Nasa scientists say it will be the weekend at the earliest before they know if they have made contact with the Mars Polar Lander (MPL).

Hopes of finding the spacecraft were raised this week after a review of data collected by a radio antenna at Stanford University showed a blip in the information record that just might have been MPL trying to contact Earth.

On Tuesday, a set of commands was sent to the Red Planet ordering the lander to send a signal to the Stanford Earth receiving station on Wednesday. But processing of data collected by the university's 43-metre-diameter dish is expected to take a number of days.

"The signal that the Stanford team detected is definitely artificial, but there are any one of a number of places it could have originated on or near Earth," said Richard Cook, MPL project manager at the Jet Propulsion Laboratory (JPL) in Pasadena, California.

"Still, we need to conduct this test to rule out the possibility that the signal could be coming from Polar Lander."

Contact was lost with MPL as it turned on its final approach to enter the Martian atmosphere. That was on 3 December 1999.

Radio antenna

Since then, project scientists have used every trick in the book to find out what happened to the $165m spacecraft.

They even employed the orbiting Mars Global Surveyor to scan the planet's surface for the entry parachute, the largest object that might have been visible from space.

But the hunt threw up nothing and all hope of ever finding MPL was abandoned on 17 January. Only when a later review of the Stanford data showed up a mysterious blip on 18 December or 4 January did JPL scientists return to the search.

Investigators are attempting to determine what might have happened to the probe. Possible explanations for the failure of the mission are:

• The three-legged lander burned up on entry
• It crashed on to the Martian surface and was destroyed
• It landed but was too damaged to make contact with Earth
• It landed on the rugged surface and fell over, perhaps down a deep gorge.

If the mysterious signal does turn out to be from MPL, it could indicate one of two problems: either the radio aboard MPL is broken, or the relay used by the Mars Global Surveyor orbiting the planet is not working.

Broken transmitter

"It is unlikely that a broken transmitter on the lander could be fixed, and unclear whether a problem with the relay could be resolved," said Richard Cook.

"Even if the signal were coming from the lander, there is little hope that any science could be returned. However, it would give the team a few more clues in trying to eliminate possible failure modes," he added.

An internal JPL board and a team of independent investigators are looking into the failure, as well as the loss of the Mars Climate Orbiter (MCO). MCO apparently burned up in the atmosphere last September because of a mix-up between English and metric units.

The investigators will also take a hard look at Nasa's entire Mars programme. At least some answers are expected by mid-March, about a year before the next Mars orbiter and lander are set to launch.