Engineers are trying to save Envisat, the European Space Agency's Earth observation satellite.
Controllers say the eight-tonne craft appears to be in a stable condition, but they are not receiving any data at all from it.
Contact was lost last weekend shortly after pictures of the Canary Islands were downloaded.
A recovery team 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 the BBC reports there is no confirmation as yet that its systems are in a "safe mode".
Envisat was launched in 2002 and is the biggest non-military Earth observation spacecraft ever put in orbit. It monitors the land, the oceans, Earth's ice cover and its atmosphere.
Information from the satellite is used daily to monitor for oil spills at sea, to check on iceberg hazards, and to provide information for weather forecasts.
The mission has already exceeded its planned lifetime by five years, but ESA had hoped to keep it operating until 2014.