Attitude and Orbit Control of the Grace Satellites at extremely low power

The two GRACE satellites were launched on March 17, 2002 by a Russian Rockot. GRACE not only was the first dual-satellite mission operated by German Space Operations Center (GSOC), but it also was the first formation flying at an altitude below 500 km. The mission was successful from a scientific point of view and the originally envisaged mission duration of five years finally became almost sixteen. A follow-on mission with the same partners started 2018 and JPL projects a new generation of satellites for gravity measurements in the twenties, so there was a strong incentive to prolong the GRACE mission as long as possible in order to minimize the gap. Infirmity comes with age and several components deteriorated towards the end or even were defunct. The scientific goals were still obtained to a very high level until June 2017, four months before decommissioning. The major challenge for operations was posed by the degradation of the NiH2 batteries. These are comprised of twenty cells packaged in the common pressure vessel (CPV) configuration. Three cells had shorted out on Grace 1 and seven on Grace 2 by the middle of 2017. The charge capacity of the operational cells was severely degraded. The longevity and the occurrence of several anomalies due to power problems depleted fuel ever more rapidly