NASA just ordered its second commercial crew flight from Boeing — one of the two private companies that is building vehicles to transport astronauts to the International Space Station. That means Boeing is guaranteed to launch astronauts at least twice on its CST-100 Starliner vehicle. NASA ordered its first crewed flight from Boeing back in May, and in November, the space agency ordered a crewed flight on SpaceX's Crew Dragon. There's still no word on which company will launch NASA astronauts first.

Boeing and SpaceX are building spacecraft for NASA through the agency's commercial crew program. The initiative is aimed at getting American astronauts back into space on American-made vehicles again. Since the cancellation of the Space Shuttle program, NASA has relied on the Russian Soyuz rocket to get its astronauts into space, which costs NASA $80 million a seat. Boeing and SpaceX's vehicles, which are in the middle of development, will serve as a much more affordable, American option.

There is still one order that has yet to be placed

As part of the commercial crew contracts, NASA is guaranteed to order at least four crewed flights from the two companies. That means there is still one order that has yet to be placed. Orders are made two to three years before the launch date so the companies have enough time to manufacture and assemble their vehicles.

NASA has already picked the first astronauts who will ride on the commercial crew spacecraft, though no definitive launch dates have been set. As of now, they're supposed to happen sometime in late 2017. Recently, there was some concern that those launches might slip to 2018 due to budget concerns. But a recent budget bill passed by the Senate fully funds the commercial crew program for the first time since its inception — meaning there's a much better chance the first launches will happen on time.