The Cassandra served as a research station where Dr. Hillary Massar works as a scientist and develops a weapon which has the power to kill Helghanhalf breeds. But the weapon had been taken by the Helghast forces. The Cassandra's true purpose was publicly covered up by the Vektan government in which the ship was stated to be a unmanned satellite station served to monitor one of Alpha Centauri's suns.
The Cassandra was attacked and hijacked by Helghast forces and her crew were killed. VSA Director Sinclair sent Lucas Kellan to destroy the ship by programming it to fly directly into the sun.