ESA’s GOCE gravity-mapper has created history by becoming the first ever satellite to fly in low Earth orbits – as low as 250 km for more than five years – thanks to the ingenious work by ESA-led team that built an electric thruster capable of continuously compensating for air drag in such low orbits.
Launched in March 2009, GOCE is the first ever electric propulsion system has shown that it is able to keep the satellite completely free from drag as it cuts through the remnants of Earth’s atmosphere – paving the way for the best gravity data ever. The primary task of the satellite was to map the Earth’s gravity and its mission life was limited by the 40 kg of xenon it carried as propellant – once that was exhausted, the mission was over.
However, that wasn’t the end for the satellite as the drag-free flight at such low Earth orbits was on the minds of scientists involved with the project. To ensure that the gravity measurements taken by GOCE are of true gravity, the satellite has to be kept stable and in ‘free-fall’. Any buffeting from residual air the satellite encounters along its orbital altitude of just 250 km could potentially drown the gravity data. This posed a technological challenge – the satellite structure had to be as aerodynamic as possible and a system had to be developed that would continually and instantaneously compensate for variations in air drag.
Along with its sleek design, GOCE is able to achieve drag-free flight by employing an electric ion propulsion system mounted at the back of the satellite, relative to its direction of flight. Unlike conventional fuel-driven engines, the system uses electrically-charged xenon to create a gentle thrust. The system continually generates tiny forces between 1 and 20 millinewtons (mN), depending on how much drag the satellite experiences as it orbits Earth.
A couple of months after launch GOCE was switched to drag-free mode as part of the commissioning and instrument calibration activities. The system was found to be working perfectly, demonstrating that the electric ion thruster-based control system automatically produces the right amount of thrust to achieve drag-free flight. This could not be demonstrated at ground level before GOCE launched since it is impossible to create exact in-orbit flight conditions in a laboratory.
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.