Space Debris Risk Refined for Hubble Repair Flight

Atlantis astronauts face an increased risk of a deadly strike from space debris during their upcoming mission to repair the Hubble Space Telescope. But NASA has taken steps to reduce the risk from levels anticipated last year, despite the growth in space junk highlighted by February's collision of two satellites.

The growing threat that debris poses to astronauts and spacecraft was the subject of a hearing Tuesday before a U.S. House subcommittee.

"It's not something that you really need to lose sleep over, but it's something that we need to be proactive about and make sure that we do understand what the risks are," Nick Johnson, NASA's chief scientist for space debris, told reporters before the hearing.

Several recent incidents have raised concern about orbital debris, which includes naturally formed micrometeoroids as well as spent rocket stages and pieces of fragmented satellites:

• In January 2007, China shot down one of its satellites during a ballistic missile test, creating a spike in debris more than 500 miles above Earth.

• This February, a commercial U.S. satellite collided with a defunct Russian satellite, creating a new debris cloud.
  
  "It's hard to believe that space has gotten that crowded," U.S. Rep Gabrielle Giffords of Arizona, who chairs the House subcommittee, said Tuesday.

• In March, residents on the International Space Station had to shelter in an escape craft after a late alert about approaching debris.

The Hubble shuttle mission faces a 1-in-221 chance of a catastrophic debris hit.

That's down from a 1-in-185 probability forecasted last fall, and comparable with the threat of a major failure by critical shuttle systems like solid rocket boosters or main engines.

By comparison, the debris risk during a mission to the International Space Station, which flies at a lower and less congested altitude, averages about 1-in-300. NASA sets the safety threshold at 1-in-200.

Mission managers improved the odds for Atlantis by deciding to fly the spaceship as much as possible with its tail forward and payload bay facing the ground, LeRoy Cain, deputy shuttle program manager, said last week.

In addition, Atlantis will dive to a lower altitude soon after letting go of the telescope near the end of the mission.

"We take it seriously, and we think we're in a position that it's acceptable to go fly," said Cain.

Atlantis is targeting a May 11 launch. The astronauts visited Kennedy Space Center for training Tuesday.

Johnson said NASA believes more than 300,000 objects measuring at least half an inch in diameter now crowd low Earth orbit.

The Department of Defense's Space Surveillance Network officially tracks more than 19,000 larger objects, from about two inches to several feet in diameter, at altitudes up to about 22,000 miles above the planet.

Spacecraft shields can protect against tiny particles.

But objects as small as 5 millimeters could penetrate a space shuttle's cabin and lead to loss of pressure, Johnson said.

The Hubble telescope itself has shown evidence during four past servicing missions of being "pummeled regularly," he said.

Debris congestion has worsened every year, as spent rocket stages and satellites accumulate, increasing the risk of collisions that worsen the problem.

Efforts to limit the impact of space debris center primarily on not making the problem worse, Johnson said.

A proposal to use high-powered lasers to clean up space junk has not proven practical, he said.

The Department of Defense plans to increase its tracking capability in an effort to give early collision warnings to owners of about 800 satellites that are capable of maneuvering out of the way.

And nations are working together to improve guidelines for limiting debris and sharing information that could help avoid collisions.

"This is a big environment, and the U.S. doing something by itself is not sufficient," Johnson said.

• Video - The Expanding Danger of Space Debris
• Video - How the Satellite Crash Happened
• The  Most Memorable Space Junk That Fell to Earth

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