Space Station Skirted Danger

The International Space Station was recently in danger of being hit and possibly destroyed by space junk, because of a blunder by flight controllers.

The U.S. space agency realized late last week the station would fly very close to a used Russian rocket.

But the trouble came when NASA tried to nudge the station into a slightly different orbit for safety.

Not only did its thrusters not fire, its computer shut down the whole steering system for 90 minutes.

Initially, the U.S. military organization that tracks objects in space predicted the rocket chunk would pass within two-thirds of a mile of the space station on Sunday. It ended up coming no closer than 4 1/2 miles.

NASA admitted the sequence of computer commands sent up by flight controllers to fire the station's engines over the weekend failed because of human error. It disclosed the incident Thursday.

The problem arose in part because NASA had to relay its signals through Russia's mission control.

Both countries promise it won't happen again.

At last count, the U.S. Space Command in Colorado Springs, Colo., was tracking more than 8,700 manmade objects in orbit, most of them junk. With an orbital speed of 17,500 mph, any one of them could do serious damage to the space station, which consists of only two compartments so far.

Measures already have been taken to prevent this from happening again, said Frank Culbertson, NASA deputy program manager for space station operations.

"I'm very glad we learned what we learned at this point in the mission before we have more hardware up there and before we have people up there," Culbertson said. But he noted that if astronauts had been aboard, they could have maneuvered the space station more easily than ground controllers.

The maneuvering command, drawn up by both U.S. and Russian engineers, was sent from the Russian control center outside Moscow on Saturday night. But the command contained faulty instructions, and the space station's computers rejected it. The station was left without motion control for an entire orbit.

Flight controllers may not have factored in the changes in the space station's mass and center of gravity since shuttle Discovery delivered two tons of gear earlier this month, or else the changes were entered incorrectly.

Culbertson said it wasn't immediately clear whether Americans or Russians were to blame.

By the time flight controllers learned of their mistake, it was too late to send up a corrected command. "But because the probability was low, we weren't too worried about it," Culbertson said.