From Halopedia, the Halo wiki
In the mid-26th century, the Battle of Gamma Pavonis VII occurred around the planet. Under the command of a young Naval officer Miranda Keyes, the UNSC Hilbert engaged a Covenant vessel. Having no weapons capable of breaching a Covenant energy shield, Miranda ordered that the Hilbert ram into the enemy vessel. The resulting collision damaged the shield, allowing the Hilbert to continue onwards and crash into the vessel, proper. As the crew abandoned ship, which was now falling into Gamma Pavonis VII's gravity well, the Hilbert's fusion core went critical. Only two of the crew members survived — Miranda included.
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