Thursday marks the final launch of Space Shuttle Discovery, the oldest of NASA's three operational orbiters and the first to be retired.Since 1984, Discovery has flown 38 missions and will notch 39 with this week's blast-off. Thus far, the shuttle has spent a total of 352 days in space, has orbited the Earth 5,600 times, and has carried 180 people along the way, according to figures released by NASA.