The US space agency says it may take days, even weeks, to get its Martian rover Spirit back into action.
The probe had short contacts with Earth on Friday, sending some engineering data that mission staff can now use to try to diagnose and fix its problems.
Spirit has not worked properly since Wednesday, when a command was sent for it to test an instrument motor.
Nasa has now formed an "anomaly team" which will attempt to work through ideas to get the rover back to health.
"I think we should expect that we will not be restoring functionality for a significant amount of time," said Mars Exploration Rover project manager Pete Theisinger.
"I expect this to go on in this mode for several days - of talking to the spacecraft, gathering more data, winnowing out theories, testing those theories against observables and continuing that process."
'Something to chew'
Spirit contacted Earth in short bursts on Friday, via an antenna of the Deep Space Network, near Madrid in Spain, and sent down engineering data at a very slow 120 bits per second.
But it was sufficient to tell Nasa that the vehicle's flight software was not behaving properly. It appears that every time Spirit tries to load the software it encounters a problem and then tries to re-boot.
Whether a piece of faulty hardware on the robot caused this or a big glitch in the software itself was impossible to say, explained Theisinger.
Spirit started to experience problems on Wednesday during a test of a motor used to move the Mini-Thermal Emission Spectrometer, an instrument that can determine the mineral composition of rocks.
For some reason, it failed to carry through a command to increase the current to the motor needed to improve its performance in the cold Martian air.
Until that point the rover had been working flawlessly, taking astonishing pictures of the planet and gathering science data on the geological make-up of its surroundings.
Some of that data - an unknown quantity - is now stuck on the rover and, for the time being, beyond the reach of mission scientists.
"I expect to get functionality out of this rover gain," said Theisinger. "I think that the chances that [this rover] will be perfect again are not good. But the chances that it will not work at all are also low. We are somewhere in that broad middle.
He added: "The engineers have got something to chew on and they're off chewing."
Lessons for Opportunity
The rover landed on Mars on 3 January on a three-month mission to explore the geological history of the planet. Its aim was to tour Gusev Crater, studying its rocks and soil for signs of water, past or present.
Meanwhile, the rover Opportunity is still on course to land on Mars on Sunday, at 0505 GMT.
The vehicle is aimed at Meridiani Planum, a plain which is near the Martian equator. It will be halfway around the planet from Gusev Crater.
The US landers are identical. Nasa will want, therefore, to trace the source of Spirit's problems because it could have implications for the way Opportunity is deployed.
After the failure in 1999 of Nasa's Mars Polar Lander and Mars Climate Orbiter, the agency opened the rover project to independent scrutiny.
Outside auditors have studied every detail in the plans, looking for any flaws which might scupper a successful mission for the vehicles.
Nasa says everything was done that was humanly possible to send the vehicles to Mars in good working order.