A New Zealand astronomer says it won't be known when or where debris from the failed Russian satellite, Phobus-Grunt, will fall to Earth, until about an hour before impact.
The satellite was launched on 8 November, bound for Mars, but it malfunctioned and never cleared Earth's orbit.
The Russian space agency Roscosmos earlier estimated the probe will fall from the sky late on Sunday (GMT). It has now cancelled all predictions of the likely crash site.
Roscosmos previously said that the debris would probably fall in the Pacific Ocean off the western coast of Chile. Two earlier updates had the fragments falling into the Indian and Atlantic Oceans.
Astronomer Grant Christie of the Stardome Observatory in Auckland says the fluctuating depth of the Earth's atmosphere makes it impossible to accurately predict the impact site.
He says by far the most likely place for debris from the satellite to land, is in an ocean.
Phobos-Grunt weighed some 13 tonnes at launch, but very little of this mass should make it through to the surface.
The BBC says estimates issued by Roscosmos suggest that only 200kg will survive the fiery plunge.