By Richard Black
BBC science correspondent
The US space agency says one of the instruments on its Mars Odyssey craft has been shut down by a solar flare.
Mars Odyssey: In orbit around the Red Planet since October 2001
The instrument, designed to assess the hazards humans would face if they ever went to the planet, has not worked since a solar storm on 28 October.
The same storm caused a blackout in Sweden, damaged two Japanese satellites and interfered with navigation and radio systems for aircraft and ships.
Nasa says it is signalling Odyssey to try to coax the equipment back online.
About a month ago, the Sun went through an intensely active period, spewing out some of the most powerful flares ever recorded.
Radiation and billions of tonnes of charged particles hurtled through space at huge velocities and enough reached Mars to shut down the instrument on Odyssey, which has been orbiting the planet for two years.
Whether the damage is terminal is not yet certain. But even if the instrument never works again, Nasa says it has fulfilled its purpose.
"Even if the instrument provides no additional data in the future, it has been a great success at characterising the radiation environment that a crewed mission to Mars would need to anticipate," said Jeffrey Plaut, project scientist for Mars Odyssey.
The US spacecraft will provide a vital link for the British lander Beagle 2 when reaches the planet's surface on Christmas Day.
Odyssey will be the first orbiter in place to relay any signal from Beagle reporting it has landed safely.