NASA and a team of external investigators will look at a number of potential causes behind Space Shuttle Columbia's explosion just 16 minutes before it was scheduled to land Saturday morning.

The age of Columbia — which has been flying since 1981 — and the wear and tear on it will also be a subject of examination, though NASA believes that was not a factor.

The very remote possibility terrorism was involved will also be examined — but that, too, has all but been discounted because of the exceptionally high speed and altitude of the shuttle when it exploded.