NASA picked the two private spacecraft to replace the Space Shuttle

NASA has selected the two spacecraft that will replace the Space Shuttle — taking astronauts to the International Space Station beginning in 2017.

In a press conference Tuesday afternoon, NASA officials announced that both the SpaceX Dragon and Boeing CST-100 will move forward as part of the Commercial Crew Program. If successful, they will be the first private spacecraft to put humans in orbit.

if successful, they'll be the first private spacecraft to put humans in orbit

The two crafts were among three vehicles competing to replace the shuttle (the third was Sierra Nevada's Dream Chaser space plane, which was not selected). Both are reusable capsules that are launched atop single-use rockets and can carry up to seven astronauts at once. The plan is for both to complete two to six missions to the space station, carrying four astronauts each time.

Since 2011, when the Space Shuttle program was retired, NASA has been entirely dependent on Russia for transporting its astronauts to and from the space station, and it now pays $70.7 million for each one-way ticket.

NASA's plans originally called for a privately-developed American spacecraft to fill the void by 2015, but annual underfunding by Congress has delayed that to 2017 at the earliest. Recent tensions with Russia have made the issue more urgent, and NASA administrators have called for more funding to ensure they hit the 2017 target date.

Currently, the plan is for both the SpaceX and Boeing crafts to continue testing over the next few years, and begin ferrying astronauts in 2017. As part of the contract, Boeing will receive up to $4.2 billion and SpaceX will get up to $2.6 billion if each company completes six round-trip flights. They'll begin by certifying each craft for human spaceflight — through a series of tests, culminating in a crewed flight to the space station with at least one NASA astronaut.

Here's a look at each of the spacecraft.

SpaceX's Dragon V2

The Dragon V2 capsule, which SapceX hopes to use to transport astronauts to and from the International Space Station. (Robyn Beck/AFP/Getty Images)

The startup SpaceX — founded in 2002 by entrepreneur Elon Musk — has been using the initial version of its Dragon capsule to ship cargo to the space station since May 2012. It unveiled the upgraded version, equipped to carry people, this past May.

Both versions of the Dragon are lifted to space by one of SpaceX's Falcon 9 rockets. Currently, the rockets are not reusable (though SpaceX eventually hopes to change this), but the capsules are — and the new version of the Dragon can actually land on Earth using a set of thrusters, instead of crash-landing in the ocean.

Someday, SpaceX hopes to use the Dragon to take astronauts to other destinations — perhaps space tourists visiting inflatable space stations in development by the Bigelow Aerospace. Some have suggested that a modified Dragon could even be used for a mission to Mars.

In the meantime, though, SpaceX will continue using the Dragon to carry cargo to the International Space Station — beginning with a fourth cargo mission this weekend — followed by testing of the crewed Dragon V2 over the next three years.

Boeing's CST-100

A rendering of Boeing's CST-100. (NASA)

On the whole, Boeing's CST-100 capsule is relatively similar to the Dragon. It's a pyramid-shaped capsule that will be launched to space atop a rocket.

Initially, this would be the Atlas V rocket — which was developed by Boeing and Lockheed Martin and has been putting satellites in space since 2002 — but the CST-100 could eventually be compatible with other rockets as well. Musk has criticized Boeing for relying on Russian-made engines in launching the rocket, and in response, Boeing is reportedly in talks with the space startup Blue Origin to develop an American-made replacement engine.

The CST-100 capsule isn't quite as far along as the Dragon: Boeing has conducted some tests of the capsule, dropping it from 14,000 feet over the Nevada desert, but it hasn't been put into space yet, let alone orbit. However, several further launch tests are planned for the next few years.

Boeing also hopes to eventually use the capsule for space tourists, and developed it in collaboration with Bigelow Aerospace partly for this reason.

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