Which near-Earth asteroids are ripe for a visit?

By John Matson

In April 2010, amid mounting criticism that his space plan lacked direction, President Barack Obama gave a speech in Florida to lay out a few ambitious goals he had in mind for NASA. The details of how those targets would be met remain somewhat sketchy even today, but the goals themselves were clear—sometime around 2025, the U.S. would perform an unprecedented feat. "We'll start by sending astronauts to an asteroid for the first time in history," Obama said.Where will the next robotic asteroid mission go, and where will asteroid-bound astronauts first touch down in the decades to come? A group of astronomers working through a census of near-Earth objects has a few suggestions