Circumference

The Circumference was a ONI stealth ship that was part of Operation: HYPODERMIC in 2552. It was likely supposed to infiltrate Covenant space and find their homeworld. Intrusions into the information net led Cortana to discover the Circumference, Applebee, and Lark were not civilian craft, as they were made to seem with their innocuous names. Circumference was docked in Reach Station Gamma, Bay 9, above planet Reach. An ONI operative, Codename: COALMINER, detected the discovery. "Coalminer" deleted the Circumference from the Reach databsae, preventing further intrusion. This intrusion prevented Doppler from deleting the Circumference's NAV database when the Covenant attacked the Planet.

Later that day, Covenant intrusion software discovered the isolation during The Battle of Reach. They sent three frigates full of Rangers to recover the NAV data, but the UNSC responded by ordering SPARTAN-117 and his Spartans to reach and destroy the data first. He suceeded, and the Circumference was destroyed by concentrated fire from a Pelican the Spartan comandeered.