1.04.2023
Multiple obstacles might make it hard for NASA to turn its dream of ice giant exploration into a reality any time soon
The planet Uranus, as seen by NASA’s Voyager 2 spacecraft in January 1986. This close encounter was humankind’s first—and so far only—visit to the lonely ice giant. Credit: NASA/JPL
Uranus is a lonely world. Nearly four decades ago, the ice giant received one fleeting visitor when the Voyager 2 spacecraft flew within 81,800 kilometers of its cloud tops. The spacecraft took thousands of photographs of the planet—revealing an enigmatic world and raising more questions than answers—before speeding off to the outer solar system. No spacecraft has ventured there since. “The Uranus system is one of the big blank spots that are left on our map,” says Francis Nimmo, a planetary scientist at the University of California, Santa Cruz. But that might be about to change.
Last year the National Academies of Sciences, Engineering and Medicine released a decadal survey that called on NASA to send its next large-scale “flagship” mission to the ice giant. Specifically, the survey recommended a $4.2 billion mission that would settle a spacecraft into orbit around Uranus for years and include a probe that would plunge into the planet’s atmosphere—which would finally help scientists better understand the origin and evolution of our own solar system as well as the galaxy’s most common type of planet. But the biggest splash of all might lurk deep within Uranus’s largest moons, which, like so many other icy bodies in the outer solar system, might host subsurface oceans of liquid water.
With Uranus now in NASA’s sights, planetary scientists across the globe have spent the past year in intense debate—discussing the key science of the mission as well as any critical questions that need to be answered beforehand. They remain hopeful but pragmatic. Congress has still not allocated funds, and the clock is ticking. The mission needs to launch in less than a decade in order to reach the ice giant during its equinox, when the sun will fully illuminate the planet and its rings and moons. That is very little time to plan and execute a mission of this scale, particularly when the space agency is already weighed down by an astronomically extensive to-do list.
And yet it could not be more crucial: If the mission arrives too late, portions of the planet and moons will slip back into darkness. So will our answers.
TANTALIZING HINTS
When Voyager 2 flew past Uranus, it revealed a tumbled-over, milky blue marble unlike any other planet in the solar system. The planet’s upper atmosphere is cold—so cold that models cannot explain why the more distant Neptune is actually warmer than Uranus. It is encircled by more than a dozen mysterious rings and harbors at least 27 moons—some of which are stitched together from a hodgepodge of materials like lunar versions of Frankenstein’s monster. But oddest of all is perhaps the fact that at some point in its history, the ice giant was knocked over, leaving it spinning on its side. “It must have been a very spectacular event like the moon-forming impact on the Earth—but on steroids,” Nimmo says.
To better understand the collision, scientists want to peer beneath the planet’s cloud tops. Because the event likely battered Uranus’s interior, an orbiter that can detect fluctuations in the planet’s gravitational field would be able to map any internal anomalies created by the giant smashup. In addition, scientists want to better understand how the ancient collision may still hold sway over the planet’s atmosphere. A world that lies on its side will undergo extreme seasons in which one pole bakes in sunshine while the other freezes in darkness. Because Uranus orbits the sun every 84 years, those seasons last for decades—and impact the atmosphere in ways scientists cannot yet imagine, with consequences for chemistry, clouds and circulation patterns. The orbiter will thus make global observations while the probe will plunge through the atmosphere itself, taking measurements at a greater depth.
Such detailed observations of the planet will help scientists understand not only the origins and histories of Uranus and its fellow ice-giant Neptune but also the evolution and cosmic context of our solar system. Both worlds may have migrated early in their lives, which would have shifted their positions around the sun and sent barrages of water-rich comets cascading toward Earth and other inner planets. More broadly, Uranus and Neptune both may represent a planetary class that abounds throughout the galaxy. Roughly 50% of known exoplanets are ice-giant-sized.
The orbiter will also peer outward to image the planet’s faint rings—which by some estimates seem to defy the laws of physics. Like Saturn’s rings, Uranus’s bangles are made of innumerable icy particles that can be sculpted by gravitational interactions with the planet and its moons. But Uranus’s rings are far more narrow than what scientists think should normally be possible. Think of them as a hoop with a radius of 50,000 km and a rim that is only 10 km wide. “That shouldn't happen,” says Matthew Hedman, a planetary scientist at the University of Idaho. “Particles should just crash into each other and spread out. Something is keeping material at specific locations.” It could be small moons nestled amongst the rings. It could be gravitational asymmetries inside Uranus. Or it could be that the rings simply do not spread out as much as we think they should. “It could be that we’re missing something fundamental,” Hedman says.
A photomosaic of Uranus’s “Frankenstein” moon, Miranda, produced from images gathered by the Voyager 2 spacecraft during a 1986 flyby of the ice-giant planet. Credit: NASA/JPL/USGS
While some scientists will use the orbiter to search for small shepherding moons, others will scrutinize the planet’s larger satellites. “Voyager provided images of the surfaces of the moons that were completely unlike anything that we expected,” says Kathleen Mandt, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory. Consider Miranda, a moon that bears deep fractures, pockmarked plains and intersecting ridges as a silent testament to being torn apart long ago only to then be poorly pieced back together. Miranda’s jigsawlike nature hints at a weak—and therefore relatively warm—crust. Warm enough, in fact, that one might not need to dig very deep to reach ambient temperatures above the melting point of water. It is thus possible that oceans lurk inside Miranda and other jumbled-up Uranian moons. To verify this tantalizing possibility, the orbiter will monitor the moons’ magnetic fields to see if any of the fields vary as a result of the internal sloshings of liquid water. It will also image the full surfaces of the moons to search for ongoing geologic activity—something that Voyager 2 failed to do.
