CAPE
CANAVERAL, Fla. - The space
shuttle Atlantis is once more poised
to launch toward the International Space
Station (ISS) carrying a six-astronaut crew determined to resume
construction of the orbital laboratory.
Atlantis' STS-115
mission is set to launch at 11:40:32 a.m. EDT (1540:32 GMT) to deliver a pair of
massive trusses and two new solar arrays to the ISS. Today's planned space
shot is NASA's second launch attempt for Atlantis in three days after an odd
fuel cell power reading prompted a Wednesday scrub.
After two
days of scrutiny, engineers concluded Thursday that the glitch would not be a
significant risk to Atlantis' STS-115 mission commanded by veteran
shuttle astronaut Brent Jett. The spaceflight's lighted launch window has
dwindled to just two days - today and tomorrow - after several delays due to
the fuel cell issue, poor
weather and a launch
pad lightning strike.
"We have a
couple of days available for launch and if we get the opportunity to go fly, I
would like to take advantage of those couple of days," shuttle program manager
Wayne Hale said.
Atlantis is
hauling a $371.8 million payload of 17.5-ton trusses and expansive solar arrays.
STS-115
spacewalkers Joseph
Tanner, Heidemarie
Stefanyshyn-Piper, Daniel
Burbank and Steven
MacLean will work two at a time to install the truss segments during the
three planned spacewalks for their spaceflight. First-time flyer Chris
Ferguson is serving as Atlantis' pilot.
Lt. Kaleb
Nordgren of the 45th Weather Squadron at Cape Canaveral Air Force
Station said Atlantis has a 70 percent chance of acceptable flight conditions
today, with forecasts improving to 80 percent on Saturday.
Fuel
cell confidence
At the
heart of Atlantis
latest glitch is a short in one of three current phases that power a
coolant pump motor in Fuel Cell 1.
Atlantis
carries three fuel cells that use liquid hydrogen and liquid oxygen to generate
the electricity used to power all the space shuttle's systems. That power,
delivered along three phase loops - A, B and C - also supports a coolant pump
motor to prevent each fuel cell from overheating.
It was
Phase A that shorted on Fuel Cell 1, though shuttle officials are confident
that the other two phases are healthy
enough to keep the generator from failing in orbit and cutting Atlantis'
11-day mission short.
Two fuel
cells can power the shuttle normally, with the third as a backup, Hale said
Thursday.
Steve
Poulos, NASA's orbiter projects manager, said Fuel Cell 1's coolant pump motor
has been in operation for a lifetime total of 3,948 hours - or less than half
of its 10,000-hour certification - despite being nearly 30 years old. A thin
wire in Phase A, and possible some degradation over time, are possible sources
of the current loop's short, he added.
"It's very
robust, it's very reliable," Hale said of the pump motor technology used in
NASA fuel cells. "This is not rocket science, this is 19th century
technology."
Hale said
he marveled at the fact that, despite its age, the motor has shown no other issues
earlier in its lifetime. It was first assembled in 1976, shuttle officials said.
"Here we
are, 30 years after we put this little motor together and finally...the first one
out of all the motors we put together 30 years ago has had a little incident,"
Hale said. "I wish my car worked that good."
NASA
will provide live coverage of Atlantis' launch preparations beginning at 5:30
a.m. EDT (0930 GMT). You are invited to follow the briefing using SPACE.com's NASA
TV, which is available by clicking
here or using the button at the left.
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