WASHINGTON -- Even if Columbia's astronauts had discarded every nonessential piece of equipment and made other changes in the re-entry plan, the shuttle would still have broken up on descent, according to a NASA report made public Thursday.

Assuming they had known the extent of the shuttle's damage, astronauts would have had to make unplanned spacewalks to cast off the science experiments, water and other equipment, including their school bus-sized Spacehab research module, to cut Columbia's mass by more than 31,000 pounds.

But those drastic measures would have reduced the searing heat that built up around the vulnerable wing leading edges during descent by just 7 percent, not enough for a safe re-entry, concluded LeRoy Cain, the flight director who supervised Columbia's return to Earth and author of the 40-page report.

Though an accident probe is still under way, investigators believe Columbia's fatal Feb. 1 breakup was triggered by a breach in the leading edge of the left wing. Most of their efforts now are focused on determining whether the wing was breached by a 2-pound chunk of foam fuel tank insulation that dislodged 81 seconds after liftoff on Jan. 16 and slammed into the underside of the leading edge of the left wing near the fuselage.

NASA Administrator Sean O'Keefe, speaking following a NASA budget hearing on Capitol Hill Thursday, said the report was prepared in response to a request from the 13-member Columbia Accident Investigation Board.

Cain was unavailable to discuss the report, NASA said. But an agency spokesman said the study was focused primarily on three options for cutting the shuttle's weight because that would have offered the most efficient means of reducing heat built up during the 32-minute plunge through the atmosphere.

Columbia began its descent at a weight of about 234,475 pounds. Only two of 112 previous shuttle flights were heavier on descent.

"The less mass, the less heat that is generated," said John Ira Petty, the NASA spokesman. "Clearly if you reduce the heating by 7 percent by jettisoning all of that gear, there would have been less heat but not nearly enough of a reduction."

Cain and his team examined three options:

· The 31,321-pound reduction to disconnect the Spacehab module and a second structure holding experiments in the cargo bay. External radiator panels as well as internal life support equipment and electronics gear would have also been cast away to obtain the 7 percent heat reduction.

· A 22,924-pound reduction attained by retaining Spacehab and throwing away more of the other equipment. Heat loss on the wing leading edge would have been 6 percent.

· A 20,387-pound weight loss accomplished by retraining Spacehab and other difficult-to-detach external gear, but increasing the internal equipment and supplies jettisoned. The reduction would also have been 6 percent.

The report also examined the viability of adjusting Columbia's orientation in space before the re-entry to point the wing edges away from the sun for a prolonged period to chill them. Such a "cold soak" would have lowered wing temperatures by 65 degrees during re-entry, where they can reach 9,000 degrees.

At most, the extra chill would have delayed the onset of maximum wing heating by about 37 seconds.

O'Keefe said the report would not alter his previous opinion that NASA would have pulled out all stops to rescue Columbia's crew had it known there was a safety problem.