More than 6,000 Americans have responded to NASA’s call for its 2013 class of astronauts, the US space agency reports. It is double the number NASA usually receives. \"We were a bit surprised, but very pleased by the overwhelming response to our recent Astronaut Candidate vacancy announcement,\" the head of NASA\'s Astronaut Selection Office, Duane Ross, commented. He said the agency usually receives between 2,500 and 3,500 submissions. In November NASA announced that it was looking for new recruits to staff missions to the International Space Station and to help develop the Orion spacecraft. The application period closed on January 27.