Here's some Halo history for ya.
December 11, 2552
Master Chief and Cortana in the aft section of Forward Unto Dawn are jettisoned on a random trajectory into slipspace. The ship exits slipspace at an unknown location near a nebula star cluster and begins drifting towards Requiem as the MC enters cryo.
December 23, 2552
The front side of the Forward Unto Dawn exits slipspace above the Excession at Voi (the Portal), at Earth and crash lands into the Indian Ocean. The Arbiter is soon recovered by an engineering crew overwatched by a UNSC Rescue Team and a Pelican.
December 31, 2552
Codename: SURGEON filed a report to Codename: USUAL SUSPECTS