Some of you may have seen this image but from Bungie's upcoming art book celebrating ten years of Halo, this image explains how Master Chief fell to Earth between the [infamous] Halo 2 "Finish a Fight" cliffhanger ending and Halo 3's "Luck" intro.
It probably doesn't sway anybody who isn't too fond of Halo's questionable physics. Bear in mind this would be the second time (canon wise) that a Spartan soldier survived falling from orbit. Tough SOBs.