Battle to save flagship Envisat spacecraft
Engineers are battling to save the European Space Agency's (Esa) flagship Earth observation mission - Envisat.
Controllers say the eight-tonne spacecraft appears to be in a stable condition, but they are not receiving any data at all from it.
Contact was lost with Envisat at the weekend shortly after it downloaded pictures of Spain's Canary Islands.
A recovery team, which includes experts from industry, is now trying to re-establish contact with the craft.
Mission managers said on Friday that they were working through a number of possible fault scenarios but conceded they had little to go on.
Radar pictures taken from the ground appear to show the satellite to be intact, but there is as yet no confirmation that Envisat has entered the expected "safe mode" of an ailing spacecraft.
This automated procedure is designed to ensure the solar panel is pointed at the Sun and that onboard power systems are prioritised above all other activity.
If this has not happened, the concern would be that Envisat's batteries could soon become depleted, denying any prospect of recovery.
"We continue to try to re-establish contact with the satellite, and in parallel to collect more information on the satellite's status by ground radar images, from optical images, from telescopes, but also from other spacecraft," said Prof Volker Liebig, the director of Earth observation at Esa.
"On Sunday, [the French space agency] will try to program [their new high-resolution imaging satellite] Pleiades to see if they can image Envisat, to give us more detailed knowledge on whether there is damage on the outside," he told reporters.
Envisat was launched in 2002 and is the biggest non-military Earth observation spacecraft ever put in orbit.
It has been at the forefront of European Earth science endeavours for a decade, monitoring the land, the oceans, Earth's ice cover and its atmosphere.
The mission, which has so far cost about 2.5bn euros (£2.1bn), has already exceeded its planned lifetime by five years, but Esa had hoped to keep it operating until 2014.
This would have given the agency time to run it alongside some of the scheduled replacements, and to cross-calibrate their data.
The first of these is Sentinel 1, which is supposed to take over the radar duties of Envisat. It should be launched next year.
Sentinels 2 and 3 will image changes on the land and over the oceans and they should follow in early 2014.
Of more immediate concern are the operational and scientific projects that rely on Envisat data.
The satellite's information is used daily to monitor for oil spills at sea, to check on iceberg hazards, and to provide information for meteorological forecasts, among a wide range of services.
All this had now been disrupted, said Prof Liebig.
"What we have done is [activate] the contingency agreement we have with the Canadian Space Agency (CSA) which we have had for many, many years. Canada has responded very positively. So, for a certain time, the CSA's Radarsats 1 and 2 will try to fill some of the gaps."
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter