When out on a long drive, if you notice you are running low on gas, you can head to the nearest gas station and fill up your tank. When you have to refuel a satellite though, there are no stations to pull into. Researchers at NASA however are working on a robotic system that could refuel satellites in orbit, extending their lives.
Presently, when you launch a satellite it will never have more than what it launched with (though there have been some serviceable satellites). This is somewhat understandable, given the circumstances, but that hardly means we cannot try to do something about it. Under the Satellite Servicing Capabilities Office (SSCO), the Remote Robotic Oxidizer Transfer Test (RROxiTT) was launched to see if it could transfer a very dangerous liquid fuel into a tank, while in orbit. It utilized many new technologies, including a new propellant hose, nozzle, and transfer system. After nine days of operations, the SSCO team declared the RROxiTT mission a success.
While the potential to extend a satellite's life by refueling it is invaluable as a means to reduce space junk, it could also have implications on the ground. The oxidizer RROxiTT transferred was nitrogen tetroxide, which is very dangerous in many ways, so building a robot to work with it instead of humans is generally a good idea.