Engineers from the European Space Agency (ESA) and an Italian company have successfully tested a prototype of an ion engine that would obtain its fuel from the thin atmosphere available in low Earth orbit, thus allowing it to operate practically indefinitely.
From the press release:
Replacing onboard propellant with atmospheric molecules would create a new class of satellites able to operate in very low orbits for long periods. Air-breathing electric thrusters could also be used at the outer fringes of atmospheres of other planets, drawing on the carbon dioxide of Mars, for instance. “This project began with a novel design to scoop up air molecules as propellant from the top of Earth’s atmosphere at around 200 km altitude with a typical speed of 7.8 km/s,” explains ESA’s Louis Walpot.
Think about it. You supply your planetary probe one or more of these engines, and once it reaches orbit around its target it has an unlimited fuel supply to do research just about forever. More important, such technology when further refined is going to enhance human exploration as well. For example, rather than use the atmosphere at it arrives, later designs could simply dive into the atmosphere to get the spaceship’s tank refilled. Such engines would make spacecraft free from the tether of Earth.