Fuel may have leaked from before lift-off until orbit was reached

A bright streak from one of the bell-shaped engine nozzles was spotted by Nasa staff on unreleased video footage of the launch, suggesting a fuel leak occurred.

This lost fuel would also explain the reduced performance of the engine and its higher temperature, as the fuel is used to cool the nozzles before it is burnt.

If the problem had been any worse, the engine could have suddenly shut down. An emergency landing would then have been attempted, either in Florida or West Africa.

Suspected damage

Nasa's mission operations representative, Wayne Hale, stressed there is no proof that one or two of the 1,000 fine cooling tubes in the nozzle of the right engine were cracked and leaking hydrogen.

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Until the shuttle returns to Earth on Tuesday night and the engine is examined, no-one will know for sure, he said.

The shuttle's commander, Eileen Collins, said: "There's always risk in space flight - our job is to minimise the risk."

It has also been revealed that a short-circuit cut out the three main engines just five seconds into the flight. It took just half a second for the back-up controllers to take over. A "popped" circuit breaker was later found to be the cause.

Mr Hale noted that because the three main engines are not used after propelling the shuttle into orbit, neither problem will affect the rest of Columbia's flight.

Observatory boost

The shuttle's primary objective, the launch of the Chandra X-ray observatory, was successfully achieved and it is now being manoeuvred into its operating orbit.

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Early on Monday, its engines were fired for over 11 minutes boosting the observatory's orbital low-point by 2,250 kilometres (1,402 miles). This was the second of five engine firings designed to put Chandra in its correct elliptical orbit.

With Chandra deployed, the five astronauts have turned their attention to other scientific tasks.

The first job was to set up an exercise treadmill and the Treadmill Vibration Information System (TVIS), which will measure vibrations and changes in microgravity levels caused by on-orbit workouts.

This is the last treadmill test before one is deployed on the International Space Station. Treadmill exercise is vital for astronauts to keep healthy, but the pounding of feet can send vibrations through a spacecraft. This can disrupt experiments and ways to isolate the vibrations are being sought.

First woman

Astronomer Steven Hawley made more observations of Jupiter, Venus and the Moon with the Southwest Ultraviolet Imaging System, after Commander Collins put the shuttle in the proper orientation for his observations.

Astronaut Catherine Coleman examines a seedling

Experiments with genetically-modified (GM) plants will also be checked. Their aim is to understand the sensitive mechanisms plants use to monitor their environment in order to develop plants that respond better to the stresses of space flight.

Back on Earth, the first American woman in space, Sally Ride, has warned Commander Collins that the hardest part of being the first woman to command a shuttle mission is yet to come.

"The hardest part of her flight is actually going to be [when she returns], with all the attention that she gets, all the demands on her time," said Ride. "She's going to be whisked from one public appearance to the next public appearance, and it's very trying, very stressful."

Collins, a 42-year-old Air Force colonel, acknowledges she is not ready for the hoopla that awaits her following Tuesday night's scheduled landing.