In the previous installment, we examined on-orbit refuelling (OOR), a concept that has existed since the Space Age.

Thanks to ongoing research by space agencies like NASA and spinoffs to the commercial space sector, OOR could soon become a reality. In addition to plans for fuel depots in orbit, which SpaceX hopes to use to service missions with the Starship and Superheavy, OOR is also being investigated for the sake of satellite refueling.

In particular, commercial space companies like Arkisys and Orbit Fab are developing commercial platforms that could refuel satellites, greatly extending their service life.

This would also significantly reduce the threat posed by orbital debris (a.k.a. space junk), which is growing out of control. If left unchecked, this junk will become a self-perpetuating problem threatening Low Earth Orbit (LEO) operations in the coming years—a.k.a. the Kessler Effect.

While many strategies are currently under development for removing or eliminating space junk, these address the symptoms, not the root cause. By eliminating the paradigm where satellites and spacecraft are examples of single-use technology, the problem of space debris could be addressed at the source.

Satellite refueling

To be clear, refueling in space is very challenging because satellites are not designed for it. Traditionally, technicians will fill a satellite’s fuel tanks through triple-sealed valves covered in protective layers. After this, a satellite’s fuel system is never accessed again. Orbital conditions also complicate the situation, where microgravity introduces various navigational challenges.

As Schmitt described: “Satellite refueling is not easy. The first thing you might think about is the complexity of grappling and robotic operations (which are indeed hard enough as is), but, in order to approach a satellite and refuel it, there is another major barrier. First, you need to identify the target using your servicer satellite – something that can only be done using advanced space navigation software.

Moreover, once you approach the target, you need to control your relative position and orientation so that it matches the configuration of your target – also something extremely challenging in the realm of space navigation. To do that, most solutions proposed use some sort of QR code in the target satellite as a visual reference – but that is not present in most satellites in space.”

Credit: NASA

The good news, says Schmitt, is that this is about to change. Today, many commercial space companies are emerging to meet the demand for on-orbit servicing for spacecraft, satellites, and perhaps (someday) space stations. Additionally, the technology for satellite refueling has been advancing thanks to pioneering experiments conducted by NASA and other space agencies. The results of these experiments have since been made available to the commercial sector, and many startups have emerged to create the necessary applications.

Development

Between 2011 and 2013, NASA conducted OOR experiments through its Robotic Refueling Mission (RMM) program, a technology demonstration jointly developed by the Satellite Servicing Capabilities Office (SSCO) at the Goddard Space Flight Center (GSFC) and the Canadian Space Agency (CSA). This mission used the Special Purpose Dexterous Manipulator (SPDM) – aka. Dextre – aboard the International Space Station (ISS) to demonstrate satellite refueling in orbit by robotic means.

The Satellite Servicing Capabilities Office at the Goddard Space Flight Center (GSFC) developed the Robotic Refueling Mission to demonstrate the technology and tools for refueling satellites in orbit. After completing the proof of concept, NASA transferred all the data to the commercial sector via NASA’s Technology Transfer Program. The experiment consisted of a box about the size of a washing machine containing robotics tools, satellite interfaces, and activity boards.

Meanwhile, similar experiments appear to have occurred with the China National Space Agency (CNSA). Between July 2 and July 6, China’s Shijian-21 and Shijian-25 satellites appeared to merge in orbit. State media later confirmed that the purpose of the Shinjian-25 was to test refueling technologies, though few details were shared.

The ESA has also pursued several initiatives for satellites and spacecraft in-orbit servicing (IOS). One key contribution is the development of the ESPRIT European System Providing Refueling Infrastructure and Telecommunications (ESPRIT) Refuelling Module (ERM) for the Lunar Gateway, which will allow for fuel transfers in space and extend the lives of missions around the Moon.

Platforms in space

Arkisys was founded in 2014 to develop the architecture to create orbital platforms similar to nautical ports. In the same way traditional ports allowed maritime vessels to be serviced, restocked, and refueled, Arkisys platforms (known collectively as The Port) will enable “space servicing.” This will include prototyping, testing, assembly, and integration, while serving as a destination for orbital transfer vehicles.

A vital component in these platforms is Orbit Fab’s RAFTI system, a cooperative docking and refueling interface that replaces a satellite’s existing fill and drain valves to enable on-orbit and ground fueling. On February 4, 2021, Arkisys and Orbit Fab announced they were teaming up to create the capabilities and technology necessary to realize satellite refueling and advance the protocols.

