The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite, operated by the European Space Agency (ESA), is one of the most sensitive observatories in Earth's orbit. It carries a highly precise gravity gradiometer aboard, which allows it to detect even the smallest fluctuations in our planet's gravity field.
But its success is entirely dependent on whether it is able to maintain its peculiar orbit or not. That's why mission controllers sighed with relief as the craft's electric ion propulsion engine was successfully ignited a couple of days ago, thus completing an important step in the post-launch phases of commissioning.
“On Tuesday last week, we fired the B-unit of the electric ion propulsion, stepping it through 1, 3 and 8.3 mNs of thrust. On Thursday, we fired the A-unit. Both are performing nominally,” Juan Piñeiro, the manager of Spacecraft Operations at the ESA European Space Operations Center (ESOC) in Darmstadt, Germany, shared.
“Next comes the 'wake up' for the instruments, which puts us in the position to start operating the first-ever gradiometer to be flown in space. After confirming they work, we will be ready for intensive testing of the satellite and of the scientific data-processing system on the ground, which will be managed at ESA's Earth Observation Center in Frascati, Italy,” GOCE Mission Manager Rune Floberghagen added.
Controlling the craft's attitude and orbit is paramount for the success of the mission. GOCE is one of the first satellites in the world to be inscribed in a free-falling orbit, meaning that it simply falls from the sky, while at the same time preserving its height. The ion thrusters are only fired with strengths measured in the millinewtons, between one and 20 millinewtons. This is comparable with the force a few drops of water exert on the surface of our planet. The engines are used to compensate for the minimal amount of atmospheric drag that the satellite encounters in the very refined environment of the upper atmosphere.
Not even top-class jet engines can stop and fire intermittently with such a small force, but the two redundant thrusters that power up GOCE can, with no sweat. Instead of burning hydrogen or oxygen, the craft relies on the power of xenon, of which it carries a 40-kilogram tank. The chemical is converted to fast-moving ions when electrons are stripped from the atoms, and then the nuclei are ejected through the rear of the spacecraft. This makes for a very smooth push, and one that can be finely controlled.