This story was updated at 8:14 p.m.
EST.
NASA is now targeting March 12 to
launch the shuttle Discovery to the International Space Station as engineers
work to replace a set of suspect fuel valves aboard the spacecraft, the space
agency announced late Wednesday.
Discovery is now slated to launch no
earlier than March 12 at 8:54 p.m. EDT (0054 March 13 GMT) — one month later
than planned — on a two-week construction flight to the space station. The
mission has been delayed several times due to concerns with the shuttle's
fuel control valves.
"Right now, we're targeting March 12
but there's a lot of work to do," said Kyle Herring, a NASA spokesperson with
the agency's Johnson Space Center in Houston, Texas.
In addition to replacing the three
suspect valves aboard Discovery, NASA engineers will spend the next week
reviewing data from recent tests and studying potential modifications to
strengthen vital plumbing connected to the shuttle's fuel valves, Herring told SPACE.com.
Herring said shuttle program
managers plan to meet on March 4 to review Discovery's status and decide
whether to press ahead toward the March 12 launch from NASA's Kennedy Space
Center (KSC) in Cape Canaveral, Fla. If so, a meeting of top NASA officials
would convene on March 6 to approve the plan, he added.
NASA initially planned to launch
Discovery on Feb. 12, but has delayed the mission four times because of fuel control
valve safety concerns. The most recent delay, announced last Friday, left
Discovery without a firm launch target.
More analysis ahead
There are three
fuel control valves aboard Discovery, one for each of the spacecraft's main
engines. They are designed to work in concert to maintain pressure in the
shuttle's liquid hydrogen reservoir inside its attached external tank.
To keep the pressure stable as a
shuttle rockets toward space, metal poppets in the valves pop up as needed —
much like lawn sprinkler heads — to route gaseous hydrogen from the shuttle's
aft-mounted engines through a set of plumbing lines and into the external tank.
When NASA's shuttle Endeavour
launched last November, one of the fuel valves aboard that shuttle cracked
and chipped. The spacecraft's two other valves compensated for the damaged
one and the shuttle reached orbit without incident.
But NASA wants to be sure that a
similar problem, should it occur during Discovery's launch, would not cause
catastrophic damage by rupturing the spacecraft's vital plumbing lines or overpressurizing its hydrogen tank. A plumbing line break
near the shuttle's aft could lead to an emergency engine shutdown, while an overpressurized tank could force open an overflow port that
would vent the flammable gas overboard during launch, according to a NASA
document.
Herring said engineers will continue
computer modeling of the fuel valves to see how they might break during flight.
Meanwhile, engineers are also working on a way to reinforce part of the curved
plumbing lines between Discovery's fuel valves and its external tank to ensure
they can withstand impacts from debris from a broken part, he added.
"We could install this modification
on the first elbow bend only, and that would basically beef it up
structurally," Herring said.
After discovering Endeavour's
damaged fuel valve, engineers replaced the ones aboard Discovery with valves
known to be in working order. It is those replacements that engineers were
removing from Discovery today. Newer valves will be installed in their place,
NASA officials said.
Two of the valves being replaced
will undergo a detailed inspection, with engineers planning to take 4,000
photographs of each one to search for evidence of cracks, NASA officials said.
NASA spokesperson Allard Beutel at
KSC told SPACE.com that when engineers replaced the fuel valves on
Discovery the first time in January, the job took about two weeks. Removing the
valves alone was a six-hour job, he added.
Launch windows
NASA must launch by around March 13
in order to complete Discovery's two-week mission to the space station before
the March 28 arrival of a Russian-built Soyuz spacecraft at the orbiting
laboratory. That Soyuz mission is due to launch March 26 to ferry an American
space tourist and a new crew to the space station.
Herring said mission planners for
both Discovery and the International Space Station are looking at ways to
extend the March launch window by a few days. But if Discovery is unable to
launch in mid-March, NASA would have stand down until after the Soyuz crew
change is complete. The next launch window would open around April 7, mission managers
have said.
Commanded by veteran astronaut Lee
Archambault, Discovery's
STS-119 mission will deliver the final piece of the space station's
backbone-like main truss and the final set of U.S.-solar arrays. Four
spacewalks are planned during the 14-day mission.
Discovery will also ferry Japanese
astronaut Koichi Wakata to the space station to join the outpost's current
Expedition 18 crew. Wakata is Japan's first long-duration astronaut and will
replace NASA astronaut Sandra Magnus as a member of the space station's crew.
Magnus has lived aboard the space
station since November and will return home aboard Discovery. Wakata is due to
return to Earth later this year during a subsequent shuttle flight.
Discovery's STS-119 mission is the
first of up to six scheduled NASA shuttle flights for 2009. The other missions
include the planned May 12 launch of seven astronauts to perform the last overhaul
of the Hubble Space Telescope and a series of space station construction
flights.
NASA's plan to launch Discovery a
month late on March 12 is not expected to affect the scheduled May launch to
Hubble or the June shuttle flight to continue space station assembly, agency
officials said.
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