NASA has ordered a second mission from SpaceX to send astronauts to the International Space Station. The order is part of the Commercial Crew Program, which was started by NASA six years ago as a way of making sure the United States continues to have human access to space. NASA ordered a first mission from SpaceX back in November of 2015, and it ordered two from Boeing — the other company involved in the CCP — in May and December of 2015.

NASA initially embarked on this program because the Space Shuttle program was scheduled to end in 2011. This left the agency with no primary space vehicle, and NASA has since had to rely on Russia's space agency, Roscosmos, to launch astronauts to the space station. That partnership has been effective, but it hasn't been cheap: NASA pays Roscosmos about $80 million per seat to get astronauts to the ISS.

The United States has been paying Russia to send astronauts to space for years

SpaceX and Boeing will be able to accomplish the same task for millions of dollars less once their spacecraft are completed and tested. NASA typically orders these missions two to three years before their launch date in order to give the companies enough time to develop the spacecraft (and the launch vehicles that will send them to space). Those first crewed CCP flights are loosely scheduled for late 2017 or early 2018, and SpaceX is currently building four versions of its crewed Dragon spacecraft in Hawthorne, California, for the missions NASA has ordered. Two of those four vehicles will be used exclusively for testing.

The Crew Dragon capsule is similar to the spacecraft that SpaceX uses to send cargo to the ISS, but it's fitted with life support and control systems. It can hold up to seven crew members, though NASA says a standard crew mission to the ISS will only carry "as many as four crew members and about 220 pounds of pressurized cargo." SpaceX plans to use a further-modified version of its Crew Dragon capsule to someday send humans to Mars.

NASA has not yet decided which of the two CCP companies will be the first to send astronauts to the ISS. In today's statement about the new order from SpaceX, the agency said that the decision will be made "at a later time."

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