In the years after the 1986 loss of the space shuttle Challenger, NASA managers repeatedly considered and then rejected installing some emergency measures aboard the shuttles that would have given crews at least a chance of surviving a catastrophic malfunction.

In the most recent examination, a year-long, $5 million study in 2001, the space agency's engineers explored installing military-style ejection seats, or systems that fire mini-rockets to propel the crew members out of a doomed spacecraft. They also considered building a detachable cockpit that could fly away as an escape pod and float down with parachutes.

But the agency rejected these proposals because "of questions about their weight, their potential effectiveness and their reliability," said NASA spokesman John Ira Petty at the Johnson Space Center in Houston yesterday.

NASA does have detailed plans for aborting shuttle missions, from countdown to blastoff and even after the ships reach low orbit. But once a craft is in space, rescuing a crew becomes exceedingly difficult.

The proposed fixes were also seen as prohibitively expensive additions to an already aging and financially strapped shuttle fleet. There were estimates, for example, that flyaway escape pods could cost as much as $1 billion to install on the orbiters. NASA engineers deemed that adding an escape capsule to each orbiter would be a major restructuring that would offer only a slim guarantee that it could save lives.

After the Challenger explosion, NASA engineers did install the "emergency egress slide," a system that allows shuttle astronauts to escape a gliding and relatively slow-moving returning orbiter by depressurizing the cabin, blowing a hatch and then sliding down a pole (to keep them from hitting the left wing) before parachuting away.

That escape route, however, would not have helped the Columbia astronauts, as the shuttle was in its early stages of reentry and was streaking at 12,500 mph when it disintegrated Saturday morning. Temperatures along the wings exceeded 3,000 degrees. Anyone exiting the craft would have died instantly.

In the end, NASA officials said, without an escape pod there was very little they could have done to save the Columbia astronauts after the first few minutes of liftoff. It was during that interval that a piece of foam insulation broke away from the external fuel tank and hit the left wing, possibly damaging the spacecraft.

There was, in essence, no way out for the Columbia crew, said NASA officials, except to attempt a return to Earth. If damage had been found, Ron Dittemore, the space shuttle program manager, said Sunday, "there was zero we could do about it."

In the past, NASA engineers considered ways to repair a shuttle's tiles in orbit, Dittemore said, but "we eventually concluded that we risked creating more damage than what we were trying to repair."

Consider the obstacles to a rescue once a shuttle has gone into orbit.

Columbia could not have traveled to the international space station because the orbiting platform was too far away. The shuttle was circling about 170 miles above the Earth, while the station was 240 miles high and in a different orbit. Columbia did not have enough propellant to bring it to the station, say NASA officials, only enough to push it into reentry.

"It would have been impossible with the fuel aboard," NASA spokesman Petty said.

Moreover, even if Columbia had enough fuel to reach the station, it did not have the docking ports that would have allowed the crew to attach it to the platform.

The other three shuttles do have such capabilities, and NASA Deputy Associate Administrator Michael Kostelnik said yesterday that, "in hindsight, that is a good thought" -- to include in future flight plans enough fuel and the right orbit to get a stricken shuttle to the station.

If ground control had concluded that Columbia was endangered by lost or damaged tiles after it had gone into orbit, it would have been extremely difficult to attempt a repair in space.

Columbia carried two suits and special backpacks that would have allowed astronauts to exit the orbiting craft and perform a spacewalk, but only within the shuttle's open cargo bay. There was no robotic arm aboard Columbia, and there were no handholds on the wings.

Leaving the cargo bay and moving along the wings would have exposed an astronaut to tremendous risk -- the stuff of movies. Even if an astronaut could reach a damaged site on an orbiter's ceramic and aluminum skin, each tile on the shuttle is unique, like a piece of a jigsaw puzzle. And the shuttle does not carry spare tiles, nor does its crew have the ability to make them in space.

Subra Suresh, head of the material science and engineering department at the Massachusetts Institute of Technology, imagined an astronaut trying to work on the bottom of a wing, without handholds or robotics, outside of the shuttle's camera range.

"These are just such tricky questions," Suresh said. "If you try to fix a tile in space, if it has been cracked or is peeling away and you pull it off, what happens if another 50 tiles fall away into empty space?"

Could another shuttle, then, have blasted off to rescue the Columbia's crew or assist in a repair?

After being in space for 16 days -- a long shuttle flight -- the Columbia crew had enough oxygen to survive for perhaps five or more days. But no two shuttles have ever been in orbit at the same time. Such a situation would put great strain on ground control operations.

Before the grounding of the shuttle fleet because of Saturday's disaster, the shuttle Atlantis had been scheduled to lift off on March 1. But the orbiter and its rocket boosters are just now being assembled at Kennedy, and the process cannot be rushed, NASA officials said.

"It's not like calling a cab," said Petty, who described a slow, cautious and controlled process to prepare a shuttle for flight. Moreover, NASA managers would have been extremely fearful of rushing Atlantis into space and risking the loss of two vehicles. Getting Atlantis into orbit in a week would "not have been feasible," Petty said.