HOUSTON — NASA's
shuttle Atlantis will be at a higher risk of suffering serious damage from tiny
space rocks and orbital trash than past missions when it launches to the Hubble
Space Telescope next month, a top program official said Monday.
NASA
shuttle program manager John Shannon said Atlantis and its seven-astronaut crew
are facing the extra risk solely because of their destination.
"It's worse
for Hubble because we fly higher," he told reporters during a briefing here at
NASA's Johnson Space Center.
Atlantis is
slated to launch
on Oct. 10 with seven astronauts aboard to begin an 11-day mission to
upgrade the 18-year-old Hubble Space Telescope.
Commanded
by veteran spaceflyer Scott Altman, Atlantis astronauts plan to perform five
back-to-back spacewalks during the flight to install new cameras, replace
batteries, gyroscopes, and guidance equipment, repair instruments never
designed to be fixed in space and add a docking mechanism and thermal insulation.
The $900
million shuttle flight is expected to extend Hubble's science mission, but
carries more risk than NASA's recent construction flights to the International
Space Station.
The orbital
observatory flies about 350 miles (563 km) above Earth, higher than the
220-mile (354 km) path of the International Space Station — the destination of
NASA's recent shuttle missions since 2005. The levels of orbital debris around
Hubble have increased in recent years due to satellite breakups or even
their intentional destruction, Shannon added.
That puts
the estimated odds of Atlantis suffering a critical strike from a
micrometeorite or orbital debris (MMOD) during its Hubble mission at about a 1-in-185
chance, up from the typical 1-in-300 chance for flights to the space station,
he added.
"It's not theoretical,"
Shannon said. "Every time we fly the vehicle back, we have MMOD damage on the
space shuttle."
The space
station and Hubble itself have also sustained minor damage from orbital debris,
but despite the increased odds, it would take a major hit to a critical area to
seriously wound Atlantis, Shannon said.
Atlantis
would not be able to reach the International Space Station, which could serve
as a refuge for shuttle astronauts if their spacecraft is seriously damaged, from
Hubble because of the space telescope's higher orbit and different inclination,
NASA has said.
Instead, the
space agency plans to have a second
shuttle poised atop its Florida launch pad to serve as a rescue ship in the
event of an emergency. But Shannon said that the likelihood that the rescue
mission — known as STS-400 at NASA — will be required at all, let alone due to
MMOD damage, is very small.
"It would
take a very rare and very significantly large-sized damage from MMOD in a
critical area to cause us to require [STS-400]," Shannon said.
Shannon
said he expects shuttle mission managers to discuss the micrometeorite threat
on Thursday during a two-day program review. The current threshold for
acceptable debris risk is about a 1-in-200 chance of a critical strike, but the
benchmark is set to ensure the risk is properly discussed and assessed, he
added.
Top agency
officials, including NASA's safety and engineering chiefs, will make a final
decision on the added risk later this month during a Flight Readiness Review, Shannon
said. They will also take into account the new heat shield repair methods and
tools developed after damage from launch debris led to the tragic loss of the
shuttle Columbia and its crew in 2003.
"I don't
get too hung up on the numbers," he said. "I want to make sure that the actions
we're taking in response to the threat are appropriate."
Since the Columbia
tragedy, astronauts have routinely used a sensor-laden extension of their space
shuttle's robotic arm to scan for damage on their spacecraft's sensitive heat
shield. Shuttle crews currently perform two scans, one just after launch and another
just before landing, to clear their orbiter for reentry through Earth's atmosphere.
NASA
initially cancelled the upcoming
Hubble servicing mission in 2004 following the Columbia accident due to
safety concerns. The agency later studied the potential to service Hubble
robotically before reinstating the astronaut-led mission in 2006.
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