Amid pessimism over the outlook for Spirit, the spacecraft's twin, Opportunity, was in a perfect trajectory meanwhile for a landing on the surface of the Red Planet this weekend, the National Aeronautics and Space Administration officials said.

Opportunity is to land at 9:05 pm Saturday (0505 GMT Sunday) on the Meridiani Planum, described by NASA as one of the "smoothest, flattest places on Mars."

Opportunity scientist Joy Crisp said that Meridiani "meets our criteria for a safe landing and is an excellent place for science."

Spirit landed on Mars on January 3 and had been functioning near-perfectly until Wednesday, beaming back to Earth spectacular color photographs of the surface of the Red Planet. But communications were abruptly cut off two days ago and only partially restored on Friday.

NASA officials expressed concern the problems could take weeks to sort out and may not ever be entirely resolved.

"The chances it will be perfect again are not good," Mars Exploration Rover project manager Pete Theisinger told reporters at NASA's Jet Propulsion Laboratory in Pasadena, California. "We have got a long way to go with the patient in intensive care.

"It is still in a critical state, but stable," he said. "We don't know what's broken and the consequences. The flight software is not working properly.

"We should expect that we will not be restoring functionality to Spirit for a significant amount of time, many days, perhaps a couple of weeks, even under the very best of circumstances," Theisinger said.

Spirit suffered a "very serious anomaly" and stopped normal transmissions on Wednesday, but a signal was received on Friday from the solar-powered rover by one of the giant antennas of the international Deep Space Network near Madrid, Spain.

"The flight team for NASA's Spirit received data from the rover in a communication session that began at 1326 Universal Time and lasted 20 minutes at a data rate of 120 bits per second," NASA said.

The transmission speed of 120 bits a second was well below the normal speed of 11,000 bits a second but even the weak signal was welcomed after two days of worrying silence punctuated by an occasional meaningless "beep."

NASA engineers plan to ask Spirit to provide further information about its condition in an effort to work out why the rover fell silent on its 19th day on the Red Planet.

The breakdown came just as the rover was to begin searching for signs that there may have been water on Mars that could have sustained life.

The 820-million-dollar Spirit and Opportunity project is the most ambitious ever to Mars and follows a number of failed voyages to the Red Planet, including a European mission whose demise was acknowledged on Friday.

The European Space Agency (ESA) said final efforts to coax a call from Europe's lost Martian lander Beagle 2 would take place this weekend, but the chances of success are negligible. The British-built mini-lab was due to have landed on December 25 but has failed to radio home.

Theories that the Red Planet was once awash with water received dramatic backing meanwhile from data relayed to Earth from Europe's unmanned spacecraft Mars Express.

First results from Mars Express sketched the vision of a planet whose surface was once sculpted by seas and glaciers and confirms indications its South Pole is capped by frozen water, ESA said.

SPACE.WIRE