With its strange rings and tilt, surprising seasonal variations, and 27 moons, Uranus is the solar system’s weirdo planet. The enigmatic ice giant has a lot to teach us, which is why astronomers are clamoring for a mission to explore the planet up close.
The first dedicated mission to Uranus is long overdue, argues Kathleen Mandt, a planetary scientist from Johns Hopkins Applied Physics Laboratory, in a Science Perspectives essay published today.
Indeed, it’s been a minute since our only, brief visit to the distant ice giant. NASA’s Voyager 2 probe flew past Uranus on January 24, 1986, capturing remarkable views of the planet and its moons before continuing on its journey to Neptune—the solar system’s other ice giant—and then to interstellar space.
Like Saturn and Jupiter, Uranus and Neptune are packed with hydrogen and helium, the lightest two elements in the universe. “We call Uranus and Neptune ice giants because they have more of the other, heavier elements relative to hydrogen than either Saturn or Jupiter,” Mandt explained in an email. “Because Saturn and Jupiter are mostly hydrogen, we call them the gas giants.”
The images that Voyager 2 captured left us wanting more, but it’s now three decades later and we still haven’t made the effort to return. In her essay, Mandt makes the case for precisely that: a dedicated NASA Uranus orbiter and probe, dubbed UOP. Parked in Uranus’s orbit, the spacecraft would take unprecedented readings of the planet, its many moons, and rings, and also deploy a probe to investigate the chemical cocktail that makes up its murky atmosphere.
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That a dedicated mission to Uranus is badly needed is hardly news. A key deficiency identified in the latest decadal review, in which NASA tasks the Academies of Sciences, Engineering, and Medicine to devise a list of priorities for the coming decade, was the “dearth of knowledge on the ice giants.” The review identified UOP as a high-priority planetary mission, a conclusion reached during previous decadal reviews, as Mandt points out. The 2013 decadal review listed the Mars Sample Return Mission and the Europa Clipper as high priorities, and those missions are now in full-on development mode. A dedicated mission to Uranus, Mandt argues, would appear to be next in line.
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Such a mission would help to unlock the many mysteries of Uranus (stop giggling, this is serious). Planetary scientists are seeking to learn more about the ice giants and how they formed and the degree to which Uranus migrated from its original, formative location. According to Mandt, up-close investigations of Uranus could detail the history of our immediate surroundings, showing how small celestial bodies were able to distribute water and other building blocks of life to the inner solar system. These investigations could also shed light on distant exoplanets and “their system architectures,” Mandt wrote in her essay. Such assessments would require detailed measurements of isotope ratios and the quantities of noble gases present within the ice giant—data that an atmospheric probe could gather with ease, according to the planetary scientist.
Other questions in need of answers include the reason for the planet’s severe tilt, an obliquity of 90 degrees that “causes extreme atmospheric seasonal variation over its 84-Earth-year orbit,” Mandt writes, adding that “observations of haze and clouds from Earth cannot provide enough information to explain atmospheric circulation and wind patterns.”
UOP could study Uranus’s magnetosphere and provide fresh insights into the planet’s internal structure (is the core solid or diffuse?). And then there are Uranus’s many moons to consider.
“The moons of Uranus are much smaller than the four large moons of Jupiter and Saturn’s moon Titan. They should be solid and inactive because they are small, and the surfaces should only show impact craters,” Mandt wrote to me. “What is exciting is that Voyager 2 images did not show many craters and instead showed features that could be formed by geological activity. We want to study the internal structure of these moons to figure out why the surfaces are so young, and we hope to find evidence for liquid water.”
Indeed, Uranus may host a moon or two with liquid water hidden beneath an icy shell. Astrobiologists should be licking their chops for a chance to investigate the Uranus system from up close.
The mission plan, Mandt argues, could draw upon pre-existing work, including the Origins, Worlds, and Life (OWL) document, released as part of the 2022 planetary science decadal survey. The plan calls for UOP to launch by 2032 to take advantage of Jupiter’s immense gravity, whereby the gas giant would slingshot the spacecraft towards its final destination. UOP would reach Uranus around 2050. These are rough approximations, but this is generally the timeframe under which this mission is likely to happen, should it be approved.
I asked Mandt about the types of instrumentation she’d like to see on the orbiter and probe.
“For the probe, the most important instrument is a mass spectrometer. This instrument is able to measure which elements and molecules are in the atmosphere based on their mass,” she replied. “We need this information to figure out where Uranus formed and how much it migrated after forming. The orbiter will need a combination of instruments that include cameras that measure different wavelengths and sensors that detect electrons, ions, and the magnetic field.”
The mission would also present some unique challenges, as no orbiter has worked so far from home. At distances reaching nearly 20 times more than Earth’s distance to the Sun, signals from UOP would take over three hours to reach ground stations on Earth; this “will make planning important with sending commands and require patience when waiting for new results,” Mandt told me. Collecting sufficient solar energy could also prove challenging. She said the spacecraft “will need radioisotope power sources like the Cassini mission had and like what is still providing power for the two Voyager spacecraft and New Horizons.”
If given the opportunity to work on this mission, Mandt said she’d like to help plan the probe’s observations, along with forging plans for studying the composition of Uranus’s moons and rings. “We are eagerly waiting to hear from NASA what the next steps will be,” she said.