This
story was updated at 10:38 p.m. EST.
CAPE
CANAVERAL, Fla. - NASA is now targeting Sunday as the earliest launch
opportunity for space shuttle Atlantis as engineers wrestle with a fuel sensor
glitch inside the orbiter's fuel tank.
Atlantis and
its seven-astronaut crew are now set to launch no earlier than 3:21 p.m. EST
(2021 GMT) on NASA's
STS-122 mission to haul the European Space Agency's (ESA) Columbus
laboratory toward the International Space Station (ISS).
"We're
thinking about our options and whether the risks are acceptable or not," Wayne
Hale, NASA's space shuttle program manager, told reporters late Friday after a
more than five-hour discussion by mission managers.
Hale said mission
managers will make a final decision on whether to press ahead with the planned Sunday
space shot after another review tomorrow afternoon.
Sensor
glitch scrub
NASA scrubbed
its attempted
Thursday launch of Atlantis after two of four critical liquid hydrogen fuel
level sensors inside the shuttle's 15-story external tank failed a standard
preflight test.
Known as engine
cut-off (ECO) sensors, the instruments serve as a backup system to shut
down Atlantis' three main engines before their supply of liquid hydrogen
propellant runs dry. NASA shuttles consume more than 500,000 gallons (1.9
million liters) of super-cold liquid hydrogen and liquid oxygen during the
short trip into space.
NASA shuttle
flight rules require at least three of the four sensors to be working properly
in order to launch. Engineers spent much of today studying whether to relax
that rule and require only two of Atlantis' four liquid hydrogen sensors to
be working, but later proposed that - due to the glitch's intermittent nature -
all four of the instruments should perform properly before a Sunday launch
attempt could lift off.
"When we
fill the tank up with cryogenic hydrogen again...our past history says they are
likely to all work," Hale said of Atlantis' fuel gauge sensors, which are
currently functioning perfectly to indicate an empty external tank. "That makes
it difficult to troubleshoot."
Three of
the NASA's seven shuttle missions that have launched since the agency resumed
orbiter flights 2005 following the Columbia accident have been delayed by
similar fuel tank sensor glitches, most recently in September 2006.
"We
believed that we had solved that problem, quite frankly," Hale said. "We've had
several flights now where we haven't had any problem, and we frankly have done
everything that we know how to do to improve that system."
In order to
further reduce the risk of the sensors failing after liftoff, while Atlantis is
still climbing toward orbit, mission managers are targeting a small, one-minute
window in which to launch the shuttle instead of the traditional five-minute
stretch.
The measure
would allow the shuttle to conserve fuel during launch as an extra level of
protection should the suspect sensors fail during flight, said Mike Leinbach,
NASA's shuttle launch director.
Launch
opportunities ahead
Shuttle
workers, today, topped off Atlantis' supplies of cryogenic reactants used by
the orbiter's three fuel cells to generate power during flight, Leinbach said. The
measure clears the way for launch attempts on Sunday and Monday before NASA
would have stand down until Thursday to again refill the shuttle's tanks, he
added.
Commanded
by veteran
shuttle flyer Stephen Frick, Atlantis' STS-122 crew will deliver the
European Space Agency's (ESA) Columbus laboratory to the ISS during a planned
11-day mission. The 1.4 billion Euro ($2 billion) Columbus module is the ESA's
largest contribution to the ISS.
The
spaceflight will mark NASA's fourth shuttle flight of 2007 and the second this
year to deliver a new orbital room to the ISS.
NASA must
launch Atlantis by Dec. 13 in order to complete the STS-122 mission while the
angles between the station's wing-like solar arrays and the sun are favorable
to support docked operations. If the shuttle cannot launch by the window's
close, NASA would likely stand down until no earlier than Jan. 2, mission
managers have said.
Current
forecasts predict a 70 percent chance of favorable launch weather on Sunday,
with low clouds and the potential for nearby rain showers as the only concern.
"Let's hope
we go fly on Sunday," Hale said.
NASA
will broadcast Atlantis' STS-122 mission live on NASA TV. Click here for SPACE.com's shuttle mission coverage and NASA TV feed.
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