The communication blackout as the capsule re-entered the Earth's atmosphere threatened a safe landing by astronaut Yang Liwei and forced ground control to use backup systems, Xinhua news agency reported.

"Yang lost every means to contact with the ground command and control headquarters as soon as he entered (the atmosphere), which fell in the worst case scenario prepared by the space mission team," Xinhua quoted Dong Deyi, head of China's control centre, as saying.

Yang's short mission aboard the Shenzhou V in October 2003 was hailed as a huge success for China's fledgling space programme, making the country the third to place a man in space after the former Soviet Union and the United States.

Some communication obstructions are normal during re-entry but Dong said none of China's radar could pick up a signal from the capsule.

Even after communications were re-established, signals remained weak enough to leave Yang at risk of "lethal impact" upon landing, he said.

"The echo signals from the spaceship were still volatile, which sufficiently threatened the safe landing of astronaut Yang," Dong was quoted as saying.

China's space command in the northern city of Xi'an ordered implementation of an optical guiding and tracking system instead of communications-guided landing control, he said.

This allowed headquarters to "properly control the slow-down parachute, which was vital to a soft landing," Dong said.

Two years after Yang's mission, the Shenzhou VI carried two astronauts into space on a five-day mission.

China has since announced plans for its first lunar probe this year and has targeted putting a man on the moon within 15 years.

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