Engineers are hoping that Spirit can still receive commands sent from Earth which could enable them to correct whatever it is that has caused the malfunction in transmissions, a Nasa spokesman said.
"It could just have gone into fault mode. It's not what we would have wanted to happen. Obviously we're concerned but it's too early to say what's happened," he said.
Spirit's failure to transmit properly had temporarily ended its mobile exploration of the red planet and the search for evidence of water.
Nasa's worst fear is that Spirit could end up like two thirds of the missions to Mars over the past 40 years that have ended in failure - including Britain's Beagle 2 spaceprobe. This has given the planet a reputation for being the "Bermuda triangle" of the Solar System.
Initially Nasa thought that the communications failure might have been due to a thunderstorm near Australia's Tidbinbilla tracking station in Canberra which was being used to receive and transmit signals on Wednesday night. The storm appeared to have weakened the signals which take nearly 10 minutes to travel the 56 million kilometres (35 million miles) to Mars but then Nasa realised that something else must be causing the interference.
Spirit is one half of a US$820m mission to Mars. Its twin rover, called Opportunity, is scheduled to land on Saturday.
Nasa last heard from Spirit as it prepared to continue its work examining its first rock, just a few metres from its lander. Since then, Spirit has transmitted just a few beeps to Earth in response to attempts to communicate with it.
It has also skipped several scheduled communications opportunities, either directly with Earth or by way of two Nasa satellites in orbit around Mars.
When Spirit was hailed a great success last week, President George Bush announced plans to take Americans back to the moon.
Speaking at Nasa headquarters in Washington, the President said the move represented a "new course for the American space programme".
"We will build new ships to carry men forward into the universe to gain a new foothold on the moon," he said, adding that the ambitious task would be carried forward "one mission, one voyage, one landing at a time."
At that time Spirit's success contrasted sharply with Beagle 2, which landed on the red planet on Christmas Day.
Beagle landed to much fanfare but controllers in Britain were unable to communicate with the craft. Scientists postponed a search due to have started for Beagle 2 yesterday and instead said they would try to communicate with the craft this weekend.
Controllers stopped trying to raise the British Mars lander on January 12. By maintaining radio silence, it was hoped Beagle 2 would be forced into an emergency communication mode that ensured its transmitter was kept on for most of the Martian day. This would give its mother ship, the Mars Express orbiter, the best chance of making contact.
The original radio silence period ended yesterday, just before a fly-past by Mars Express. However, mission scientists decided not to hail the lander immediately.
A statement from the Beagle 2 team said: "We are erring on the side of caution as we cannot confidently predict the precise ending of the 10-day slot. This is because the absolute accuracy of the timer on Beagle 2 could have been affected by the temperature on Mars, making the clock run slightly faster or slower than predicted."
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