Refilling in orbit, the untested step both Moon landers depend on

Drawn from 16 sources: NASA Marshall Space Flight Center (5), NASA (3), NASA Goddard Space Flight Center (2), NASA Office of Inspector General and 5 others. About 6 minutes. Checked October 1, 2026.

Both of the landers NASA has bought to put astronauts on the Moon arrive in Earth orbit too short of propellant to go farther. SpaceX’s lander is the upper stage of its own rocket, launched on a Super Heavy booster, and NASA’s lander program says that refueling it in low Earth orbit is necessary for it to head to the Moon with a useful load.^2,1 Blue Origin’s Blue Moon crew lander will take on propellant in low Earth orbit and again on the way.¹ Either way, the first crewed landing waits on a capability no one has demonstrated: moving large amounts of super-cold propellant from one spacecraft to another in orbit.

NASA calls the problem cryogenic fluid management, a suite of technologies that store, transfer and measure ultra-cold fluids such as liquid hydrogen, liquid oxygen and liquid methane.³ NASA stated the payoff plainly in 2018, as it prepared its Robotic Refueling Mission 3 for the space station: the ability to resupply cryogenic fuel in space could minimize the amount of fuel spacecraft must carry from Earth’s surface, making it possible to travel farther into space for longer periods.⁴

The sequence: tankers fill a depot, the depot fills the lander

SpaceX’s plan uses three versions of Starship: a tanker that carries propellant to low Earth orbit, a storage depot that holds it there, and the lander that receives it.¹ The steps below follow one campaign.

The tankers launch. Before a landing mission, SpaceX will put the depot into low Earth orbit, then launch more than ten tankers after it. The tankers will fly from Starbase in Texas and from Kennedy Space Center in Florida, at a target rate of one every six days. The campaign begins more than 200 days before the crew launches, to leave margin in the schedule.¹

Each tanker docks and transfers. Each tanker rendezvouses with the depot, docks and moves its liquid oxygen and liquid methane across, a process the inspector general calls propellant aggregation. It repeats until the depot holds enough.¹ The connection itself is new hardware: NASA’s cryocoupler project describes the problem as fitting a nozzle to a fuel tank, except that the coupler has to attach and detach repeatedly without a spacewalk, seal at hundreds of degrees below zero and tolerate two spacecraft that are not perfectly aligned.⁹

The lander fills and departs. When the depot is full, an uncrewed lander lifts off from Kennedy, docks with the depot, refuels and continues to lunar orbit, where it can wait up to 100 days for the crew.¹

Loading the ships

Illustration to scale: each Starship is about 52 m long. Docking and transfer as described by NASA and SpaceX.

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A depot in low orbit

For each Artemis landing, SpaceX plans to place a storage Starship — a depot — in low Earth orbit, about 400 kilometers up, to collect propellant.

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A tanker arrives

A tanker Starship launches from Texas or Florida, full of liquid oxygen and liquid methane, and closes in on the depot.

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Docked tail to tail

The two ships join at their aft ends, where the propellant connections are. The supercold liquids are pushed across from one tank to the other.

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Again, and again

The tanker undocks, flies home and another takes its place. NASA’s Inspector General counts more than ten tanker flights per landing, beginning more than 200 days before the crew launches.

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The lander fills

With the depot full, the lunar Starship docks and loads the propellant it needs to reach lunar orbit, land and climb back up to Orion.

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To the Moon

Fully loaded, the lander leaves Earth orbit. None of this has flown yet: the first ship-to-ship transfer is still to be demonstrated.

Blue Origin’s version is spread over more orbits. It will first launch a transporter to low Earth orbit, essentially a depot; a fleet of refuelers launches, meets it and transfers propellant. The Blue Moon lander then launches and takes fuel from both a refueler and the transporter before flying to lunar orbit. The transporter stays behind to collect more propellant, climbs to a higher “stairstep” orbit for a final aggregation, and follows the lander to lunar orbit, where it gives the lander a last transfer before the crew arrives.¹

10+

Starship tankers planned for each landing, launched about six days apart¹

200+ days

Time the refilling campaign starts ahead of the crew’s launch¹

12–24 days

Turnaround SpaceX must reach on its launch pads, which it had not shown by March 2026¹

100 days

How long a fueled Starship lander can wait in lunar orbit for the crew¹

Boiloff: in space a cold tank keeps warming

Settling and measuring: finding the liquid in a weightless tank

On Earth, liquid sits at the bottom of a tank and vapor at the top. In orbit it floats, so before propellant can be drawn out of one tank and pushed into another, it has to be settled at the outlet, and the quantity has to be measured without gravity to show where the surface is.

Starship’s third test flight, on March 14, 2024, took the first step. During its coast, the ship transferred thousands of pounds of cryogenic propellant between its own internal tanks, a demonstration funded through NASA’s 2020 Tipping Point awards; the transfer was of liquid oxygen, between a header tank and a main tank. To understand how super-cooled propellant sloshes when the engines shut down and how that movement affects the ship’s stability, engineers studied the performance of the thrusters that control its orientation, and how the fluid could be settled to make the transfer efficient and to give the Raptor engines the conditions they need to restart in orbit.^7,2

Storing and transferring cryogenic propellant in orbit has never been attempted on this scale before.

Measuring is its own technology. On Intuitive Machines’ first lunar lander in February 2024, a NASA radio frequency mass gauge measured how much cryogenic propellant remained in the lander’s fuel and oxidizer tanks in low gravity, data meant to help predict fuel use on future missions.⁸ NASA engineers working with the lander companies have helped them develop propellant gauges for low gravity.¹

Smaller demonstrations since 2018, from the space station to Starship

NASA’s 2020 Tipping Point awards for cryogenic flight demonstrations¹⁰

Show the numbers

Company	Award
Lockheed Martin	$89.7M
United Launch Alliance	$86.2M
SpaceX	$53.2M
Eta Space	$27M

Where the ship-to-ship test stands in October 2026

The next step for SpaceX is a large-scale, long-duration transfer between two Starships in low Earth orbit. The first to launch will be the “target”; a second, the “chaser,” will dock with it and transfer cryogenic propellant into it.² SpaceX said in October 2025 that the first ship would spend an extended time in orbit so that its performance and boiloff could be measured before the second arrives, and that the ships would find each other with the same sensors that dock Dragon capsules to the space station.¹⁴

NASA’s lander program considers this transfer one of the most significant technical challenges SpaceX faces: the technologies and processes are entirely new and have never been done between vehicles. The test was planned for March 2025 and slipped to March 2026.¹ In July 2026 GAO listed a long-duration flight and the propellant transfer test as Starship’s next major milestones for the lander, both scheduled for 2026, and called SpaceX’s cryogenic technology a top program risk.¹³ A transfer needs two ships in orbit, and no Starship had reached orbit until September 28, 2026, when the 14th flight did.¹⁵

When the first transfer between two Starships was due¹

Neither company’s plan will be fully flight-tested before astronauts depend on it. The uncrewed demonstration landers will not carry life support, airlocks or, on Starship, the elevator, so they will be lighter than the crewed landers and need fewer tanker flights; the inspector general concluded that the end-to-end propellant campaign will not be fully flight-tested before the first crewed missions.¹ Blue Origin plans to show its own transfer during its uncrewed demonstration mission, expected in February 2029, after ground tests.^1,13 NASA’s notional plan for its 2027 Artemis III flight lists supporting missions to demonstrate a lander’s propellant transfer and aggregation in Earth orbit.¹⁶