For sure, it was a far-reaching recommendation: The planet Uranus and its moons should be NASA's highest-priority new flagship mission for startup in the decade 2023-2032.
The proposed mission, known as Uranus Orbiter and Probe (UOP), would conduct a multiyear orbital tour to yield knowledge of ice giants in general and the Uranus system in particular, doing so through flybys and the delivery of an atmospheric probe. The payoff: "transformative, breakthrough science across a broad range of topics."
UOP was advocated within the pages of "Origins, Worlds, and Life — A Decadal Strategy for Planetary Science and Astrobiology 2023-2032." Released in 2022, that heady document came from the prestigious U.S. National Academies of Sciences, Engineering, and Medicine and was sponsored by NASA and the National Science Foundation.
Improving our knowledge
Underscoring the importance of Uranus, the Keck Institute for Space Studies (KISS) at the California Institute of Technology in Pasadena held a workshop late last year to look into how best to improve our knowledge of Uranus' internal structure in the context of a future mission that includes an orbiter and a probe.
Related: Uranus up close: What proposed NASA 'ice giant' mission could teach us
Spearheading that KISS workshop was Mark Hofstadter, a planetary scientist working at NASA's Jet Propulsion Laboratory (JPL) in Southern California. He's not solar system shy and admits Uranus is his favorite planet.
Space.com caught up with Hofstadter to discuss what exploration of Uranus — that far-flung, ice giant of a world with a mass roughly 14.5 times that of Earth — can offer. The planet has been visited only once, by NASA's Voyager 2 spacecraft during a brief flyby back in 1986.
Within the planetary community, many people are very much focused on a Uranus mission, Hofstadter said. As for the KISS study, it was focused on one aspect of the Uranus system, he said: Understanding the current structure and composition of the planet's interior, in order to learn about its formation and evolution.
"I personally believe learning about the interior is the most important reason to go to Uranus," Hofstadter said. But he also stressed that Uranus is a complex system, consisting of the planet's interior, its atmosphere, its rings and small moons, larger satellites, its magnetosphere and its interactions with the solar wind.
Thus, studying Uranus has the potential to produce transformative, breakthrough science across a broad range of topics, as the Decadal Survey pointed out, said Hofstadter.
"You could even expand that idea of 'system' to include how Uranus, over its lifetime, gravitationally altered the paths of other objects in the solar system, potentially influencing the Earth," Hofstadter added. "Every component of that system has features that violate some of our ideas about how planets work."
Ice giant
That's why the UOP's plan to put an orbiter into the middle of that system — a spacecraft equipped with a wide range of instruments, as well as a probe to enter the atmosphere — is expected to dramatically increase our understanding of our entire solar system, as well as planets that orbit other stars, Hofstadter advised.
Why does he rate Uranus' interior as the most compelling investigation for a mission?
"Of the thousands of planets we have discovered in our galaxy, most are roughly the size of Uranus and Neptune. That makes Uranus an important type of planet, and we'll apply what we learn at Uranus to all those other faraway planets we cannot study in detail," responded the JPL planetary scientist.
Complex system
Hofstadter emphasized that there are many unknowns about the composition and structure of both Uranus and Neptune. For example, scientists aren't sure if Uranus has a deep, solid region.
"We call them 'ice giants' because we originally thought they formed primarily from water ice, but recently some have argued they should be mostly rock," Hofstadter said. "Learning basic things about the interior — something we can only learn by going there with a spacecraft — will help us understand how this type of planet forms and evolves."
Related: What is Uranus made of?
Naive Earth-centric view
There's also the question of Uranus' astrobiology potential. The history of reconnoitering the outer solar system — planets beyond Mars — has been one of surprises, Hofstadter said.
Indeed, our naive Earth-centric view originally indicated that sunlight is needed to provide the energy for life, and we thought places beyond Mars would be cold, inactive, dead worlds.
"We now know that there are other ways to heat up icy objects in the outer solar system and create liquid oceans," said Hofstadter. "And we still believe liquid water is critical for life, and that the chemistry of life elsewhere might work differently than that on the Earth."
Moons of Uranus
Given that, some of the moons of Jupiter and Saturn are now considered the most likely places in our solar system to find alien life.
"We don't know enough about the Uranian moons to say whether or not they also harbor liquid oceans, but theoretical studies have shown they might," said Hofstadter. "That, combined with Voyager's images showing that some of Uranus' moons have geologically young surfaces, makes it plausible that life could exist under the icy crust of one of Uranus' moons."
Putting on his more speculative hat, Hofstadter said that some have asked if life might exist within Jupiter, Saturn, Uranus or Neptune.
"Such life would probably have a very different genesis than that of the Earth, and I don't think anyone can say if it is plausible. Nor is it clear how one might detect such life," Hofstadter said.
Weird world
Meanwhile, Earth-based work is also critical for understanding Uranus. Laboratory and theoretical studies, the KISS study explains, as well as Earth-based and space-based observations of Uranus can enable researchers to properly interpret future on-the-spot spacecraft measurements and guide what measurements a visiting craft should make, as well as where they should be made.
Uranus is the seventh planet from the sun and the third-largest world in our solar system. It is definitely and defiantly a weird world, appearing to spin sideways, rotating at a nearly 90-degree angle from the plane of its orbit.
The surprises that the Uranus Orbiter and Probe mission, should it indeed fly, will reveal at that enigmatic world will likely put scientists in a sideways spin, too.
