NASA managers cleared the shuttle Atlantis for blastoff Friday, weather permitting, to ferry a giant $617 million observatory into orbit to probe the most explosive objects in the universe.

The year's first shuttle flight also features the first spacewalk by American astronauts in more than five years, and NASA officials are especially eager to get Atlantis off the ground.If all goes well, commander Steven Nagel, 44, co-pilot Kenneth Cameron, 41, Linda Godwin, 38, Jerry Ross, 43, and Jay Apt, 41, will blast off on the 39th shuttle mission at 9:18 a.m. Friday, kicking off a five-day mission.