Mission Summary of Cassini Spacecraft Guidance and Control Hardware Health and Performance

The Cassini-Huygens mission ended on September 15, 2017, after nearly two decades
in
ight. The well-designed Cassini spacecraft had robust hardware that permitted two
extended missions, lasting nine years longer than the expected prime mission. At the end
of the mission, the Attitude and Articulation Control Subsystem (AACS) was using two
pieces of redundant back-up of hardware, one reaction wheel and the hydrazine thruster
branch, due to hardware anomalies earlier in the mission. The back-up hardware performed
nominally through the rest of mission. The prime reaction wheels at the end of the mission
had reached more than 130% of the consumable limit for number of revolutions. No
thruster on either thruster branch accumulated more than 45% of the consumable limits.
The inertial reference unit slightly exceeded the pre-launch requirements on bias error,
but as the software continuously estimated this value in
ight, the attitude estimation
was not adversely a ected. The star trackers performed nominally, and though there was a
spacecraft anomaly in 1998 related to the star trackers, the origin was not in hardware itself.
The Sun sensors and accelerometer both performed as expected and met all requirements
throughout the mission. Ultimately, the lifetime of the Cassini spacecraft was not limited
by hardware performance. Planetary protection requirements necessitated the end of the
mission as the spacecraft's propellant reserves depleted, and Cassini plunged into Saturn's
atmosphere with a healthy attitude control system.