Nasa's Mars Global Surveyor spacecraft has been out of contact with Earth for more than a week.
Mars Global Surveyor was launched in November 1996 (Image: Corby Waste/Nasa)
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.
The spacecraft, designed to systematically map Mars, is one of four spacecraft orbiting the Red Planet. Its companions include Nasa's Mars Reconnaissance Orbiter and Mars Odyssey and the European Space Agency's Mars Express.
A motor component controlling one of Surveyor's two solar arrays apparently malfunctioned last week. The spacecraft may have gone into a power-saving safe mode by turning so that the array faced the Sun.
Thomas Thorpe, MGS project manager, said that might have brought the craft's antennas and transmitters out of alignment with Earth.
Without telemetry, controllers do not know the angle the panel failed at or the orientation of the main body of the spacecraft.
Another difficulty is that the spacecraft changes position quickly, zooming around Mars twice every two hours. Signals cannot reach it when it is in eclipse for 40 minutes of that orbit time.
"You have to know which receiver to command to, you have to address (precisely) where the commands are going," Mr Thorpe said.
If it has not been receiving commands, the spacecraft is programmed to eventually change its position so that one of its low-gain antennas is pointing at Earth.
However, this may mean putting one solar panel in the dark, leaving just one to power the probe's systems.
"One panel is not sufficient to keep the spacecraft alive for very long," Mr Thorpe said.
In order to get a better idea of Surveyor's position, controllers had asked to have it photographed by the Mars Reconnaissance Orbiter, which began circling the Red Planet on 10 March, Mr Thorpe said.
"It's not clear how long it will take us to exhaust possibilities or come to a conclusion that we're not going to hear from it," Thorpe said. "If it's getting sufficient power, the spacecraft could stay healthy for years," the MGS project manager added.
Mars Global Surveyor was originally launched as a $247m mission to study the planet's surface for one Martian year, roughly two Earth years.