NASA has pulled a spacewalk
dedicated to heat shield repair techniques from the schedule of its next space
shuttle flight to make way for additional inspections with the orbiter's
robotic arm, the U.S. space agency said Monday.
Mission planners decided to remove
the third of three planned spacewalks for NASA's
STS-121 shuttle mission to the International Space Station (ISS) - to
launch aboard Discovery no earlier than July 1 - to allow time for in-depth
scans of the orbiter's heat shield, NASA officials said, adding that the
extravehicular activity (EVA) is not being abandoned altogether.
"The crew and the team are going to
continue training for the third one in the event that the consumables would
support an extra day for the mission, which is likely," NASA spokesperson Kylie
Clem told SPACE.com of the decision, which was first reported by CBS
News.
The third planned EVA for STS-121
spacewalkers Michael Fossum and Piers Sellers called
for the astronauts to test a series of reinforced carbon carbon
(RCC) repair techniques.
The repair
methods, which use a black putty-like material to fill in RCC cracks, were
first tested during an hour-long procedure during an STS-114
spacewalk last summer. That flight, also aboard Discovery, was NASA's first
shuttle mission following the 2003
loss of the STS-107
astronauts aboard Columbia, which sustained heat shield damage at launch.
That accident led to the RCC repair tests, as well as checks of a dabbed-on
gray material to coat broken tiles, during the STS-114 mission. STS-121 will
be the second post-Columbia test flight.
Clem said a revised STS-121 schedule
now calls for the mission's six-astronaut crew, commanded by shuttle veteran Steven
Lindsey, to use a 50-foot (15-meter) boom extension to Discovery's own
50-foot (15-meter) robotic arm in extra heat shield scans before and after
undocking. Inspections were already planned for the second day of the 13-day
mission, NASA said.
Discovery's mission could be
extended an extra day to allow the heat shield repair spacewalk depending on
the amount of consumables available to power the orbiter's fuel cells, Clem
added.
Sellers and Fossum
will continue to train for that EVA along with the two others, first of which
is aimed at testing the stability of the boom extension with an astronaut
standing on the end. The test, which NASA station managers have said will be
conducted over the orbiter's payload bay, is designed to determine whether the
boom can be used as a repair platform if needed.
The second spacewalk is dedicated to
space station work, with Fossum and Sellers repairing
a cable
system that transfers power, data and video to and from the orbital
laboratory's mobile transporter railcar. The railcar is used to move major
components along the length of the station.
Meanwhile, shuttle engineers at
NASA's Kennedy Space Center spaceport in Florida are replacing one of
Discovery's three main engines in preparation for its STS-121 launch.
Shuttle engine checks at NASA's Stennis Space Center in Mississippi discovered a crack in a
solder joint associated with the engine's controller, prompting orbiter
managers to swap Discovery's lower left engine because of the possibility that
it has the same problem, NASA KSC spokesperson Jessica Rye told SPACE.com.
Engineers are expected to remove the
engine from Discovery's aft Tuesday, and have the replacement installed by April
22, Rye added.
"The end result is no impact to the
shuttle's rollover date," Rye added.
Discovery is currently scheduled to
roll from its hangar-like Orbiter Processing Facility to NASA's massive Vehicle
Assembly Building, where its solid rocket boosters and external tank reside, on
May 12.
The mission's launch window
stretches from July 1 to July 19.
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