Moscow  - A communications problem Thursday delayed a harrowing spacewalk by two Russian cosmonauts who were to leave the International Space Station
(ISS) to retrieve an explosive bolt expected to provide clues to why the Soyuz capsule was badly off- course on its last two jolting re-entries.

International Space Station Commander Sergei Volkov and Flight Engineer Oleg Kononenko were to conduct a six-hour spacewalk starting at 1812 GMT.

A communications breakdown to controllers on Earth prompted the postponement, Russian space officials said. The Interfax news agency gave no further details.

Experts say it is an unorthodox mission for the Soyuz craft, docked outside the station.

The pyrotechnic bolt is one of five meant to explode and decouple part of the craft as it shuttles back to earth. The release ensures proper aerodynamics for the remaining capsule to reenter Earth's atmosphere.

The cosmonauts will use a serrated knife to cut through thermal lining over the bolt, uncouple it from a mess of detonating cables, force it out with a wrench and seal it in a blast-proof canister to send back to Earth.

The operation rides on the suspicion that the targeted bolt is that which failed to explode the last two times, crippling the aerodynamics of the capsule and causing the Soyuz to slam into the atmosphere backwards, hatch-first, leaving the crew unshielded from the heat of impact.

But Russian and US experts said all precautions had been taken and the cosmonaut duo was well-rehearsed. The station's only US crew member will provide support from within the ISS.

The capsule has crash landed on its last two re-entries though the crew have been un-harmed. They landed however kilometers from base out in the Kazakh steppes.

But Russian experts hope Thursday's spacewalk will help resolve the anomalies, not wishing to chance a third entry when Volk and Kononenko return to Earth in October. (dpa)