Uranus should be NASA’s top planetary target, influential report finds

After decades in the shadow of the other planets, Uranus should become NASA’s focus of exploration, a panel of planetary scientists reported today in the field’s long-awaited “decadal survey,” a priority-setting report the agency will use to makes its case to congressional funders. If the scientists get their wishes, NASA in the early 2030s will launch a $4.2 billion orbiter and atmospheric probe to Uranus, seeking to understand the formation and composition of this ice giant. Intermediate between the rocky planets and gas giants in size, Uranus and its neighbor Neptune “represent a unique planetary type that we poorly understand,” says Ravit Helled, a planetary scientist at the University of Zürich, one of 130 scientists who contributed to the survey.

The decision to favor Uranus over Neptune ultimately came down to celestial opportunism, says Robin Canup, a planetary scientist at the Southwest Research Institute and co-chair of the report, which was overseen by the National Academies of Sciences, Engineering, and Medicine. If launched on a Falcon Heavy rocket in 2031 or 2032, the orbiter could get a gravity assist from Jupiter and arrive in 13 years; Neptune would take far longer. “This mission is technically ready to go,” Canup says. “We advocate that it be started right away.” But whether that can happen depends on NASA figuring out a budget that has been strained by the pandemic and soaring mission costs.

It was Uranus’s turn. The last decadal report, in 2011, ranked an ice giants mission third, following a set of missions to return rock samples from Mars and a visit to Europa, Jupiter’s icy moon—missions that are now underway or in development. So perhaps the survey’s biggest surprise is its recommendation for what comes after Uranus: a $4.9 billion mission to Enceladus, the tiny moon of Saturn that spews organic-rich plumes of water out of fissures in an icy cap—ready-made samples of a subsurface ocean that might host microbes. “Enceladus is probably the best place to look for evidence of life that we can do today,” says Philip Christensen, a planetary scientist at Arizona State University, Tempe, and the report’s other co-chair. (The recommendation will mark an end for plans to put a lander on Europa’s surface, which had previously been advanced as a top future mission.)

The report also lists targets for a set of competitive missions, called New Frontiers. Some concepts are familiar from past surveys: a Saturn probe, a comet sample return, a lunar geophysical network. Others are new: sample return from Ceres, the water-rich dwarf planet in the asteroid belt; an orbiter and lander to a Centaur, one of the small bodies between Jupiter and Neptune believed to capture the composition of the early Solar System; a Titan orbiter; a Venus lander; and an Enceladus plume sampler. (Enceladus’s inclusion in two different mission categories stresses its importance, Christensen says.)

NASA should also continue programs dedicated to exploring the Moon and Mars, the panel recommends. After the agency builds the Mars sample return missions, the panel calls for it to develop a $1.1 billion robotic lander, called the Mars Life Explorer, that would drill 2 meters into midlatitude ice deposits.

For the Moon, the panel endorses the Artemis program, funded by NASA’s human spaceflight division, which plans to return astronauts to the surface. But it suggests science should drive the choices of what to do, rather than being an afterthought. “It’s not just flags and footprints,” says Bethany Ehlmann, a planetary scientist at the California Institute of Technology and co-author of the report. The report calls for a $1.5 billion long-range large robotic rover called Endurance-A that could cover 1000 kilometers, drill 100 kilograms worth of samples, and return them to astronauts who would eventually bring them back to labs on Earth.

Those ambitions will strain NASA’s planetary science budget, now $3.1 billion per year—the highest since the Viking missions to Mars in the 1970s. The Mars sample return campaign, which will retrieve rocks collected by the Perseverance rover, will cost more than $7 billion and consume one-fourth of the planetary budget in the next few years. The cost of Europa Clipper, which after launch in 2024 will swoop past the moon nearly 50 times, has grown from $4.25 billion to $5 billion. And several cost-capped competitive missions have seen their budgets more than double because of the time needed to reach their remote destinations, a factor not included in their spending limits.

The budget overruns have led NASA to postpone missions: The ambitious Dragonfly rotocopter to Titan, Saturn’s methane-rich moon, will now launch in 2027 instead of 2025, and the next New Frontiers selection will be delayed by several years. To stop this cycle, NASA needs to face reality and raise the cost caps for the two competitive mission lines, New Frontiers and Discovery, to $1.65 billion and $800 million, respectively, while also forcing those missions to fully account for lifetime costs. Those measures should still allow NASA to select five Discovery missions over a decade, but only one New Frontiers mission.

Although the planetary science budget has grown to accommodate big missions, the scientists who advance that work have not seen the same gains, the report stresses. The share of the budget spent over the past decade on research grants has fallen from 14% in 2010 to 7.7%. Progress has been made in recruiting more women to the field, but underrepresented racial and ethnic groups, notably Latino and Black scientists, make up just 5% and 1% of its workforce, respectively. “We have untapped talent and we’re missing out on great people and great ideas,” Canup says. The report recommends collecting better demographic data and expanding predoctoral programs that support students from underrepresented communities.

Students entering the field now could constitute the scientific heart of the mission targeting Uranus, which humanity first saw up close with the Voyager 2 flyby in 1986. That survey prompted many scientists to think of the ice giants as anomalies: stunted gas giants that accumulated only a couple Earth masses’ worth of hydrogen and helium before stopping, either because of a lack of gas or late formation. But since Voyager, astronomers have found thousands of planets around other stars, and many are Uranus-size, says Jonathan Fortney, a planetary scientist at the University of California, Santa Cruz. “Nature loves to make planets of this size.”

Uranus also holds its own individual appeal. Its spin axis lies nearly horizontal—likely the result of a giant impact early in its history that tipped it over. Compared with the other planets, it is also surprisingly cold, suggesting it either cooled quickly or that its atmosphere has put a lid on any heat escape. It has two sets of rings, along with a densely packed set of primordial moons and oddball objects, likely trapped comets or objects from a region beyond Neptune called the Kuiper belt. “Some may still have water on the inside,” says Kirby Runyon, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory (APL).

For the lure of an ocean, however, it’s hard to top tiny Enceladus, just 504 kilometers wide. In 2005, NASA’s Cassini spacecraft spotted plumes of saltwater erupting from rifts in its icy surface. Subsequent flights through those plumes revealed abundant organic molecules, necessary to build life, along with silica and hydrogen gas, a sign that the ocean feeding the plumes probably has hydrothermal vents in its depths, a potential energy source for microbes.

The survey endorsed a hybrid “orbilander” mission to Enceladus, which would sample the plume and survey the moon’s surface for a couple of years before turning on its side and landing, a relatively easy task in a place with weak gravity and no appreciable atmosphere. It would target a place where the erupting water falls as snow, which its instruments could sample. Two would explicitly be aimed at detecting life: a DNA sequencer and a microscope. Enceladus has checked off all the requirements for habitability, says Shannon MacKenzie, a planetary scientist at APL who led a study developing the idea. “The next question is: Is Enceladus inhabited?”