The space shuttle, still plagued by the problems that doomed Columbia, is slated to fly again in May. The shuttle program is due for retirement in 2010. What will the next era of human spaceflight look like?
Tom Jones, planetary scientist; former NASA astronaut; author, Sky Walking: An Astronaut's Memoir
Elon Musk, chief executive officer, Space Exploration Technologies (SpaceX)
Rick Tumlinson, co-founder, Space Frontier Foundation
George Whitesides, executive director, National Space Society