When that historic spacecraft swung past the ice giant in 1986, it saw only half of each moon. At the time, the planet and its many moons were at southern summer solstice, with their south poles fully lit and their north poles shrouded in darkness. “There are regions on those moons of Uranus, which no eyes—human or robotic—has ever been able to see before,” says Leigh Fletcher, a planetary scientist at the University of Leicester in England. Naturally, scientists would like the next Uranus mission to reach the system during its equinox, when sunlight bathes the entire planet and its moons from the north pole to the south pole. That date is in 2050. And while that might sound far in the future, a mission would need to get off the ground relatively soon in order to traverse the vast distance to Uranus. In fact, the decadal survey recommended launching the mission by 2032—a timeline that allows the spacecraft to use Jupiter’s massive gravity to gain speed out to Uranus and arrive well before the equinox, allowing a full view of this epochal seasonal transition.
A RACE AGAINST THE SUN
Meeting a 2032 launch deadline will require a monumental push. And whether the scientific community can be ready in less than a decade is an open question. If not, scientists risk missing the opportunity to view the system in its full splendor—which is particularly worrisome because the next chance won’t occur until 2091.
“There are no Uranus orbiters sitting on a shelf in Walmart that you can just swipe a credit card, buy and put together,” says Casey Dreier, chief of space policy at the Planetary Society. Although the decadal survey recommended a spacecraft that looks similar to NASA’s highly successful Cassini mission, which explored Saturn’s system from 2004 to 2017, the ice giant is a different beast. Scientists will need better telescopic data about the planet’s upper atmosphere before they can design a parachute and a heat shield that will protect the probe and slow it down to a science-maximizing entry speed. They will also need to scout for hazardous debris between the upper atmosphere and the rings—a region the mission must traverse to successfully enter orbit around the ice giant. As such, many observatories, including the James Webb Space Telescope, will swivel their gaze toward Uranus over the next few years.
“Because Uranus is much harder to study from our earthly vantage point, we need to throw everything we’ve got in our arsenal at it to try to characterize the environment before our big, expensive, chance-of-a-lifetime spacecraft actually gets there,” Fletcher says.
Ultimately, however, the make-or-break factor for NASA’s Uranus mission will not be telescopic observations or rocket technology, but rather funding. In NASA’s presidential budget request released early last month, the agency stated that it would not even request money for the mission until 2025, after which the amount of funding requested would grow modestly. “That is not going to get you to a launch in 2032,” Dreier says.
Nevertheless, the planetary science community trusts that the space agency will eventually launch this mission for the sheer reason that the US federal government traditionally holds decadal-survey recommendations to be sacrosanct. The last survey, for example, recommended that the community prioritize two flagships—one to return samples from Mars and another to explore Jupiter’s icy moon, Europa. Both of those priorities have manifested into actual missions. But such inviolable decadal-scale “to do” lists are also problematic. With two flagships already in the works and ahead of a Uranus mission in the queue, NASA’s planetary science division is laboring under an enormous amount of budgetary stress. “It’s an extraordinary commitment,” Dreier says. The plans for the Mars sample return are particularly worrisome, he says, because of that complex mission’s multiple phases (and multibillion-dollar budget) involving international partners operating on relatively time-sensitive deadlines. For such a mission, some degree of overruns and delays seem inevitable and could have catastrophic effects for a planetary science portfolio stretched too thin. “Because of that, it’s going to be difficult—not impossible, but difficult—to grow funding for a Uranus flagship,” he says.
Moreover, Mars and Jupiter are not the only locations in the planetary lineup. NASA will soon launch missions to Venus, Saturn’s moon Titan and even a distant asteroid—pushing the workforce to its limit. The Psyche mission that was going to rendezvous with an asteroid, for example, was delayed in June 2022, partially because of understaffing. The spacecraft was ready to launch, the rocket was available, and the weather was divine. But there weren’t enough workers available to fully test the spacecraft’s software in time. “It’s an overabundance of riches,” says William McKinnon, a planetary scientist at Washington University in St. Louis. “We have approved missions, we have talent, and we have fantastic targets. But just trying to get everything going at once has proven to be a challenge.”
Still, McKinnon and his peers are hopeful that once the mission to Europa launches, it will open a wedge—both in the workforce and the budget—to focus on Uranus. And Dreier notes that “just because it feels somewhat dour right now, doesn’t mean it can’t change.” He points toward NASA’s Europa mission, now called Europa Clipper. For much of the 2010s, the space agency’s budget had been shrinking; visiting Europa would risk breaking the bank and was widely seen as an almost impossible dream—at least, that is, until the mission’s many scientific advocates found an unlikely political champion in the form of then representative John Culberson of Texas, a Republican who almost singlehandedly allocated hundreds of millions of dollars to the effort. Today, Europa Clipper is ready and waiting for a launch in October of next year.
Time will tell whether the ice giants find their champion, too. “It’s an uphill battle, but we’ve done this before,” Dreier says. “We shouldn’t just assume we’ll be in our graves by the time we get data back from a Uranus mission.”
Quelle: SCIENTIFIC AMERICAN