The second and last available computer needed to operate the shuttle Columbia's $150 million telescope payload shut down early Thursday in a major crisis for the hard-luck astronomy mission.

At the same time, flight controllers in Houston were monitoring an unexpected buildup of carbon monoxide in Columbia's crew cabin. It was not immediately clear what caused the levels to rise or if the two problems were related, but NASA officials said the concentration was well below the danger level."The surgeons . . . they're not concerned at this point," said a NASA spokeswoman.

The crippling computer shutdown came about 7:15 a.m. after the crew noticed a burning smell similar to the odor that preceded the failure of a similar computer shortly after launch Sunday.

The second shutdown stopped the crew's astronomical observations in their tracks, and officials said if one of the two computers cannot be reactivated, the astronauts would be unable to operate their high-tech space observatory in what could be a major setback for NASA and for hundreds of scientists who have devoted eight years or more to the "Astro" astronomy mission.

Engineers and scientists on the ground were looking into two plans for salvaging the mission: reactivating the first computer, if possible, or rigging Columbia's payload so it could be operated by remote control from the ground.

"We're working on a plan for commanding the (Instrument Pointing System) and the instuments from the ground and having you work together with them to continue to get science data," astronaut James Voss radioed Columbia from mission control in Houston. "We'll let you know when we get that plan put together."

The goal of Columbia's mission is to study a smorgasbord of violent stars, explosive galaxies, mysterious quasars and other high-energy targets with a $150 million battery of telescopes sensitive to energetic X-rays and ultraviolet light that cannot penetrate Earth's atmosphere.

But the astronauts have had problems getting their sophisticated space observatory up and running, losing valuable data along the way.

Columbia blasted off Sunday from the Kennedy Space Center, and since then, the crew has been working around the clock in two shifts to operate three ultraviolet telescopes. An X-ray telescope is being operated by remote control from the ground.

Four astronomers are on board - Robert Parker, 53, Jeffrey Hoffman, 46, and civilian researchers Ronald Parise, 39, and Samuel Durrance, 47 - and three astronauts responsible for shuttle operations: commander Vance Brand, 59, co-pilot Guy Gardner, 42, and flight engineer John "Mike" Lounge, 44.