• The foam from one of the external orbiter tanks that flew off during shuttle launch and struck the left wing and the outer protective tiles.

• The left wing, landing gear, hydraulic system and tire pressure — whose sensors and tracking systems were all lost in the minutes before Columbia broke apart.

• The outer tiles that insulate the vehicle from the extreme heat it’s exposed to upon launch and re-entry into the Earth’s atmosphere. There are concerns the tiles might have broken off or been more severely damaged than was initially thought as a result of the foam impact on liftoff.

• The angle of the rockets and shuttle upon re-entry from orbit. Though the computer controls most of the shuttle operations, if something went wrong and put Columbia slightly off track — even by just a few degrees — that could have severely rattled the shuttle and caused it to disintegrate.

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.

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