Red-faced NASA officials have revealed the reason their Mars Climate Oribiter smashed into the atmosphere of Mars last week.

NASA's Jet Propulsion Laboratory found that one team used Imperial units while the other used metric in calculating manoeuvres required to place the Mars Climate Orbiter in the proper orbit.

The bad numbers had been used ever since the spacecraft's launch last December, but the effect was so small it went unnoticed. The difference added up over the months as the spacecraft journeyed toward Mars.

It was a $125-million mix-up.

The Orbiter travelled without a glitch until September, when it began its manoeuvres to enter the planet's orbit. Instead of staying on course, it flew too close to the Martian atmosphere and broke up in space.

The craft was supposed to receive data from the Mars Polar Lander and establish Earth's first link to monitor Martian weather and look for signs of water.

The Lander is still expected to arrive in December. Investigators are now trying to determine whether NASA made the same mistakes with that spacecraft. If so, there may still be time to correct them.