It is feared that if a reliable link-up cannot be established, Mir may begin to tumble in space and re-enter the atmosphere in an uncontrolled manner.
The intermittent communication with the unmanned space station Mir raises questions about Russia's ability to control the platform's planned destruction in late February.
Space officials have now regained contact with the station but at the moment they cannot fully explain what lay behind the communications breakdown.
It is feared that if a reliable link-up cannot be established, Mir may begin to tumble in space and re-enter the atmosphere in an uncontrolled manner.
Mir's large size - the platform has a mass of 140 tonnes - means large pieces are bound to make it through to the surface of the planet. Populated areas would be at risk from falling debris.
Current plans include the launch of an unmanned Progress supply craft around 10 January. This would dock with Mir two days later and lower the platform's orbit to prepare it for a controlled descent over the Pacific Ocean at the end of February. The target is a remote area 1,500-2,000km east of Australia.
But, according to the latest estimates, if all contact with Mir was lost, the platform would re-enter much sooner - about the end of January. It could then come down in an area south of 51.6° N, where most of the world's population lives.
In the UK, chunks of Mir could come down anywhere south of a line joining Bristol, Reading and London.
Two cosmonauts, Salizhan Sharipov and Pavel Vinogradov, are on standby to fly to Mir to carry out emergency repairs ahead of any descent procedure.
But if the platform loses all electrical power then no spacecraft, manned or unmanned, will be able to dock with it.
The next few days in the life of the 14-year-old space station will be critical.