STAR CITY - A computer software error likely sent a Russian spacecraft into a rare ballistic descent that subjected the three men on board to chest-crushing gravity loads that made it hard to breathe, space experts said Tuesday.

If that proves to be the case, it should be an easy repair and the two new residents of the international space station should have nothing to fear when it's their turn to ride a Soyuz capsule back to Earth this fall.

A cosmonaut whose own Soyuz landing two years ago was steep but not ballistic, Talgat Musabayev, said Russian space experts believe the problem was caused by software in the guidance computer that was installed in the Soyuz TMA-1 spaceship. It was the first time the new, modified spaceship had been used in re-entry. Astronaut Kenneth Bowersox, the commander of the 5 1/2-month mission, said he and his crewmates knew what was coming when the computer display suddenly switched from a normal to a ballistic entry Sunday.

The capsule came in considerably steeper than planned, and the men endured more than eight times the force of gravity, double the usual amount.

"It was easier than I thought it was going to be," Bowersox told reporters at cosmonaut training headquarters outside Moscow. "There's a lot of pressure on your chest and when you come back from space, just one-G makes you feel heavy.

"So it's hard to breathe and your tongue sort of slips in your head and toward the back of your throat."

Astronaut Donald Pettit noted: "For me, for a moment, it felt like I was Atlas and I had the weight of the whole world on my shoulders."

Their capsule landed nearly 500 kilometers (300 miles) off-target in Kazakhstan. Two hours passed before anyone knew where they were or how they were doing - indeed, whether they were even alive.

During a crowded news conference, the Russian cosmonaut who was in charge of the Soyuz, Nikolai Budarin, said no one on board did anything to trigger the backup computer program for a ballistic re-entry.

"It's for the specialists to figure out what was the cause," Budarin said. "Let's wait and see, but now I can say that it was not our own doing."

Bowersox, a test pilot who assisted Budarin during the descent, said he does not believe the crew made any errors, but acknowledged, "You just never know for sure."

"In these types of situations, everything happens fast, sort of a blur, and it's best not to be too positive," Bowersox said. "The tape recorders are much better at analyzing the truth than the humans are."

Pettit still looked weak and a little shaky. He had to be supported following Sunday's landing and could barely walk.

He said he was glad to have some privacy at touchdown.

"We'd been prepared that the landing site was going to be a bit of a mob scene with lots of people and hustle and bustle and everything, and I was actually relieved to ooze out of the spacecraft and lay on Mother Earth and just have a solitude moment in which to get reacquainted," he said.

The spacemen actually had four hours by themselves; that's how long it took helicopters to arrive.

Pettit said being tall and skinny, and a first-time space flier, made his transition to gravity all the more difficult.

"I've had a little more trouble walking around than others," said the 165-pound 6-footer (75-kilogram, 1 meter, 80-centimeter), "but I guess I fall in both of those categories."

Bowersox and Pettit will recuperate at Star City for two more weeks before flying back home to Houston with their wives, who sat in on the press conference and beamed with pride.

They are the first NASA astronauts to return to Earth in a Russian spacecraft.