The US space agency announced just as the rocket was about to blast off early Thursday that the launch had been put back to Friday. It came as a new blow soon after the return of the Discovery space shuttle mission which was beset with difficulties and delays.

The Mars Reconnaissance Orbiter (MRO) is being sent to step up the search for signs of water on the Red Planet and help pick sites for possible landings on Mars in the future.

A technical problem with the rocket, which is being used for the first time by NASA, had been blamed for postponing a launch on Wednesday.

On Thursday, space officials blamed something else, a faulty sensor on a fuel pump and related computer software for the 55-meter-tall (180-foot-tall), 335.6-tonne Atlas V.

"We have scrubbed for at least 24 hours," a NASA official said. The next two-hour launch window starts at 7:43 am (1143 GMT) on Friday.

The MRO is to start circling Mars in a low orbit from March, taking close up images of the planet to help scientists understand what happened to the planet's water -- a key sign of life -- and to pick out possible landing sites for future missions.

"Mars Reconnaissance Orbiter is the next step in our ambitious exploration of Mars," said Douglas McCuistion, director of NASA's Mars exploration programme.

"We expect to use this spacecraft's eyes in the sky in coming years as our primary tools to identify and evaluate the best places for future missions to land," he added.

NASA experts say the orbiter will send back more data about Mars than all previous US missions combined.

The MRO will use a spectrometer that can detect minerals linked to the existence of water, a radiometer that analyzes atmospheric dust, water vapor and temperature and an Italian radar that can look under the ground to detect water.

One of the three cameras on the MRO is the largest-diameter telescopic camera ever sent to survey a planet. A second camera will prepare high-resolution images and a third will draw up a Mars weather map.

All the information will be sent back using the biggest antenna ever sent to Mars.

NASA said the craft can transmit about 10 times as much data per minute as any previous vehicle, conveying details of the Martian surface, subsurface and climate.

The MRO will join two American orbiters, the Mars Global Surveyor and Mars Odyssey, and one European orbiter, Mars Express, that are already looking for signs of water and ice.

"Dramatic discoveries ... about recent gullies, near-surface permafrost and ancient surface water have given us a new Mars in the past few years," said NASA's Mars exploration chief scientist Michael Meyer.

"Learning more about what has happened to the water will focus searches for possible Martian life, past or present," he added.

A Phoenix module is to head for Mars in 2007, followed by a Mars Science Laboratory in 2009.

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