Space shuttle launch in doubt as fuel tanks are filled

In the past few hours Nasa's launch team have filled the external tanks of space shuttle Atlantis with more than 535,000 gallons of liquid hydrogen and liquid oxygen in preparation for launch on Friday morning local time. The chances of launch are still low, around 30%, due to adverse weather conditions that are expected to come into Cape Canaveral in the next few hours.

Filling the shuttle's famous 45-metre-long orange external tank with fuel, a procedure called tanking, started at 2am (EDT) and took just under 3 hours, finishing ahead of schedule at 4:48am Florida time. The pressurised liquid hydrogen is kept in the tank at -253C, and the liquid oxygen at -183C until they meet and burn in the orbiter's main engines.

There are no technical impediments for the scheduled launch at 11:26am (4.26pm BST) but Nasa officials are closely monitoring the weather. In the small hours of Friday morning the four astronauts who will fly on the last ever shuttle mission, designated STS-135 and intended to take supplies to the International Space Station, ate breakfast and completed final medical examinations. A short time after 6am, they will make their way to the launch pad, to be strapped into the Atlantis orbiter.

On Thursday Nasa said it was investigating the effects of a possible lightning strike that occurred a third of a mile from the launch pad amid torrential rain. Engineers reviewed data, the agency said in a statement, and inspected the rotating service structure, which provides access to the orbiter on the launchpad and has to be rolled back before launch. There is no indication, however, that this has had any impact on the launch time. The countdown can be aborted up to 9 minutes before the scheduled launch.

Countdown commentator for Nasa, Allard Beutel, told CBS News that some people might call it "silly to try and play in the rain this morning, but he said we're going to absolutely try for tanking. But throughout the overnight and closer to the dawn early morning hours, between six and seven o'clock, they'll keep their eyes on the forecast, of course, all night long, but around that time, they'll start getting a very focused look on the weather and see whether it's the right thing to proceed towards launch or whether the forecast really is getting worse. We'll look at perhaps standing down at that point."

If Atlantis misses its launch window on Friday, there are additional opportunities to launch on Saturday and Sunday mornings, when the chances for favourable weather increase to around 40% and 60% respectively. If the delay continues after that, the next window for launch is likely to be Saturday 16 July.