In the Halo Reach Trailer one of the Spartans recognises an MIA Spartan, whom we'll call the Lone Wolf and says, "Didn't think anyone survived Pegasi, Sir."
What is this Pegasi reference?
51 Pegasi is a system approximately 50.1 lightyears away from Earth, in the constellation Pegasus. The stars planetary system was controlled by the Covenant during 2545, and was home to 51 Pegasi B and Pegasi Delta, used by the Covenant in mining deuterium and tritium in the planetoid's rich oceans.
That's the boring crap, what about the Lone Wolf Spartan? What happened on Pegasi?
Oh yeah, him! The Lone Wolf might have been involved in a mission known as Operation Torpedo:
Most of the following information is gleaned from the Halo novel, The Ghosts of Onyx which was written by Eric Nylund.
The aim of Operation: Torpedo was to destroy a Covenant deuterium/tritium refinery. These chemicals were used in the Covenant's plasma reactors, therefore making this refinery an excellent refueling plant for Covenant ships, right on the edge of UNSC space. The destruction of this supply point would triple the length of Covenant resupply time frames and buy the UNSC some valuable time to recover from the ravages of the war it had found itself in.
A tactic of using long range nuclear bombardments was met with complete failure as the Covenant ships shot the weapons out of the sky. The operation was then assigned to the newly commissioned Spartan-III Beta Company, attached to the UNSC All Under Heaven.
The outcome was a Beta Company victory, but at great cost. All Spartan IIIs, with the exception Tom-B292 and Lucy-B091 were killed. However, all Covenant forces groundside as well as several Covenant ships were also eliminated.
That last line is telling - the Lone Wolf might thus be one of those Spartans - or it could be a totally new character unheard of in the Halo story before.....it could be you.....
Most the info for this page was borrowed from Halo Wikia