[noarchy]

Some of them may have been conscious: "At least some of the astronauts were likely alive and briefly conscious after the breakup, as three of the four Personal Egress Air Packs (PEAPs) on the flight deck were found to have been activated. Investigators found their remaining unused air supply roughly consistent with the expected consumption during the 2 minute 45 second post-breakup trajectory."

Source: http://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disast...

[nickff]

It was mentioned in the MIT OCW 16.885/ESD35J lecture series that they were likely conscious briefly while still ascending, when they turned on those PEAPs, (after the shuttle had disintegrated); but they were later unconscious, because of the high altitude and low pressures the non-airtight cockpit was subjected to.

Source: http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-8...