Russia's only space radio telescope is no longer responding to commands from Earth, officials say.
Astro Space Centre chief Nikolai Kardashev said some of the Spektr-R satellite's communication systems had stopped working.
But it was still transmitting scientific data, RIA Novosti news agency reports.
The telescope has been operational way beyond its expected five-year lifespan, Russia's space agency Roskosmos says.
Specialists had repeatedly tried and failed to fix the lost connection, Mr Kardashev said.
Yuri Kovalev, head of research for the Spektr-R project, said the link went down on the morning of 11 January, but added that "there is still hope".
Spektr-R was launched into space in 2011.
A new Russian-German satellite, Spektr-RG, is scheduled to be launched this year.