At 4:50 PM ET, the space shuttle Discovery will lift off. Finally. That is, if everything goes plan–an important caveat, given the fact that the launch has been repeatedly delayed since November 5th, thanks to poor weather conditions, electric problems, and a number of leaks found in the shuttle’s fuel tank.Fueling for the launch began today at 7:25 ET. The process takes three hours to complete.AdChoices广告The launch marks the final mission for the shuttle after 27 years. The mission will last eleven days, during which the crew will help stock the International Space Station with supplies and a new robot helper named Robonaut 2.