While NASA may appear to have its pick of thousands of known asteroids for a manned mission, only two are good targets within the next 20 years.rnAn asteroid mission requires a large-enough destination that astronauts could reach within a few months of launch from Earth, says Lindley Johnson, head of NASA's Near-Earth Object program in Washington. Other limits to such an ambitious undertaking include the viewing range of ground-based telescopes.rn"They don't come all that close all that often," Johnson said at a near-Earth object (NEO) workshop NASA held in August.
展开▼
