Nasa's Mars Global Surveyor spacecraft has been out of contact with Earth for more than a week.
Engineers have been trying to re-establish communication with the probe, which could be showing its age after 10 years in space.
The spacecraft has entered "safe mode", awaiting further instructions from controllers on Earth.
Since then, the spacecraft has not confirmed receiving a command to point one of its transmitters to Earth.
MGS was launched in November 1996, operating longer than any Martian craft.
Carrying a powerful camera that has returned thousands of images, it has discovered features suggesting water once flowed on the desert world, and it has scoped out potential landing sites for future exploration.
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