Envisat, launched in 2002 by the European Space Agency, has been using its 10 on-board instruments to gather data on environmental factors such as air quality, the extent of arctic sea ice and oil spills.
However, on April 8 Envisat stopped sending data, and all efforts by scientists to re-establish contact have been in vain, NewScientist.com reported Monday.
The cause remains unknown but power to the communications system has likely failed, mission manager Henri Laur of the European Space Research Institute in Frascati, Italy, said.
Researchers had hoped Envisat would continue to operate until replacement satellites are launched in a year or so, but say they now face a significant gap in their ability to monitor the Earth's environments.
The chances of restoring contact with Envisat -- 10 years old but designed to operate for only five -- are slim, researchers said.
"The chances are low, but as long as we believe there's a chance we will carry on," Laur said.