Few people at Nasa's Jet Propulsion Laboratory, the home of US planetary exploration, believe that their continuing efforts to find the lost Mars Climate Orbiter probe will be successful.

The powerful radio-telescopes that comprise the Deep Space Network tracking system are making a final "full sweep" of the region around Mars in the desperate hope that the spacecraft is still there.

Gloom: Mars Climate Orbiter mission control

But soon the search will end and only one question will remain. What happened to cause the spacecraft to burn up in the Martian atmosphere?

"Everybody's scratching their heads over this one... because it's so hard to explain," said Ron Greeley of Arizona State University.

Problems with the spacecraft have been ruled out. "We are looking at human error and software," said mission controller Richard Cook.

Too close

It is believed that in its pass behind Mars the spacecraft came too low into the atmosphere.

Solar panels: ripped off

The original plan was to pass 200km (125 miles) above the Martian surface. On Wednesday, mission controllers believed that the pass-distance would be 150km (93 miles), which was acceptable.

But in reality the Mars Climate Observer passed just 60km (37 miles) above Mars. At this low altitude, friction with the atmosphere would almost certainly have destroyed the spacecraft.

In all probability, its solar panels would have been torn off. The spacecraft would have become a "shooting star" in the Martian sky and fragments of it would have been scattered over a wide region.

Suspicion is focusing on the 15 September engine firing that was its final course-correction prior to reaching Mars.

One scientist involved in the mission suspects human error. "It is almost as if a decimal point slipped in the calculations," he said.

At present an unnoticed navigation error seems the most likely cause of the loss. Tracking data from the spacecraft suggests there were indications that it was on the wrong course in the hours before it was lost.

Mr Cook adds: "It is a very significant change. That is why we are so shocked."

Cutbacks

Political reaction to the failure has bemused many American scientists.

They say they cannot understand supportive statements from Washington about the loss - just as the US Government plans dramatic cuts in Nasa's space science budget.

Although the loss of the Mars Climate Orbiter is a disaster for the particular scientists involved, especially since it seems to have been due to human error, it is "science delayed and not science lost," according to Nasa's Dr Carl Pilcher.

These days Nasa operates a so-called "faster, better, cheaper" policy towards its spacecraft. Missions are smaller and more numerous.

The theory is that just as much science can be carried out without the risk of putting all hopes into one large spaceprobe.

The next mission to Mars, the Mars Polar Lander, will touch down in December. Following that, Nasa plans to launch Mars missions every two years from 2001 to 2009.