Elon Musk and Jeff Bezos are trying to figure out how to pump gas in space.
The billionaire space rivals are working on ambitious missions to the moon or Mars, and a crucial design element for each venture is using spacecraft that take on additional fuel while orbiting Earth. Vehicles that could grab propellants in orbit would be less weighed down at liftoff, letting planners design missions to travel farther from Earth with more cargo, scientific gear or crew members,
Elon Musk and Jeff Bezos are trying to figure out how to pump gas in space.
The billionaire space rivals are working on ambitious missions to the moon or Mars, and a crucial design element for each venture is using spacecraft that take on additional fuel while orbiting Earth. Vehicles that could grab propellants in orbit would be less weighed down at liftoff, letting planners design missions to travel farther from Earth with more cargo, scientific gear or crew members, advocates say.
Having depots or in-orbit refueling that give spacecraft something like a truck stop to pull into may sound like science fiction. It is also a concept that engineers have been working on for years.
One of the biggest challenges to making it a reality: moving and storing massive amounts of supercold propellants that are prone to boil off in the void of space. Both SpaceX and Blue Origin have much to prove.
Some space-industry officials are skeptical that either will have moon landers that depend on orbital fueling ready to meet National Aeronautics and Space Administration timelines. The agency has worked closely with contractors to understand the challenges of in-space refueling, a NASA spokesman says.
SpaceX conducted a fuel-transfer demonstration inside a Starship spacecraft during a 2024 test flight and next year is aiming to move propellants between two vehicles. That test has been delayed as the company has faced setbacks with the massive rocket, including an explosion during a ground test in Texas in June.
Blue Origin is developing a transporter vehicle that would take on propellant near Earth. Then, it would fly to a lunar orbit, where the transporter would prepare a lander that would take astronauts arriving on a different ship down to the surface of the moon. That mission depends on the company’s New Glenn rocket, which the company launched for the first time in January.
Engineers have long built rockets and spacecraft to take on all the fuel they need while still on the ground. Those designs have proven their value. They also have their limitations.
Take NASA’s Apollo missions to the moon. Saturn V, the rocket that sent agency astronauts there, weighed 6.5 million pounds at liftoff, and around 5.5 million of those pounds were fuel.
Combined with reusable rockets, fueling in space could make deep-space flights cheaper over time and ease mission logistics, supporters say.
“It has got to be done,” says Dallas Bienhoff, who formerly studied setting up a low-Earth-orbit propellant depot for Boeing and now works for Pasadena, Calif.-based robotic-mining company OffWorld. “Otherwise, we’re going to be limited with what we can achieve in space.”
Musk put it this way during a conference talk in 2017. Starship, which then went by a different name, was expected to loft a spacecraft carrying a large amount of mass to low-Earth orbit. The ship wouldn’t be able to travel farther without more fuel, he said.
“If you send up tankers, and refill in orbit, you can refill the tanks all the way to the top,” Musk said then. That would allow the ship to continue out to Mars, he added.
From Apollo to the present
Orbital refueling isn’t a new idea. In the early 1960s, NASA officials considered such operations as they tried to figure out how to land astronauts on the lunar surface before the Soviets.
The idea didn’t go forward then, but it didn’t go away. A range of in-space fueling experiments or operations have been conducted over the years, including by Russia. In 2007, the Pentagon’s Defense Advanced Research Projects Agency and partners conducted more than a dozen fuel transfers between two satellites.
But no U.S. company has put in-space propellant transfers at the center of their deep-space plans as SpaceX and Blue Origin have.
Part of the reason is making it all work is difficult. Spacecraft will need to rendezvous in orbit, connect with each other and flow large amounts of propellants from one vehicle to another.
The nature of the propellants SpaceX and Blue Origin have proposed using makes transferring them especially tricky. The fuels are chilled to supercold levels to keep them liquid, and have a tendency to boil off.
“It wouldn’t be easy to do on Earth. But now you have to do it in space, pumping it from one big refrigerator to another,” says Thomas Cooley, formerly chief scientist for a space-vehicles group at the Air Force Research Laboratory who now has a space-consulting business.
Dave Limp, chief executive at Blue Origin, said in a social-media post in July the company had made major progress maturing technologies that aim to prevent propellants from boiling off in space.
Fuels also behave in odd ways in space. When you pump gasoline into your car’s tank, the gas flows in a predictable manner but that isn’t necessarily the case in orbit. There, companies will contend with what William Notardonato, chief executive of Eta Space, calls “liquid-acquisition issues.”
“You don’t know where your liquid is stored. The propellant might be up at the top of the tank. And it brings up the question, what is the top in microgravity?” says Notardonato, whose Rockledge, Fla.-based company is developing an in-space propellant depot and working on a mission where it will track how a supercold propellant behaves in a satellite tank using a camera.
Launching, time and again
Another unknown for in-orbit refueling: the number of launches it will take SpaceX and Blue Origin to fuel up vehicles for deep-space missions.
SpaceX has shown launches can be conducted much more frequently than in the past, but rockets are still a long way from flying as easily as commercial jets. Meanwhile, Starship, the company’s vehicle that will use orbital fueling, remains in an experimental phase and Blue Origin is still ramping up New Glenn.
During a presentation earlier this year, a Blue Origin executive didn’t specify how many refueling flights the company may need for a NASA mission that has it transporting astronauts from lunar orbit to the surface of the moon.
For its NASA astronaut moon missions, SpaceX has been planning to conduct multiple launches of a tanker variant of its Starship spacecraft to fuel up a Starship depot in low-Earth orbit, according to NASA officials. Then the company would send a moon-lander Starship to the depot to take on fuel. After that, the lander would fly out to the moon.
An executive at SpaceX early last year estimated a moon mission for NASA could take around 10 flights, while a different leader at the company last fall predicted around 16. Some current and former space-industry officials say the number of launches needed may be higher.
One former NASA leader during a congressional hearing in February said as many as 20 could be required. A technical paper that has circulated among some industry officials and was viewed by The Wall Street Journal suggests it may take up to 40.
Musk, during a recent presentation about SpaceX’s plans for Mars, didn’t discuss the number of flights that would be needed to send a Starship to the red planet. Staff at the company have talked about trying to send a basic, uncrewed ship out there next year with as few as three refueling flights, the Journal has reported.
In a recent post on his social-media site X, Musk said a Starship flight to Mars in 2026 would be tough, giving it just a slight chance of happening.
Micah Maidenberg is a reporter in the Chicago bureau of The Wall Street Journal. Email him at micah.maidenberg@wsj.com.
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