Starship is the largest rocket ever built, yet it cannot reach the Moon or Mars on a single tank of fuel. To make those journeys, SpaceX plans to refuel Starship in orbit, transferring propellant from one spacecraft to another hundreds of kilometres above Earth. As of late 2026, this has never been attempted between two Starships. It may be the most important milestone in SpaceX's entire program.
Why Starship needs refuelling
Rockets spend most of their fuel just escaping Earth's gravity and reaching orbit. By the time Starship reaches low Earth orbit, it has burned through much of its propellant. To travel onward to the Moon or Mars, it needs far more.
Rather than building an even bigger rocket, SpaceX's approach is to launch a Starship into orbit and then send additional Starships, configured as tankers, to fill it up. Once its tanks are topped up, the ship has enough propellant for the long journey.
An analogy helps. Imagine a large truck that uses most of its fuel climbing a steep mountain road. At the top, a series of tanker trucks meet it and refill its tanks, so it can continue across a long, flat highway. In space, the "mountain" is reaching orbit, and the "highway" is the trip to the Moon or Mars.
How many tanker flights are needed?
Estimates vary and have changed over time. For a lunar landing mission, public discussions, including those by NASA officials and analysts, have suggested that a significant number of tanker launches may be needed, potentially more than ten. The exact number depends on how much propellant each tanker can deliver, how much boils off during the waiting period and the final mission design.
That is why SpaceX's push for rapid launch cadence matters so much. If refuelling requires many tanker flights, they need to launch close together, before the propellant waiting in orbit starts to evaporate.
Why it is so difficult
Refuelling a car takes minutes. Refuelling a spacecraft in orbit is one of the hardest challenges in spaceflight.
Cryogenic propellants. Starship burns liquid methane and liquid oxygen, both stored at extremely low temperatures. In space, sunlight warms the tanks, causing propellant to boil off and escape over time. Managing this is difficult.
Microgravity. On Earth, gravity holds fuel at the bottom of a tank. In orbit, liquids float. Engineers must find ways to settle the fuel so it flows into the transfer lines correctly, for example by giving the spacecraft a gentle push to move the liquid toward the outlet.
Rendezvous and docking. Two enormous spacecraft must meet in orbit, line up precisely and connect safely, all while travelling at about 28,000 kilometres per hour.
Scale. The quantities involved are huge, far larger than anything transferred in space before.
NASA's Office of Inspector General has described vehicle-to-vehicle cryogenic propellant transfer as one of the most significant technical challenges for the lunar lander effort.
What has been tested so far
In March 2024, during Starship's third test flight, SpaceX transferred liquid oxygen between two tanks inside the same ship. Reports put the amount at around three to five tonnes. This was a valuable experiment, but it was an internal transfer, not a transfer between two separate vehicles.
Starship test flights in 2026, including Flight 12 in May and Flight 13 in July, focused on testing the new V3 design, deploying Starlink satellites and relighting engines in space. Neither attempted ship-to-ship refuelling. SpaceX's June 2026 IPO prospectus confirmed that in-orbit refuelling had not yet been attempted.
What a full refuelling demonstration involves
To prove the concept, SpaceX needs to show several steps working together:
- Launch a target Starship into orbit.
- Launch a tanker Starship to meet it.
- Rendezvous and dock the two vehicles.
- Transfer a meaningful amount of cryogenic propellant.
- Separate safely.
- Repeat this reliably, because one success is not enough for a mission that may need many transfers.
SpaceX has also discussed using an orbital depot, a Starship that stays in orbit and collects propellant from multiple tankers before transferring it to a ship heading to the Moon or Mars.
Why it matters for Artemis
NASA's Artemis program aims to return astronauts to the Moon. NASA selected a version of Starship, called the Human Landing System, to carry astronauts from lunar orbit down to the surface and back. That lander must be refuelled in Earth orbit before heading to the Moon.
This means Artemis's lunar landing timeline depends heavily on SpaceX achieving reliable orbital refuelling. Any delays in refuelling tests can ripple through NASA's plans.
Why it matters for Mars
Musk's long-term goal is to send Starships to Mars and eventually build a human settlement. A Mars trip requires even more propellant than a Moon mission. Without orbital refuelling, Starship simply cannot reach Mars with meaningful cargo or crew. SpaceX also plans to eventually produce propellant on Mars itself for the return journey, but that is a separate, even longer-term challenge.
The bottom line
Orbital refuelling is the key that unlocks Starship's biggest ambitions. Without it, Starship is a powerful rocket for reaching Earth orbit. With it, Starship becomes a vehicle for the Moon and Mars. The challenge is enormous: cryogenic propellants, microgravity, precise docking and the need for many rapid launches. SpaceX has made real progress with Starship V3, but the first ship-to-ship transfer is still ahead. When it happens, it will be one of the most important moments in modern spaceflight.
