A hypothetical future air-breathing space mission in low orbit around Earth: propelled at around 7.8 km/s, the satellite would ingest air molecules from the top of the atmosphere (left) to fire its ion thruster (right), providing thrust to overcome atmospheric drag, allowing it to stay in low orbit indefinitely. This image is for illustrative purposes only.
ESA’s GOCE gravity-mapper flew as low as 250 km for more than four years thanks to an electric thruster that continuously compensated for air drag. However, its working life was limited by the 40 kg of xenon it carried as propellant – once that was exhausted, the mission was over. Replacing onboard propellant with atmospheric molecules would create a new class of satellites able to operate in very low orbits for long periods.