The head of the U.S. space agency says it was a mistake to switch to the space shuttle and International Space Station programs after the end of the Apollo moon missions of the 1970s.
NASA administrator Michael Griffin tells the USA Today newspaper the space shuttle was a design that was "extremely aggressive and just barely possible."
Fourteen astronauts have died since the first space shuttle flight in 1982.
Mr. Griffin also says he would not have gone ahead with the current space station "in the orbit we're building it in."
NASA unveiled a plan last week for the United States to send astronauts back to moon by 2018, utilizing elements of both the Apollo and space shuttle programs.