Arkisys CEO Dave Barnhart told Interesting Engineering via email: “Orbit Fab took the lead in developing the critical technology to enable refueling, namely the gas pump and spigot! Their RAFTI system is advanced and is planned to be tested on a mission coming up within the next two years to validate its operations in orbit. Arkisys Port is looking at Orbit Fab’s refueling “interfaces” as they are called along with some others for our first mission as well.

You can think of satellite refueling in two ways; one is how Arkisys is proposing which is a fixed or stable platform in an orbit (ie. our Port Module) that satellite comes to and “fill up” similar to a fixed gas station on earth. The other is how aircraft are refueled on the runway by a fueling truck driving up to them; in space that would be a space servicing spacecraft that flies up to a satellite and provides the robotic refueling arm and technology.”

Both of these approaches, said Barnhart, extend satellite lifespans, reduce the number of defunct satellites in orbit, and promote sustainability. In the same way that maintaining cars or planes instead of scraping them helps reduce the waste we create, orbital refueling will help keep orbital pathways clear and safe. Given the high volume of traffic LEO is expected to have in the coming years, clearing the road of debris and junk is highly advisable.

Guidance and economics

Naturally, some challenges must be overcome before this vision of sustainability in space can be achieved. These include the technical challenges of making safe rendezvous in orbit and the economic challenge of making refueling systems and servicing platforms cost-effective. Overcoming the first challenge is necessary to avoid collisions in orbit, which not only threaten functioning satellites but can cause explosions that lead to more space debris.

Barnhart said, “Imagine city roads becoming jammed, not just from the cars parked there, but from vehicles weaving across lanes to enter and exit. Each orbital altitude is like a traffic lane on a multilane highway. The solution lies in changing the mindset. Just like modern cars are now built to be easily serviced, future satellites need to be designed for post-launch maintenance.”

To address this, developers are working on guidance and navigation software to allow safe rendezvousing and collision avoidance. This includes Translunar, the company Schmitt co-founded earlier this year through Purdue University’s New Venture Challenge. “[We are] developing advanced AI models to navigate relative to any satellite, regardless of the presence of pre-positioned markers,” said Schmitt. “This also includes long-range navigation, which allows us to approach any target in space from a distance of dozens of kilometers!”

As Barnhart explained, the second challenge may be more difficult—i.e., the business case. In short, servicing satellites could be complicated by the proliferation of small satellites (CubeSats) in orbit. These low-mass satellites are very cost-effective and can be launched in small swarms, but are strictly designed as a single-use technology.

Barnhart explained, “The value of “refueling” makes the most sense on larger spacecraft and those that stay up for 10 or more years.  Their “value” is higher, and thus the ROI goes up with longer life. Alternatively, the new advent of LEO and small satellite constellations takes almost the opposite approach: optimize the cost and efficiency of each of the satellites to last only a few years, and then replace them. It makes less sense to extend their life as there are hundreds to thousands of them and would take a long time to “refuel”, thus their value proposition for refueling is different.”

He added that the solution lies in changing the mindset. “Just like modern cars are now built to be easily serviced (think plug-and-play parts or EV charging ports), future satellites need to be designed for post-launch maintenance. Europe is already leading here: by 2035, all EU satellites will be required to support in-orbit servicing.”

“So the analogy? It depends on your “vehicle”: Are you driving a Maserati or a Pinto, and are you on a racetrack or a backroad? Each scenario calls for different logistics—but better planning keeps everyone safer, longer, and more efficient,” he added.

Several satellite developers are expected to hit crucial milestones in the coming years, and government agencies are getting on board. In 2023, the U.S. Space Force awarded a $1.6 million SpaceWERX Small Business Innovation Research (SBIR) contract to a team led by Arkisys to demonstrate robotic satellite assembly. Per this contract, Arkisys, Novawurks, Motiv Space Systems, Qediq, iBoss, and Texas A&M University will demonstrate how they would assemble a three-axis stabilized satellite with the robotic arm on the Arkisys Port Module.

At the 40th Space Symposium held in April, the Space Force also announced that it had contracted with space startup Astroscale to conduct an orbital refueling demo. Per their agreement, the company’s APS-R spacecraft will launch in the summer of 2026 to geosynchronous orbit, rendezvous with a Space Force satellite, and refuel it. The APS-R will then refuel itself from another spacecraft built by Orbit Fab, then refuel another Space Force satellite. 

Though these efforts are still in their infancy and challenges remain, there is no shortage of people and startups dedicated to making solutions happen. Along with countless partners in the public sector, academia, and research institutions, these efforts are united in a single purpose: to make space sustainable and open to future generations.