Plans for a NASA orbiter mission to Uranus’ moons beckon

uranus side ways

Uranus is a strange world, lying on its side and with an asymmetric magnetic field. Its moons may be even stranger, and NASA is planning an orbiter mission to Uranus’ moons, according with

Earlier this year, the United States The National Academies of Sciences, Engineering, and Medicine recommended that the next flagship mission in planetary science, which could cost up to $4 billion, be launched to Uranus in the 2030s. Such a mission would be the second to peer into the Uranian system, following only Voyager 2’s flyby in 1986; it would also be the first spacecraft to spend an extended period of time in the area. While the atmosphere and interior of the ice giant would be top priorities for the mission, there’s more to the picture.

“In terms of the science questions that we can entertain with an orbiter and a probe at Uranus, this list is really long,” Richard Cartwright, a planetary scientist at the SETI (Search for Extraterrestrial Intelligence) Institute in California, said during a presentation at the annual meeting of the American Astronomical Society’s Division of Planetary Sciences held earlier this month. “And I’m just thinking about the moons — specifically, the five large ones.”

An image of Uranus' moon Ariel taken by Voyager 2 in 1986.
An image of Uranus’ moon Ariel taken by Voyager 2 in 1986. (Image credit: NASA/JPL)

Uranus has a total of 27 known moons. The inner ring moons, which Cartwright noted are the most densely packed system of satellites in the solar system and may be exchanging material with the rings, are closest to the planet itself. The irregular moons, which orbit backward and may be captured asteroids, are the furthest away, all beyond 2.7 million miles (4.3 million kilometers) from Uranus.

“There are so many open questions about the origins of the irregular satellites,” Cartwright said, noting that mission personnel could arrange for a spacecraft to fly past one as it approaches Uranus itself. “We don’t know much about these guys.”

However, the remaining five moons, known as the classical moons, are where a spacecraft could really shine. These are the moons that were large enough for astronomers to see from Earth by 1950.

Miranda, the smallest and most recently discovered of these moons, is about 310 miles (500 km) wide and embodies the mysteries surrounding the Uranian moons. “Miranda’s really strange,” Cartwright observed.

Uranus' moon Miranda taken by Voyager 2 in 1986
An image of Uranus’ moon Miranda taken by Voyager 2 in 1986. (Image credit: NASA/JPL)

Images from Voyager 2’s flyby show difficult-to-decipher geological features, he said. According to NASA, Miranda has canyons 12 times the depth of the Grand Canyon on Earth, and the moon’s surface is unusually thick. Miranda is home to three large “corona” regions that are unlike anything scientists have seen before, as well as volcanoes that erupt slushy ice “lava.”

“There are craters that appear to have been filled in by something, and then there are craters that do not appear to have been filled in by something, and in many cases, these craters are right next to each other,” Cartwright explained. “So, something very interesting occurred in Miranda’s geologic past, possibly multiple times.”

Miranda may be the oddest of the classical moons, but she’s not alone.

Ariel appears to have the most recently formed surfaces of the five classical moons. Umbriel has the most ancient and darkest. Voyager 2 only saw Umbriel and the two largest moons, Titania and Oberon, but all four of the largest moons may have oceans beneath their icy crusts, possibly spitting plumes of water into space.

“Clearly, we need better coverage of these moons,” Cartwright said, referring to their northern hemispheres, which Voyager 2 couldn’t see at all. Furthermore, Voyager 2 only saw a snapshot, one taken during the spring season in the southern hemisphere.

Much of the work Cartwright envisions for these worlds could be accomplished with instruments that any Uranus orbiter would carry, such as cameras. However, he encouraged mission planners to consider including a dust analyzer capable of identifying compounds based on their weight as an instrument that would be especially useful in understanding Uranus’ moons.

“We could actually collect material that gets ejected off the surfaces of these moons, dust grains, and then sweep it up with the dust analyzer and characterize the composition,” he said.

There’s still time to propose instruments for the spacecraft. NASA has said it could start early studies of what a mission could look like in the coming year. However, Cartwright encouraged scientists not to dawdle.

“It’s important we start this mission as soon as possible so that we can get that Jupiter gravity assist window, hit that window between 2030 and 2034 so we can get to Uranus faster, before the system transitions back into southern spring in 2050,” he said.

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