But that changed after engineers grew concerned that a rough landing could damage the rover and the drill mechanism. They decided to open the box and mount one bit in the drill as a hedge to ensure success of one of the most promising scientific tools aboard Curiosity. The drill is to bore into rocks looking for clues that life could have existed on the planet. Even if a damaged mechanism couldn't load a drill bit, at least the rover would have one ready to go.

Under agency procedures, the box should not have been opened without knowledge of a Nasa scientist responsible for guarding Mars against contamination from earth. But planetary protection officer Dr Catharine Conley was not consulted. "They shouldn't have done it without telling me," she said. "It is not responsible for us not to follow our own rules."

Those rules required sterilisation of any part of Curiosity that will touch the surface of the planet, including the drill bits and all six of the rover's wheels. The precaution was taken to preserve the ability to explore water or ice - even if the chances of finding it were remote.

Conley, a microbiologist, said she learned about the unsealing of the box just before launch. By then, it was too late to fix.

Other Nasa officials said the decision to open the box of drill bits was a calculated risk. "Water or ice near the surface in Gale crater was not a significant probability," said David Lavery, programme executive for solar system exploration at Nasa headquarters. "We weighed that against the risks of not having a bit mounted in the drill prior to launch, and the spectre of not being able to drill any holes at all on Mars."

The bits were unsealed in a near-sterile environment, he said. Even so, the breach was enough to alter aspects of the mission and open a rift at Nasa between engineers and planetary protection officials.

Curiosity was first proposed in 2004 under a mission category that would have allowed it to explore a region with ice and water. That category called for sterilising portions of the spacecraft that would contact the surface of Mars to avoid contamination of moist areas where microbes - from earth or from Mars - have the best chances of survival.

After learning that the drill bit box had been opened, Conley had the mission reclassified to one in which Curiosity could touch the surface of Mars "as long as there is no ice or water".

Conley's predecessor at Nasa, John Rummel, a professor of biology at East Carolina University, said, partly in jest: "It will be a sad day for Nasa if they do detect ice or water. That's because the Curiosity project will most likely be told, 'Gee, that's nice. Now turn around'."

If water is found, Curiosity could still conduct tests from a distance with gear including a laser and spectrometers.

About 250,000 bacterial spores throughout Curiosity are assumed to have survived the landing, officials said. Nearly all of them are believed to have perished within minutes of exposure to the harsh Martian conditions in Gale crater - freezing temperatures, ultraviolet radiation and an atmosphere of carbon dioxide.

But some earth life forms can live in space and in at least some of the conditions found on Mars. The European Space Agency discovered that lichens launched on a Russian Soyuz rocket in 2005 survived several days of full exposure to the vacuum of space and ultraviolet and cosmic radiation.

Just this year, Dr Andrew Schuerger, a plant pathologist and expert on the survival of terrestrial micro-organisms under Martian conditions, found a bacterium species capable of growing in conditions present on the surface of Mars.

Contaminating another planet is an ethical concern for scientists, as well as a practical one. "So wouldn't it be tragic if some future expedition were to discover life on Mars only to discover later that it had actually discovered life from earth?" said Bruce Betts, a spokesman for the Planetary Society in Pasadena