Battle of Chi Ceti
From Halopedia, the Halo wiki
En route to the Damascus Materials Testing Facility, the Commonwealth was attacked by a Covenant craft. The frigate successfully fended off the Covenant ship, blasting a hole in it with a MAC and then hitting it with a Shiva warhead. The ship retreated to the edge of the system.
The Spartans on board then landed on Chi Ceti 4, where they donned their MJOLNIR armor for the first time.
