Agency taking steps to avoid collisions with space junk

WASHINGTON -- Space shuttles have been returning to Earth with a larger-than-expected number of dings from space junk, prompting NASA to make changes to better avoid potentially catastrophic collisions, space agency documents show.

As NASA explores whether damage from space debris could have caused the Columbia disaster, its own documents detail replacements of nicked windshields and dents caused by space debris collisions that have become more frequent than NASA's computer models had predicted.

A NASA report disclosed last May that the computer modeling the space agency was using to assess the dangers "was underpredicting" the number of collisions and urged a revision to the way NASA calculated the risks.

The space agency made the changes after a decade of seeing shuttles return to Earth far more banged up than expected.

The number of dings and dents "was in the hundreds for about 20 shuttle missions," said NASA consultant Donald Kessler, who kept track of space debris for the agency during a three-decade career.

For example, after a long 1996 mission by the now-lost Columbia shuttle, two windows were replaced and scientists later found 51 pits in them. They contained meteoroids, paint chips, aluminum, stainless steel, silver, copper and plastic, according to NASA incident reports reviewed by the Associated Press.

"The manmade materials probably came from solid rocket motors and fragments of old spacecraft," explained David S.F. Portree, a former NASA technical writer who chronicled collisions with space debris in a 1999 report.

Kessler said the collisions are frequent enough that NASA on average now replaces a window after each shuttle mission.

But Kessler and Portree say they doubt the Columbia tragedy has anything to do with space debris. "NASA taken this issue very seriously," Kessler said, and the shuttle was flying below the altitude where space junk is more prevalent when it broke apart.

NASA has long been aware of the dangers of space debris collisions -- eight times during the first five shuttle missions in the 1980s astronauts were warned that objects would pass close by a space craft, reports show.

In 1982, Columbia passed within six miles -- a harrowing close distance when traveling at high speed -- of a section of an old Soviet rocket, according to a NASA document that chronicled four decades of strikes and close encounters with space debris.

U.S. officials say they are carefully tracking a growing pile of natural and manmade debris in the space around the Earth -- everything from meteoroid fragments to remnants of old rockets and satellites.

"We track approximately 9,000 (manmade) objects in space -- anything from about four inches" on up but "anything smaller than that, it goes beyond our capability," explained Air Force Lt. Col. Andy Roake, spokesman for the Air Force Space Command.

But even small collisions could have consequences for a spacecraft that relies on special thermal resisting heat tiles to protect it from burning up as it re-enters the volatile atmosphere of the Earth.

One space agency report noted that a marble-size object traveling at 22,000 miles per hour would strike a shuttle with the same force as a 400-pound safe traveling at just 60 miles per hour -- the speed limit on many highways.

For nearly 20 years, the government has used radar, telescopes and a satellite deployed by the Challenger shuttle in 1984 to build computer models that assisted NASA scientists in steering astronauts clear of space junk.

In last year's report, NASA made improvements to the model to better calculate the number of likely collisions.

Also, shuttles frequently fly upside down and backward to reduce the chances of exposing vulnerable parts of the craft to debris, and NASA flight managers consult with the Air Force Space Command to steer clear of any larger debris.

"We work with NASA so that if they have a flight path coming up, we'd look at that and see if any space object is going into a hypothetical box around the shuttle," the Air Force's Roake said.

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The AP's Frank Bass in New York contributed to this report.