Perhaps outer space's most defining characteristic is the lack of air. The vacuum of space, largely devoid of molecules, makes the European Space Agency's new efforts to build an air-breathing thruster a significant challenge. The space agency recently hit a milestone developing a the new propulsion technology, however, building and testing a thruster in space-like conditions on the ground.
The ESA has been studying air-breathing electric thrusters (ABET) for over a decade. In 2007, a study concluded that the technology "could provide a promising innovative solution" for low Earth orbit (LEO) travel. In this low orbit around the planet, the upper atmosphere is actually thick enough to drag on spacecraft, requiring fuel thrusters to periodically lift satellites to keep them in orbit. An air-breathing thruster could collect these molecules and accelerate them out the back to achieve propulsion, allowing satellites to orbit for longer.