A US space agency (Nasa) mission to map the world's ice sheets is in trouble.
By Helen Briggs
BBC News Online science reporter
A laser in the satellite's sole scientific instrument is not working and operations have been put on hold for the time being.
The flagship US mission was launched six months ago to track changes in the Greenland and Antarctic ice sheets.
The Ice, Cloud, and land Elevation Satellite (Icesat) was sending back promising data before the problem arose. A review panel has been set up to decide how to proceed.
The satellite uses a laser beam to measure distance
Icesat project manager, Jim Watzin, of the Nasa Goddard Spaceflight Center, hopes a decision will be made this month.
"Right now the mission is temporarily on hold," he told BBC News Online. "Prior to the anomaly, we were getting excellent results."
The problem is with a laser used to measure the topography of the Earth's major ice sheets.
Without it, the instrument cannot carry out its detailed mapping of ice surfaces and their underlying geology.
It's a huge disappointment if there's a questionmark over its future
David Vaughan, British Antarctic Survey
Scientists have been waiting decades for the launch of Icesat because the satellite should give the best picture yet of what is happening to ice in the Arctic and the Antarctic.
There has been heated debate about whether major ice sheets have been shrinking or growing and what impact this might have on global sea levels and climate.
"It's a huge disappointment if there's a question mark over its future," says David Vaughan, a scientist at the British Antarctic Survey in Cambridgeshire.
Glaciologists have been gleaning data from similar but less advanced satellites since the early 1990s.
It has allowed them to pinpoint areas like the Pine Island Glacier, one of the biggest on Antarctica, which may be on the verge of slipping into the sea.
This has given "a mere glimpse" of the changes that are taking place in the Antarctic, says Vaughan.
He describes it as like looking at the ice "through a thick pair of glasses".
Icesat, and Cryosat, set for launch by the European Space Agency in 2004, have higher resolutions and should give a clearer picture.
"Neither is the answer to our dreams but together they get us really close," says Vaughan.
Jim Watzin says the problems on Icesat will have "no effect on the minimum mission".
He says Icesat carries two lasers and a back-up, so, in the worst case scenario, engineers could switch to the second.
"To lose one of the three is not good - it shortens the overall life span of it," says Vaughan.
Icesat is a laser altimeter mission and Cryosat is a radar altimeter mission. They both work by firing pulses at the Earth and calculating distance from the time taken for the pulses to return.