NASA SLS megarocket shortage causes tug-of-war between moon missions, Europa exploration

The core stage of the SLS rocket for NASA's Artemis 1 mission, complete with four engines, seen in Louisiana in January 2020 before shipment to Mississippi for testing.
The core stage of the SLS rocket for NASA's Artemis 1 mission, complete with four engines, seen in Louisiana in January 2020 before shipment to Mississippi for testing. (Image credit: NASA/Jude Guidry)

NASA is choosing between human missions to the moon and a robotic mission to Jupiter's icy moon Europa as the agency manages its limited supply of megarockets in the coming years.

The agency began developing its Space Launch System (SLS) in 2010, intending for the rocket to be the agency's primary vehicle for crewed and deep-space missions. But work has been slow, and NASA and Boeing, which builds the vehicles' two main stages, are only now testing the core stage of the first SLS. It won't fly until late next year, when it makes the first flight of NASA's Artemis lunar-exploration program — an uncrewed trip around the moon known as Artemis 1. The schedule will therefore be tight for the third Artemis launch, which aims to land two astronauts near the moon's south pole in 2024.

Meanwhile, engineers are building the Europa Clipper spacecraft, designed to learn enough about the moon's ice shell, subsurface ocean and geology to help scientists determine whether the hidden ocean may suit the needs of life as we know it. And Congress has mandated the agency also use an SLS rocket to launch Europa Clipper — without consideration for whether one may be available.

"We have a requirement right now by law that says that we're going to launch this vehicle on an SLS rocket," NASA Administrator Jim Bridenstine said during a steering committee meeting of the U.S. National Academy of Sciences' planetary science decadal survey, an independent report that determines NASA's priorities, held virtually on Oct. 16.

Related: NASA's 1st SLS megarocket core loaded onto barge for key engine test

And that's a major scheduling problem. NASA can't just whip up an SLS rocket from thin air: The agency needs to contract with different companies for different parts of the megarocket and allow time for manufacturers to deliver on those contracts.

For each massive core stage at the heart of an SLS rocket, for example, Boeing needs more than a year of preparation time plus three years of manufacturing and testing time, according to a NASA document reported by NASA Spaceflight. That means even if NASA signed its next contract today, the resulting rocket likely wouldn't be ready to fly until 2025.

The initial NASA-Boeing contract covered the core stages for Artemis 1 and 2; an agreement made in October 2019 transferred money for Boeing to begin work on a third core stage, according to a NASA announcement made at the time. The agreement was meant to pave the way for a full contract for core stages for up to 10 Artemis missions to be finalized within a year's time, the announcement continued, while allowing Boeing to begin work soon enough to meet the 2024 timeline for Artemis 3, the first crewed lunar landing since Apollo 17 in 1972.

And future SLS core stages are in the works, the company has confirmed. "The team is not just working on one rocket," John Honeycutt, NASA's SLS program manager, said during a news conference held in conjunction with the International Astronautical Congress on Oct. 13. "We've made great progress on Artemis 2 and 3."

But with NASA prioritizing Artemis 3, that still leaves Europa Clipper waiting for a ride. As of a September status report, the Clipper team is planning to have the spacecraft ready for launch in early 2024, the same year Artemis 3 is targeting launch.

In terms of rocket science, right now, Europa Clipper can launch on a commercial vehicle, like SpaceX's Falcon Heavy or United Launch Alliance's Delta-IV Heavy rocket, although the mission would then need a longer cruise time to reach its destination.

But in terms of the law, NASA's hands are tied.

"Because of that, we're planning to build the Europa Clipper and then put it into storage, because we're not going to have an SLS rocket available until 2025," Bridenstine said. "That's the current plan. I don't think that's the right plan, but we're going to follow the law."

Photos: Europa, mysterious icy moon of Jupiter

Storing Europa Clipper, which is a $3 billion project, will cost the agency between $3 million and $5 million per month, according to calculations made by NASA's Office of Inspector General and reported to congressional leaders in August 2019.

In addition to getting the spacecraft off the ground sooner, a commercial launch for Europa Clipper would save the government money — $1 billion according to NASA's Office of Inspector General or $1.5 billion, according to a document summarizing changes included in President Donald Trump's budget request for the fiscal year 2021, which started at the beginning of October although the year's budget is still in congressional limbo.

Typically, NASA is free to launch its missions on whatever rockets the agency deems fit. But in the case of Europa Clipper, Congress wrote it into the appropriations bills funding the agency that the mission must launch on SLS.

That edict has led to the agency's current pinch: with the moon missions accelerated to land humans in 2024 as part of the Artemis program also relying on SLS rockets, the agency doesn't have enough launch vehicles to meet the schedules of both programs.

"We have a lot of demand for the SLS rocket going forward; when we think about doing a mission to the moon every year, we don't have extra SLS rocket sitting around," Bridenstine said. "So when we take an SLS rocket to launch Clipper, for example, then that takes away a moon mission. And there are other opportunities to launch Clipper, but there are not other opportunities at this point to launch moon missions with humans."

The Europa Clipper team has warned that continuing uncertainty about the launch vehicle may begin to slow work on the mission, as engineers have to postpone work that would make the spacecraft incompatible with one launch vehicle or another.

Later in the same decadal survey meeting, when the steering committee heard from staffers supporting congressional committees that deal with NASA issues, Europa Clipper's launch vehicle woes came up again.

"All I can really say is, it's the law," Joel Graham, a professional staff member for the Senate Committee on Commerce, Science, and Transportation, said during the meeting when asked about the situation. "We fully support putting it [Europa Clipper] on SLS."

Graham hinted that frustration with the arrangement should be directed to the companies building SLS components rather than Congress. "I would certainly hope that our suppliers can meet that and provide the vehicle for it," he said. "I think we can all agree that's the best vehicle to send it on."

A House committee representative was more open to flexibility on the matter. "Our authorization bill includes language on just looking at the options and sort of the pros and cons of the various options," said Pamela Whitney, a staffer with the House Committee on Science, Space and Technology. "Obviously, there is policy in place, but we also understand there are various other factors that are in play right now."

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Meghan Bartels
Senior Writer

Meghan is a senior writer at and has more than five years' experience as a science journalist based in New York City. She joined in July 2018, with previous writing published in outlets including Newsweek and Audubon. Meghan earned an MA in science journalism from New York University and a BA in classics from Georgetown University, and in her free time she enjoys reading and visiting museums. Follow her on Twitter at @meghanbartels.