Circumference

Summary
The Circumference was a ONI stealth ship that was supposed to infiltrate Covenant space and find their homeworld, when it was destroyed at Reach. Because the crew was not on the ship, the AI and navigational information had to be taken offline by a Spartan task force, otherwise, the Covenant would have found the location of all of humanity's colonies and Earth.

Intrusions into the informantion net led Cortana to discover the Circumference in Reach Dock Bay Gamma-9.