Feasibility Study of Two Cassini Reaction Wheel/Thruster Hybrid Controllers

As the first spacecraft to achieve orbit at Saturn in 2004, Cassini has collected science data throughout its four-year prime mission (2004–2008) and has since been approved for a first and second extended mission through September 2017. Cassini carries a set of four reaction wheels for attitude control when stable pointing of a science instrument is required. In 2002–2003, a prime reaction wheel exhibited signs of bearing-cage instability and was replaced by the backup wheel. To date, all Cassini wheels have accumulated >3.5 billion revolutions each. As such, in spite of careful management of the wheel rates by the operation team, there are observed symptoms of anomalous wheel-bearing drag torque. To prepare for the contingency scenario in which two reaction wheels had degraded, the feasibility of controlling Cassini using the two remaining wheels and four thrusters to meet the spacecraft pointing requirements is evaluated. Our results indicate that the hybrid controllers can achieve better attitude-control performance and lower hydrazine consumption rate relative to those of an all-thruster control system.