July has always been a big month for America's space agency.

That's no different this year as NASA Administrator Charles Bolden announced the four American astronauts that have been selected to be the first to train to fly to space on commercial crew vehicles as part of the agency's plan to return space launches to U.S. soil.

"These distinguished, veteran astronauts are blazing a new trial, a trail that will one day land them in the history books and Americans on the surface of Mars," Bolden wrote on his blog Thursday.

The commercial crew astronauts are as follows: Air Force Col. Robert Behnken, until recently head of the astronaut office; Air Force Col. Eric Boe, part of shuttle Discovery's last crew; retired Marine Col. Douglas Hurley, pilot of the final shuttle crew; and Navy Capt. Sunita Williams, a former resident of the International Space Station.

The four astronauts will be on capsules built by SpaceX and Boeing. Each has test pilot experience and has flown twice in space. The companies are aiming for test flights to the space station by 2017. It will be the first human launches from Cape Canaveral since the shuttles retired in 2011.

NASA is on a mission to send Americans to Mars by the 2030s.

According to NASA, it currently costs $76 million per astronaut to fly on a Russian spacecraft. On an American-owned spacecraft, the average cost will be $58 million per astronaut.

"Our commercial crew initiative makes these parallel endeavors possible," Bolden wrote. "By working with American companies to get our astronauts to the ISS, NASA is able to focus on game-changing technologies, the Orion spacecraft and the Space Launch System rocket that are geared toward getting astronauts to deep space."

Information from The Associated Press was used in this report.