One of the four fuel tank sensors went on the fritz this morning while the huge external tank was filling up. NASA spokesman Kyle Herring says they could launch with just three working fuel sensors but mission managers decided they'd rather be safe than sorry.
"Well it's something that we've seen before. A couple of times before we've seen this same signature, and so we understand it, it's just a question of whether we wanted to take another 24 hours to look at the data, to ensure ourselves of being comfortable with that situation."
This mission to the International Space Station has already been delayed by other problems and weather, and this new delay until tomorrow is putting NASA in a tight spot with the Russians. Herring says they really need to launch tomorrow.
"At the present time tomorrow is the last launch opportunity. We have other vehicles on the runway, if you will, with the Soyuz launch in Russia, from Baikonur in Kazakhstan, and that's scheduled for around the 18th of September. There are rules that you don't have two vehicles at the station like that at the same time."
Tomorrow's launch is set for 10:15 AM Houston time, and Herring says if they have to scrub again, they won't be able to launch until October. This is a major mission for the space station, because Atlantis is carrying a large segment of the station and a solar array, which the Atlantis astronauts will install with at least three lengthy space walks.