On Earth, no one would scrap a car because its tank is empty. In space, that has long been the rule: once a satellite runs out of propellant, its mission usually ends. In-space refueling aims to change that. Together with propellant depots, tanks that store fuel in orbit, it could turn spacecraft into vehicles that can be serviced, extended and sent further. This guide explains why refueling matters, what NASA has already demonstrated in orbit, how depots fit into lunar plans and what operators should watch.
Why refueling matters
- Longer satellite life: propellant is often the resource that ends a mission while the rest of the satellite still works. Refueling extends that life. See in-orbit servicing and life extension.
- More capable transport: a vehicle that can be refueled in orbit does not need to carry all its propellant from the ground, which frees launch capacity for payload.
- Exploration: missions to the Moon and Mars need large amounts of propellant. NASA states that the ability to replenish and store cryogenic fluids can help enable long-duration journeys to destinations like the Moon and Mars.
What NASA has demonstrated
Robotic Refueling Mission (2011)
NASA's Robotic Refueling Mission (RRM) tested tools, technologies and techniques for refueling and repairing satellites aboard the International Space Station. NASA photos from 12 July 2011 show astronaut Mike Fossum holding the RRM payload during a spacewalk, with the station's Dextre robot nearby. Its installation was one of the main tasks of that six and a half hour spacewalk.
Robotic Refueling Mission 3 (2018 to 2019)
RRM3 built on the first two phases. It arrived at Kennedy Space Center in 2018 and flew to the ISS on SpaceX's 16th commercial resupply mission. NASA describes it as demonstrating the transfer of xenon gas and liquid methane in microgravity, including the first transfer and long-term storage of liquid methane, a cryogenic fluid, in microgravity. Astronauts Anne McClain and David Saint-Jacques installed it on the Kibo airlock slide table in February 2019, as shown on the cover.
Propellant depots for the Moon
Depots are the large-scale version of this idea. In NASA's 2024 description of SpaceX's Starship Human Landing System, SpaceX was to launch a storage depot to Earth orbit before the crew launches. For Artemis III and Artemis IV, SpaceX planned to complete propellant loading operations in Earth orbit to send a fully fueled Starship lander to the Moon. NASA has since announced changes to the Artemis campaign architecture, so check the latest NASA information before relying on these details. The concept shows the scale of what depots are meant to do: refuel a vehicle in orbit so it can continue a mission that would be impossible with the propellant it launched with.
Commercial refueling for small spacecraft
Refueling is also moving into smaller vehicles. NASA's Small Spacecraft Technology report mentions Atomos's Quark-LITE, launched in spring 2024. Despite communication and tumbling issues, it demonstrated rendezvous, docking and refueling capabilities, which NASA describes as groundwork for more advanced missions in geostationary orbit. The report also notes that the status of the planned follow-up missions is unknown.
Challenges
- Interfaces: most satellites in orbit were not designed to be refueled. RRM tested tools that can work with such existing hardware.
- Cryogenic storage: fuels like methane and oxygen must be kept extremely cold, as RRM3's long-term storage test addressed.
- Rendezvous and docking: the servicer must approach and connect safely, as Quark-LITE's issues show.
- Business case: refueling only pays off if enough customers need it and the service costs less than replacing the satellite.
What this means for satellite operators
For new missions, it can be worth checking whether a satellite should be prepared for future servicing, for example with accessible fill valves or docking features. For launch planning, refueling and orbital transfer vehicles can change which orbit you need to reach on launch day. Our guides to space tugs and electric vs chemical propulsion cover related trade-offs.
Frequently asked questions
Can satellites be refueled in space?
NASA has demonstrated refueling tools and techniques on the ISS with its Robotic Refueling Missions, and commercial demonstrations have started. Most existing satellites were not designed for it.
What is a propellant depot?
A tank in orbit that stores propellant so other spacecraft can refuel there. NASA's 2024 description of SpaceX's lunar lander included a depot in Earth orbit.
Why is cryogenic propellant hard to store in space?
It must stay extremely cold. NASA's RRM3 demonstrated the first transfer and long-term storage of liquid methane in microgravity.
Which propellants can be transferred?
RRM3 demonstrated the transfer of xenon gas and liquid methane.
Related reading
- In-orbit servicing and satellite life extension
- Space tugs and orbital transfer vehicles
- What is space logistics?
Sources
- NASA (Spanish): NASA strengthens Artemis, adds a mission and refines its overall architecture
- NASA: Robotic Refueling Mission payload during STS-135 spacewalk (2011)
- NASA: Robotic Refueling Mission 3 arrives at Kennedy (2018)
- NASA: RRM3 at the Payload Hazardous Servicing Facility (2018)
- NASA: RRM3 installation aboard the ISS (2019)
- NASA: Starship HLS propellant depot concept (2024)
- NASA: Small Spacecraft Technology State of the Art, Integration, Launch and Deployment
Cover image: Astronauts Anne McClain and David Saint-Jacques install Robotic Refueling Mission 3 on the Kibo airlock slide table aboard the ISS, February 2019. Credit: NASA (iss058e015664), public domain.