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One of these three vehicles will be NASA's spacecraft of the future

NASA is currently weighing three options for the vehicle that will replace the Space Shuttle as its sole means of putting astronauts into space.

On Thursday, the agency said its decision is imminent: sometime during August or September, it'll award one (or perhaps two) contracts to the three companies in competition.

Since 2011, when the Space Shuttle program was retired, NASA has been entirely dependent on Russia for transporting its astronauts to and from the International 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.

We're now at a critical juncture, as we're about to finally have a firm idea of what the craft will look like. We know it'll be capable of carrying at least seven astronauts to and from the Space Station — but beyond that, it could take one of three different shapes.

Here are the three vehicles in development, built by SpaceX, Boeing, and Sierra Nevada, respectively. At this point, it seems that SpaceX and Boeing are in the lead, but it's still hard to say which design will be selected.

SpaceX's Dragon V2

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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. Reusability is a key strategy that could help drive down the cost of space travel.

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.

Boeing's CST-100

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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 would 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.

The CST-100 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. Working in Boeing's favor, though, is the company's decades of collaboration with NASA on a huge range of projects, including the Space Shuttle and the International Space Station.

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

Sierra Nevada's Dream Chaser

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The Dream Chaser, shown before a test at Edwards Air Force base in California. (NASA)

The Dream Chaser's design is radically different than the other two capsules in contention. It was developed by the Sierra Nevada Corporation, an aerospace electronics company that bought SpaceDev, a private spaceflight company, in 2008.

In many ways, the Dream Chaser is a scaled-down version of the Space Shuttle: it's a spaceplane that would be lifted to space by a rocket, but would execute a controlled landing on a runway upon return.

The Dream Chaser has also been put through a few flight tests — both tethered to helicopters and dropped in free flight — but the latter ended poorly, as the Dream Chaser's landing gear did not deploy, leading it to skid off the runway.