This
story was updated at 11:17 a.m. EDT.
CAPE
CANAVERAL, Fla. - Last minute glitches continued to plague NASA's space shuttle Atlantis after an
errant fuel tank sensor prevented the orbiter and its six-astronaut crew from
rocketing towards the International
Space Station (ISS) Friday.
For the
second time in three days, Atlantis stood down from a planned
mid-day liftoff due to technical issues. The spacecraft's STS-115
astronauts - commanded by veteran spaceflyer Brent
Jett - were buckled inside Atlantis and set to launch spaceward at 11:40:37
a.m. EDT (1540:37 GMT) before shuttle managers scrubbed the attempted space
shot.
"We had a
lot discussion...and we follow the rules," NASA launch director Michael Leinbach
told Jett, adding that the scrub falls in line with shuttle flight
restrictions.
"We
understand and we concur 100 percent," Jett replied. "That whole plan was given
a lot of thought by a lot of smart people...it's the right thing to do."
Launch is
now set for 11:15 a.m. EDT (1515 GMT) tomorrow, with weather forecasts offering
an 80 percent chance of favorable flight conditions, NASA said.
"We've got
a lot of confidence in tomorrow," Jett said.
NASA has
only one
remaining day - Sept. 9 - to launch Atlantis' scrub-ridden mission to jump
start ISS construction before settling into an extended delay to allow time for
a Russian Soyuz mission to swap out station crewmembers. The next available
launch attempt can occur no earlier than late September or early October,
shuttle officials have said.
Atlantis' STS-115
mission will deliver a $371.8 million pair of new
trusses and solar arrays to the ISS. The spaceflight will mark NASA's third
shuttle mission since the 2003
Columbia accident and the first major ISS
construction flight since late 2002.
Joining
Jett aboard Atlantis STS-115 spaceflight are pilot Chris
Ferguson and mission specialists Joseph
Tanner, Daniel
Burbank, Heidemarie
Stefanyshyn-Piper and Steven
MacLean - representing the Canadian Space Agency.
Sensor
issues
Today's
scrub comes after a series of delays for Atlantis that culminated in the
failure of one of four liquid hydrogen fuel gauge sensors in the orbiter's
external tank.
Known as engine
cutoff (ECO) sensors, the small detectors measure the amount of fuel
remaining in a shuttle's external tank, and are designed as a backup system to
shut down an orbiter's three main engines before propellant runs out.
ECO sensor
No. 3 inside the liquid hydrogen vessel at the bottom of Atlantis' 15-story
external tank failed a standard prelaunch check, in which launch controllers direct the
fuel gauge to read 'dry' - indicating an empty tank - but the sensor continued
to read 'wet'.
If
the more than one ECO sensor reads wet when the fuel tank is actually dry
during launch, which would require a number of sequential failures, the engines
could rip
apart and cause catastrophic damage. A false 'dry' reading when fuel
actually remains could shut down Atlantis' engines prematurely before the
shuttle reaches its intended orbit.
NASA
flight rules typically call for four working sensors in both the liquid oxygen
and liquid hydrogen fuel tanks, but the regulations do have an exception that
would allow a Saturday launch. That approval hinges on the results of
additional sensor checks as engineers drain Atlantis' external tank tonight and
refill it early Saturday.
If
all four sensors work fine, or if the problem occurs exactly as seen today,
Atlantis could launch on Saturday, NASA spokesperson June Malone told SPACE.com.
That
same rationale allowed NASA's space shuttle Discovery to launch
its STS-114 return to flight
mission in July 2005 despite a 13-day
delay due to a similar ECO sensor problem. The sensors, all tracked to a
batch manufactured in 1996, were later replaced with new versions in future
shuttle external tanks.
Malone said
the external tank draining and refilling process will give shuttle engineers a
critical look at the health of its ECO sensors.
"They
are very valuable," NASA spokesperson June Malone told SPACE.com of the
additional sensor checks. "It's simply more data to measure the sensor
performance."
Delays a
plenty
Atlantis
has suffered a series of inopportune delays from its initial Aug. 27 launch
date. The flight has been postponed due to a lightning
strike to the shuttle's launch pad, weather threats from a tropical
depression and - on Wednesday - a power
glitch in a pump motor that helps cool one of the spacecraft's three vital
fuel cells. The fuel cell issue led shuttle officials to scrub
a Sept. 6 launch attempt, but was not in violation of any flight
rules.
There were
no additional fuel cell pump motor issues akin to that which prevented a Sept.
6 launch attempt for Atlantis, NASA spokesperson George Diller said.
"It's an
extremely complicated machine, the space shuttle," NASA astronaut Robert Satcher told SPACE.com
Thursday. "It takes a lot of people with expertise to make it work, and they do
a very good job at it."
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