Russia's only space telescope is no longer responding to Earth's commands, officials say.
Astro Space Center chief Nikolai Kardashev said some of the Spektr-R satellite communication systems had stopped working.
But it was still broadcasting scientific data, reports the RIA Novosti news agency.
The telescope was operational far beyond the expected five-year life span, according to Russian space agency Roskosmos.
Experts have repeatedly tried and failed to repair the missing connection, Kardashev said.
Yuri Kovalev, head of research for the Spektr-R project, said the call fell on the morning of January 11, but added that "there is still hope."
Spektr-R was released into space in 2011.
A new Russian-German satellite, Spektr-RG, is scheduled to be launched this year.