunning, up-close views of Uranus's many moons.
The one and only spacecraft to visit either of them was the Voyager 2 spacecraft, which briefly zipped by in the late '80s, more than three decades ago. When it did, the probe turned up more questions than answers. And given that space probe technology has dramatically improved since then, there's a lot we still don't know about these so-called "ice giants," including how they formed, what their interiors are like, and what role they play in the positions and orbits of the outer planets.
NASA seems to understand this gap in our knowledge and is prioritizing a mission to Uranus. In a new editorial in the journal Science by Kathleen E. Mandt, a planetary scientist at Johns Hopkins University Applied Physics Laboratory, lays out the case for traveling to Uranus, how we'd get there and what we'd potentially learn.
"The space science community has waited more than 30 years to explore the ice giants, and missions to them will benefit many generations to come," Mandt writes.
Every decade, NASA employs the National Academies of Sciences to survey scientists about the most juicy, intriguing space missions to start planning. The most recent survey spanned the course of a year, in which the surveyors reviewed over 500 white papers, held 176 meetings and sat through more than 300 presentations, all of which were made even more difficult by the early days of the COVID pandemic. In spite of all this work, the survey is a good strategy to really understand what space targets experts think we should concentrate on. A trip to Uranus or Neptune emerged as a top contender.
"The "dearth of knowledge on the ice giants" was identified as a problem of highest priority for resolving in the coming decade," Mandt writes. "It was ranked as the third-highest priority flagship in the 2013–2023 decadal survey, Visions and Voyages, following Mars Sample Return and the Europa Clipper, two missions now well advanced in their development."
In other words, as other space missions proceed, Uranus and Neptune are becoming primary next objectives. The mission to Uranus is still in the early stages of development, but the basic design has already been fleshed out. Known as the Uranus Orbiter and Probe (UOP), the mission will most likely involve a cone-shaped probe that will dive down into Uranus's atmosphere, coupled with a kind of satellite that will get stunning, up-close views of Uranus's many moons.
We stand to learn a considerable amount about how these massive planets formed, as well as how our Solar System took shape approximately 4.5 billion years ago. A leading hypothesis is that the solar system began as a massive cloud of interstellar dust and gas that collapsed, perhaps from the shockwave of a supernova triggered by an exploding star. This collapse may have formed a nebula of swirling material, generating tremendous pressures and energy from which our Sun was born.
Leftover gunk orbiting this whirling spiral of chaos began to clump together, warped by gravity into spherical shapes we call planets. Depending on their location and the materials available, the planets formed into either rocky worlds like Earth, Theia or Mars, or giant balls of gas and ice.
Neptune and Uranus are called "ice giants" because they consist of far more frozen water, ammonia, and methane compared to Jupiter and Saturn, the "gas giants." We've obtained plenty of up close data from Jupiter and Saturn the past few decades, which has taught us a lot about how these behemoths formed. But there's still a lot we don't know about the composition of the atmosphere of Uranus and Neptune.
"Studying all four giant planets (Uranus, Neptune, Jupiter, and Saturn) is vital for reconstructing Solar System history," Mandt writes. Investigating these objects can reveal some of the violent history of our Solar System's past. "The Uranus system is especially interesting because it could bear testimony to a massive collision that tilted Uranus on its side."
While NASA hasn't formally announced a launch date yet, it will probably blast off between 2028 and 2038.
Did something massive smack into Uranus years ago, altering its orientation compared to the sun? We might know more if we take a closer look. And other surprises could be in store, though it's not very likely that Uranus harbors extraterrestrial life. It is probably far too cold and chemically volatile to play host to even the most hardy of life, not even so-called "extremophiles." However, the moons of Uranus could have liquid water, which is believed by most astrobiologists to be necessary for life to exist.
"The five largest moons, predicted to be cold dead worlds, all showed evidence of recent resurfacing, suggesting that geologic activity might be ongoing," Mandt explains. "One or more of these moons could have potentially habitable liquid water oceans under an ice shell, making them 'ocean worlds.'"
While it's probably more likely that life, if it exists somewhere other than Earth, is hiding somewhere like Europa, a moon of Jupiter, the intriguing possibility is worth taking a gander in the further reaches of the Solar System. Studying Uranus and Neptune could also inform our quest to detect habitable exoplanets in other galaxies.
A potential mission is going to take a very long time to get to Uranus, because the ice giant isn't exactly close to us. Approximately 1.6 billion miles (2.6 billion kilometers) away from Earth at its closest orbit, it takes sunlight 2 hours and 40 minutes to travel from the Sun to Uranus. Sending a probe there will take between eight and 15 years, especially if we want it to get there during a window that makes Uranus's moons the most visible. While NASA hasn't formally announced a launch date yet, it will probably blast off between 2028 and 2038, according to recommendations published last month in the journal Acta Astronautica.
That means we might not being gathering data on Uranus until the 2040s at the earliest. But when a probe does get there, it will spend many years surveilling Uranus — unlike Voyager 2 which spent only about six hours there before continuing on its way. As for Neptune, an orbiter may not arrive until 2049. Two decades or more is a long time to wait for more info on these mysterious ice giants, but given all the mysteries these two worlds are hiding, it will undoubtedly be worth the wait.
