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September 08, 2006

Try Try Again--Saturday Launch?

This time it was a fuel sensor that kept the STS-115 crew from taking off on their mission to add a new solar array to the International Space Station. Their mission was originally scheduled for May 2003 and postponed because of the Columbia accident. Last summer’s STS-117 mission showed that the foam problem had not been solved, so the shuttles were grounded until this July. With STS-121 successful, STS-115 was cleared to resume the assembly of the ISS. The first launch attempt in August was scrubbed because of a lightning strike to the launch pad. Then Ernesto threatened the Florida coast, and the shuttle was almost to the hangar when that threat was cleared. Atlantis was put back on the pad and then a fuel cell pump caused another scrub Wednesday.

Friday was supposed to be the last day of this launch window, but the Russians agreed to hold their Soyuz flight one more day. Both partners would like to see the new solar arrays installed as soon as possible, but the Soyuz, that transports crew and also serves as the escape ship if there were an emergency, must be changed out every six months.

NASA plans to empty the fuel from the external tank and refill it overnight. If the sensor again fails the test, the computers will be instructed to ignore it (so it won't report that there is no fuel and shut down the engines prematurely—leading to possible disaster), and listen to the other two sensors. My understanding is that if the sensor passes the test, then the launch will likely be scrubbed to investigate why it failed before. So ironically, we should all hope that the sensor fails so the crew can launch.

If you find this rather confusing, you are not alone. When asked how it could be that a failure could actually give a “go” for launch, Miles O’Brien of CNN quipped, “After all, it IS rocket science!”

Hopefully, the rocket and Mother Nature will cooperate and let this batch of very patient astronauts get on with their mission. So far the weather forecast is promising for the 11:15 a.m. EDT launch window.

Marianne Dyson

Posted by m_dyson at September 8, 2006 01:25 PM



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