NASA: Atlantis Launch Scrub Decision Down to the Wire

CAPE CANAVERAL, Fla. - NASA put offFriday's launch scrub decision for the space shuttle Atlantis as long aspossible to determine whether a fuelsensor issue warranted keeping the orbiter Earthbound for one more day, topshuttle officials said today.

According to NASA's own flightrules, questions over a problematic engine cutoff sensor in Atlantis' externaltank made today'slaunch scrub inevitable, though shuttle officials held out on the offchance that more data could make the space shot possible, NASA shuttle programmanager Wayne Hale told reporters here at the Kennedy Space Center (KSC).

"At the end of the day, we decidedthat staying with the plan we established...was the prudent thing to do," Halesaid.

Atlantis and its six-astronaut crew- which had already boarded the spaceplane before today's scrub- are now slated to launch towards the International SpaceStation (ISS) Saturday at 11:15 a.m. EDT (1515 GMT). Weatherforecasts predict an 80 percent chance of favorable flight conditions tomorrowfor the shuttle's STS-115mission to resume construction of the orbital laboratory.

Early Friday morning, engineersdetected an errant reading in one of four enginecutoff sensors (ECO) near the base of the liquid hydrogen portion ofAtlantis' 15-story external tank. The sensor, part of a backup system design toshut down Atlantis' three main engines if the orbiter's fuel tank runs dry,failed a standard preflight test to give a simulated 'dry' - or empty - readingwhen directed by launch controllers.

NASA has seen similar situations inthe past - an ECO sensor glitch delayed the launch of Discovery's STS-115return to flight mission for 13 days in 2005 - and designed a plan thatrequires a 24-hour scrub to retest the afflicted part as engineers drain, thenrefill, a shuttle's external tank.

"If everything is performing as weexpect and we just have one sensor continue to be a bad actor, we'll launchtomorrow," said Hale, who defended the decision to wait on the scrub decisionsince the astronaut and launch teams were in place and ready to discuss theissue. "We feel really good that we really need only three of those enginesensors to work."

But some mission managers andshuttle engineers felt there was a technical case for launching Friday despitethe fuel sensor anomaly and its related regulations, though they wereeventually overruled. Ken Bowersox, head of NASA's FlightCrew Operations Directorate, and STS-115 Mission Management Team (MMT) chairmanLeRoyCain - who carries the final say - opted to follow the flight rules, a movealso supported by several other non-voting members.

"We feel good about the fact that weexecuted the plan that we put in place," Cain said.

A tight turnaround

Tucked inside Atlantis' payload bay isa 17.5-ton pair of new trusses and two packed solar arrays awaiting delivery tothe ISS. The $371.8 million payload is slated to be the first major addition tothe ISS since late 2002. Space station construction was stalled in 2003 as NASAworked to recover from the Columbia accident.

The lighted flight window forAtlantis' STS-115 mission opened on Aug. 27 and has slowly ebbed away due to alaunch pad lightningstrike, a tropical depression, a fuel cell cooling system glitch and - mostrecently - the ECO sensor anomaly.

Saturday marks the last availableday to launch Atlantis, under the current lighted conditions required by flightrules, before NASA must stand down to avoid conflicts with the planned Sept. 18liftoff of an ISS-bound Russian Soyuz spacecraft carrying a new crew and onetourist to the orbital laboratory.

But shuttle padworkers have their work cut out for them to once more ready Atlantis for flightand avoid anticipated afternoon thunderstormsthat could stall launch pad activities later today.

Along with NASA's normal operationsassociated with a launch scrub - draining Atlantis' fuel tank, testing its ECOsensors, then ramping back up for more tests and the refueling process Saturday- shuttle officials also hope to replace a lost thruster cover near thespacecraft's nose to prevent rain water from collecting in its nozzle. But thatprocess would have strain NASA's already strained scrub turnaround timeline.

"If we don't get unlucky in terms ofthe timing of the weather, it sounded like we maybe could pull it off," Cainsaid. "It's very tight."

Meanwhile, Atlantis' STS-115 crew -commanded by veteran spaceflyer BrentJett - are scheduled to wake up at 11:50 p.m. EDT (0350 Sept. 9) tonight to prepare for their next launch attempt.

Mission managers plan to meet at 12:30 a.m. EDT (0430 GMT) tomorrow to decide whether to refuel Atlantis external tankand press ahead with a Sept. 9 space shot. If approved, external tank fueling would begin atabout 1:20 a.m. EDT (0520 GMT), according to a NASAschedule.

NASA will provide live coverage ofAtlantis' launch preparations beginning at 5:00 a.m. EDT (0900 GMT). You are invited to follow the briefing using SPACE.com'sNASATV, which is available by clickinghere or using the button at the left.

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