Cassini Attitude Control Operations - Guidelines Levied on Science to Extend Reaction Wheel Life

Abstract

The Cassini spacecraft was launched on October 15, 1997 and arrived at Saturn on June 30, 2004. It has performed detailed observations and remote sensing of Saturn, its rings, and its satellites since that time. Cassini deployed the European-built Huygens probe, which descended through the Titan atmosphere (Saturn's largest moon) and landed on its surface on January 14, 2005. The Cassini mission has recently been approved by NASA to continue through September of 2017. This 7-year extension is called the Solstice mission and it presents challenges to the spacecraft operations team and its ability to maintain the health of the spacecraft. To keep the spacecraft healthy for 7 more years, the spacecraft team must carefully manage hydrazine use (about 48% of the 132 kg launch load remains as of January 2011). A vital part of conserving hydrazine is to use the reaction wheel assembly (RWA) control system for precise pointing and slews wherever possible. In any given week, the Cassini spacecraft is commanded to use RWA control about 99% of the time, with about 1% of the time requiring reaction control system (RCS) thruster control (to perform Delta V course corrections or to bias the RWA momentum). Such extensive use of the RWA hardware throughout the mission requires that the RWAs be operated in a way that minimizes degradation in the RWA electronics, DC motor, and spin bearing for each reaction wheel. Three consumables in particular have been identified for the RWAs: (1) Total number of revolutions for each RWA. (2) Time spent at very low wheel speeds. At these low speeds, good elasto-hydrodynamic (EHD) film lubrication may be compromised. (3) Total number of on/off power cycles. The second of these consumables, minimizing the time spent at very low wheel speeds, is especially important to keep the spin bearing healthy and well-lubricated. These consumables are actively managed by the attitude control operations team throughout the mission. One vital management technique is to predict individual RWA momentum (given the pointing and slews that are needed to collect the best science) and to bias the RWA momentum in a way that reduces both the total number of revolutions as well as the time spent below EHD wheel speed. Another strategy to protect RWA health is to alter the planned pointing of the spacecraft (which can affect science collection) so that the RWA consumables are conserved. This paper focuses on why this second technique is needed, and discusses how guidelines have been developed by the attitude control team which affects the planned science pointing, so that science data can be most optimally collected while still minimizing RWA consumable usage.