A pair of German-American Earth observation satellites are due to be retired this November and put on trajectories that will see them burn up in Earth’s atmosphere in 2018.
The two Gravity Recovery and Climate Experiment (Grace) satellites were launched on 17 March 2002 from Plesetsk Cosmodrome, in north-west Russia. They fly in formation, about 140 miles (220km) apart, and measure the gravitational field of the Earth.
The pull of gravity changes as water moves around the planet in seasonal patterns, and as ice deposits change thickness. With its monthly gravity map, Grace has revealed rapid ice loss from Greenland and Antarctica and mapped seasonal changes in water levels in the Yangtze river in China.
On 4 September, one of Grace’s battery cells stopped working and its satellite ceased communication with ground controllers. The link was restored four days later, and the battery was found to be back to a workable voltage, but it was a clear sign that the twin spacecraft are ageing.
Grace is now on its final data collecting run and in November both spacecraft will be sent onto different orbits and rendered inert. The separate orbits will ensure that they do not collide and create space debris.
They will re-enter Earth’s atmosphere and burn up early in 2018, with only small fragments expected to make it to the ground. Nasa says these pose a very small risk to people, and are well within their safety requirements for re-entering spacecraft.
The mission’s end comes just a few months before its replacement, Grace Follow On, is due to launch on a SpaceX Falcon 9 rocket from Vandenberg air force base, California.
