NASA is building a robotic spacecraft refueling system, to prevent a Gravity-like orbital debris cascade

NASA is preparing to take the next logical step after in-flight refueling between two aircraft — robotic refueling of orbiting satellites. This could extend the lifetime of many satellites indefinitely, and could play a very important role in preventing a Gravity-like scenario, where fragments of a single satellite cause a cascade of debris that destroys almost every satellite in Earth orbit.
The program, which has the delightful acronym of RROxiTT (Remote Robotic Oxidizer Transfer Test), essentially consists of a special robotic arm and a cannister of nitrogen tetroxide. Nitrogen tetroxide (NTO) is a very strong oxidizer, and it combusts automatically when combined with fuel. Because no ignition source is required, NTO is often used in spacecraft rocket engines (Space Shuttle, most geostationary satellites), and in their launch vehicles (Russia’s Proton, China’s Long March). Basically, spacecraft can only carry a limited amount of NTO — and when they run out, they lose the ability to maneuver. In the case of satellites, which have to constantly jiggle around and boost themselves back into a higher orbit, running out of fuel is usually the end of its mission. These dead satellites then become part of the growing problem of orbital debris.

With RROxiTT, however, NASA wants to give those old spacecraft a new lease of life — saving money, and reducing the amount of debris (i.e. dead satellites) stuck in orbit. There are two key problems that RROxiTT needs to be overcome: Safely transporting and transferring highly volatile oxidizer, and then unscrewing the spacecraft’s fuel cap (which was never designed to be removed). NASA’s Goddard Space Flight Center, which has experience in robotics, is handling the second problem, and the Kennedy Space Center was drafted in to help with the first bit.
A further layer of complexity is that the refuelling craft will be unmanned and controlled from Earth. Remotely controlling a spacecraft and complex robot arm is an innately complex task — but once you add in some latency, it becomes even harder. Presumably the spacecraft and refueling nozzle will have some level of autonomy — but the final task of actually unscrewing the satellite’s fuel cap and inserting the nozzle will most likely be done by hand. As you can see in the video embedded above, extensive testing will be carried out here on Earth before NASA actually goes ahead with a launch. It’s worth noting that this same tech might also be used to fill up spacecraft here on Earth — a hazardous task that is currently performed by humans.
If NASA can successfully perform in-space refueling of spacecraft, it would be a pretty huge boost for commercial satellites, which currently have a fairly short lifetime — but also potentially for space exploration. We still don’t quite know how we’re going to power long-distance space journeys. There are pretty strict limitations on just how much fuel we can easily lift off the surface of the Earth. It’s not too crazy to suggest that, in the future, manned trips to Mars or Europa might involve a few refueling stops along the way.