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:
- 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.
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