Just weeks after NASA’s Europa Clipper mission quietly received a formal commitment to a final cost and timeline from the agency, it looks increasingly like the spacecraft will not fly on its legally mandated megarocket, the Space Launch System (SLS)—at least, not in the timeline outlined by Congress—documents and experts confirm.

Because of the severe radiation challenges of the Jovian system, Europa Clipper is one of the most ambitious flagship missions ever attempted by NASA, with seismic implications for the agency’s search for life beyond Earth. Europa—with its deep, ancient ocean locked beneath an icy crust—is seen by some astrobiologists as the solar system’s most promising site for harboring alien biology. In search of further signs of habitability, the Europa Clipper spacecraft will enter orbit around Jupiter and encounter the moon multiple times. With each flyby, it will collect data on Europa’s ice shell and subsurface ocean, remotely sounding the unseen fathoms below.

According to Curt Niebur, the mission’s program scientist at NASA Headquarters in Washington, D.C., Europa Clipper could answer the question of whether the right conditions exist on the icy moon to support life as we know it. If those ingredients—which include organic molecules, as well as potential energy sources such as hydrothermal vents on the ocean floor—are found on Europa, Niebur says, “we're going to want to explore further and see if life actually has arisen under the ice.” A second mission, now in development, would land on Europa to excavate and collect samples in search of native organisms.

Beyond the science, the mission has an unusual political element: Europa Clipper is the first and only space mission to be married to a specific spacecraft in any appropriations bill, says former Republican representative John Culberson of Texas, a longtime Europa advocate who previously chaired the Commerce, Justice, Science, and Related Agencies subcommittee of the House Committee on Appropriations. The current appropriations bill mandates Europa Clipper use the SLS and requires a “launch no later than 2023” on the rocket.

It was a move that Culberson, an attorney and political consultant, used to ensure Europa Clipper would one day reach the launchpad. He says tying the mission to the SLS, which is being built in Alabama, garnered the support of Republican Senator Richard Shelby of Alabama, chair of the Senate Committee on Appropriations, without which the mission might not have been mandated by law. (Attempts to reach Senator Shelby for this article were unsuccessful. A staffer reported that he was unavailable for comment.)

While Europa Clipper’s development has proceeded apace, however, the SLS rocket has remained mired in setbacks and was arrogated by the Artemis lunar program instigated by the Trump administration. Before it could carry the Europa craft to space, the SLS would fly two Artemis moon missions—the first of which has reportedly been pushed back from 2020 to 2021, according to Senate testimony by NASA Administrator Jim Bridenstine earlier this year. For Europa Clipper to launch on time, the second Artemis launch, now scheduled for 2023, would have to occur without error, with a third SLS rocket ready to go—a tall order, considering the program’s lengthy development, three-year delay and zero rockets completed thus far.

A report issued this past May by NASA’s Office of Inspector General supports that the SLS “is unlikely to be available” for Europa Clipper in 2023. And it adds that NASA “continues to maintain spacecraft capabilities to accommodate both the SLS and two commercial launch vehicles.” Moreover, even NASA’s own 2019 budget encourages “a Europa Clipper launch readiness date in 2025” and further proposes “to launch the Clipper on a commercial launch vehicle” to save money. Each SLS launch is estimated to run more than $1 billion.

In its Europa Clipper Key Decision Point-C memorandum, the formal commitment signed at NASA Headquarters earlier this month, the spacecraft is scheduled for a launch that ranges from 2023 to 2025, confirms Thomas Zurbuchen, NASA’s associate administrator for the science mission directorate. But if the mission were to launch in 2025—or on something other than the SLS—it would be in violation of current law, which means the law must change or a working SLS must suddenly appear in order for Europa Clipper to take off in accordance with federal statute.

“We’ve been working very hard to follow the appropriation law,” says Joan Salute, program executive of Europa Clipper. But she admits the mission “may or may not” be possible with the SLS in 2023. And if it’s not, she says, “we can’t follow that [appropriations law].”

Both Salute and Zurbuchen say forthcoming appropriations bills may need to be updated. “Congress will follow the development of the SLS, and that language should be updated as we know more about the SLS’s availability to us, or not, in 2023,” Salute says. And NASA will talk to Congress in order to “make sure the right solution happens for the American taxpayer,” Zurbuchen adds.

Zurbuchen points out that the appropriations bill’s language had been changed previously to accommodate Europa Clipper. Indeed, the 2017 appropriations bill called for a 2022 orbit and 2024 landing.

Democratic Representative Adam Schiff of California, whose district includes NASA’s Jet Propulsion Laboratory, its research-and-development center, says, “When Congress returns in September, we will continue to work to pass an appropriations package across the House and Senate that preserves our strong support for [Europa Clipper’s] goals and timeline. We will also continue to monitor the mission’s progress as part of that process, including the potential launch vehicle.”

Another vehicle that could take Europa Clipper to Jupiter’s moon is SpaceX’s Falcon Heavy—though it would do so with an asterisk attached. Falcon Heavy has already achieved three successful launches, but using the SpaceX rocket—which is less powerful than the SLS—would add at least three years of travel time to the planned two-year mission. And while using SpaceX’s rocket would save hundreds of millions of dollars on launch costs, it could add to Europa Clipper’s operations budget because of its longer cruise time to Jupiter. “It’s vital that the [Europa] Clipper be launched on the SLS,” Culberson says.

The SLS has an undeniable advantage over Falcon Heavy: it enables a direct flight from Earth to Jupiter. Falcon Heavy will require gravity assists from other planets, and unless it uses an add-on “kicker stage”—an additional upper stage for extra loft—one of those gravity assists will require an encounter with Venus. According to Salute, a Venus flyby introduces “a riskier environment, radiation and temperature. And so we would like to avoid flying closer to Venus with this direct trajectory that SLS affords us. Right now, SLS is the only launch vehicle that can give us that trajectory, and that’s why it’s so advantageous to us.”

The idea that Europa Clipper might not run on the SLS is almost unthinkable to Culberson, who says he remains optimistic the mission will fly on the SLS in 2023. “Heaven forbid SLS is not ready in time,” he says. “But in the event that it’s not, the most important thing for the mission is to ensure [it arrives] safely. The rocket, whatever it is, has to be reliable and has to be powerful enough to get the Clipper to Europa in a timely fashion.”