Life with a weak Heart; Prolonging the Grace Mission despite degraded Batteries

Abstract

The two Grace satellites were successfully launched on March 17, 2002 by a Russian Rockot launcher. GRACE not only was the first dual-satellite mission operated by GSOC, but it also was the first formation-flying occurring at an altitude below 500 km. The mission is extremely successful from a scientific point of view and the originally envisaged mission duration of 5 years has more than doubled by now. A follow-on mission is planned by the same partners for 2016 and JPL projects a new generation in the twenties, so there is a strong incentive to prolong GRACE and try to bridge the gap. Infirmity comes with age and several components have deteriorated or are defunct. Nevertheless, the scientific goals can still be obtained to nigh on 100%. The major challenge for operations is posed by the degradation of the NiH2 batteries. These are comprised of 20 cells packaged in the common pressure vessel (CPV) configuration. However, two cells have shorted out on Grace 1 and one on Grace 2. The available capacity of the operational cells is also severely degraded. The current operational capacity of the batteries is limited to ≤ 3 Ah as compared to the original nameplate capacity of 16 Ah. This paper describes the special operations needed to prolong the mission despite the considerable power constraints. The first Section gives a general overview with emphasis on the components relevant to this paper. The battery, its current state and the mission specific circumstances which require special handling are described in Section 2. The several threats to and failure mechanisms of NiH2 batteries are also presented here. The third Section then contains a detailed description of all measures taken to pamper the batteries. This includes heater and parameter settings, special on-board macros, orbit-to-orbit charge regulation, but also physical actions such as turning the satellites away from the Sun to force battery discharging and subsequent charging. The fourth Section, finally, presents conclusions, recommendations and an estimation of how long the Grace mission can be prolonged.