Faulty batteries have delayed the launch of an unmanned spacecraft needed to help bring Russia's Mir space station out of orbit.

The launch, which was due to take place at 0656GMT from the Baikonur cosmodrome in Kazakhstan, has been postponed until 21 January.

The plan is for the Progress M1 supply vessel to dock with Mir and then gradually ease the space platform back into the Earth's atmosphere.

Much of Mir will burn up, but some debris is expected to land in the Pacific Ocean between Australia and South America.

Power levels

Officials called off the launch after plunging voltage levels on Mir caused the space station's gyroscopes to slow, making the station too unstable for docking.

The low power also knocked out Mir's central computer.

Last month, an abrupt loss of power caused mission control to lose contact with Mir for more than 20 hours.

This time, flight controllers were prepared and quickly switched off non-essential equipment when they saw the voltage falling, preventing a loss of communication.

"Taught by bitter experience, we didn't allow loss of contact with the station," mission control chief Vladimir Solovyov said.

He said the voltage drop was most probably caused by Mir's old batteries, which with age have become unpredictable and less capable of retaining power.

Solovyov said that experts had suggested bypassing the unstable batteries and connecting the station's most vital equipment directly to its solar panels.

Breaking up

The 130-tonne space station was originally built to last for only five years, but it has been orbiting Earth now for 15. It is being scrapped because it is proving too costly for Russia's space authorities to maintain.

Mir is scheduled to crash back to Earth on Tuesday 6 March.

But the final decision on when to "scuttle" the platform depends on when the Progress ship is able to dock with Mir.

After fuel from the Progress is taken on board Mir, Russian ground controllers will fire rockets on the station to slow its orbit and send the platform hurtling into the thicker layers of Earth's atmosphere.

As it re-enters, Mir is expected to break up into thousands of pieces, some weighing as much as 700 kg (1,500 pounds).