If all goes well, Opportunity should touch down at 0505 GMT Sunday on the Meridiani Planum, described by the National Aeronautics and Space Administration as one of the "smoothest, flattest places on Mars."

Spirit, which functioned flawlessly after its January 3 landing in Gusev Crater on the other side of the planet, has been plagued with communications problems since Wednesday.

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

But NASA is being extremely cautious.

Analysis of Spirit's descent through Mars' atmosphere prior to the Gusev landing led to a decision to open Opportunity's parachute at a higher altitude than initially planned, said Wayne Lee, the Jet Propulsion Laboratory's chief engineer for development of the rover's descent and landing systems.

The two golf-cart-sized solar-powered rovers were to study the dusty Martian surface's geological characteristics for three months to determine whether the red planet ever had conditions conducive to life.

The European Space Agency (ESA) meanwhile said final efforts to coax a call from Europe's lost Martian lander Beagle-2 would also be made this weekend, but the chances of success were deemed negligible.

The British-built mini-lab was to have landed on Mars December 25, but has not been heard from.

Theories that the Red Planet was once awash with water got dramatic confirmation from data relayed to Earth by Europe's unmanned spacecraft, Mars Express.

Initial results from Mars Express sketched an image of a planet whose surface was once sculpted by seas and glaciers and confirmed indications that its South Pole is capped by frozen water, ESA said.

Until its communications problems began Wednesday, Spirit had been beaming back to Earth spectacular color photographs of the Martian landscape.

NASA officials worried that the problems could take weeks to sort out, and may never 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 here. "We have got a long way to go with the patient in intensive care.

"We made good progress overnight and Rover has been upgraded from critical to serious," he said Saturday. "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 was guarded about the landing of Opportunity.

"You should not expect the events of tonight to go necessarily as well as they did last time," he said, referring to Spirit's picture-perfect landing January 3.

On Wednesday, Spirit suffered a "very serious anomaly" and stopped normal transmissions, NASA said, 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.

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

The Meridiani Planum, where Opportunity was to touch down, is a zone of grey hematite, an iron oxide. Scientists plan to use the robot's research instruments to determine whether the grey hematite layer comes from sediments of a former ocean, from volcanic deposits altered by hot water or from other ancient environmental conditions.

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 of past life-sustaining water on Mars.

The 820-million-dollar Spirit and Opportunity project is the most ambitious ever sent to Mars.

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