Moscow  - A Russian investigation into what sent the Soyuz spacecraft badly off-course as it crashed back to earth Saturday found it sheer "luck" that the crew survived "whole and unharmed."

"Everything could have ended much worse. The whole situation, you could say, was on the razor's edge," an expert close to the investigation was quoted by news agency Interfax as saying Tuesday.

The Soyuz capsule hit the atmosphere upside down, unleashing a probable host of fatal incidents for the crew: South Korea's first astronaut Yi So Yeon, US astronaut Peggy Whitson and Cosmonaut Yuri Malachenko.

"The Soyuz collided with the atmosphere not from the side protected by a heat screen but hatch door forward," the expert said.

The perilous entry scorched the hatch and melted down the craft transmitting device, cutting off communication.

The valve regulating pressure with the outside was also burned off which could have led to cabin temperatures soaring beyond bearing, he said.

"On top of that, if the fire had caught on to the hatch, which is near the container containing brake parachute, the crew could not have survived," the expert, who would not be named for security reasons said Tuesday.

Yi, returning from nine days in space, admitted Tuesday, "I thought we were going to burn."

"I saw flames around the ship and at first I was really scared," the 29-year-old bio-engineer said.

Yi and the two other International Space Station (ISS) crew members experienced a major gravitational pull and a hard impact as the capsule errored on reentry.

Marooned 400 kilometres from home base, the desert Kazakh steppes, the crew met astounded local farmers but could have been stranded for hours without communication.

But sensitive US Department of Defence surveillance systems helped find the craft, the expert told Interfax.

Malachenko and Whitman were back from six months on the ISS where Whitman broke the record for cumulative days in space.

No flight preparation could insure against such "abnomality" and "obvious technological failings," the expert said.

"There is no guarantee that the same the fate is not awaiting the crew of the Soyuz that are meant to fly back to Baikonur in half a year," he stressed.

The 17th ISS Expedition crew, Sergei Volkov and Oleg Kononenko, will return from orbit next fall. (dpa)