Countdown to lift-off

The Guardian's science editor Tim Radford explains the scale of the challenge facing the crew of the Discovery, which blasts into space tonight.

Right now — it's 12.30 BST — engineers in Florida are carefully pumping 256,000 gallons of superchilled hydrogen and oxygen into the external fuel tank of the space shuttle Discovery. Space.com describes it as a three-hour job.

It isn't one to hurry, because a lot rides on this mission.