NASA will make a major announcement on the future of U.S. human spaceflight today (Sept. 16), and you can watch it live online.

Space agency officials will hold a press conference at 4 p.m. EDT (2000 GMT), and could unveil the selection of one or more private space taxis by aerospace companies to move forward on the path toward providing astronaut transportation services to International Space Station for NASA.

You can watch NASA's commercial crew announcement live on Space.com, courtesy of NASA TV.

According to a NASA statement, today's announcement will discuss "the return of human spaceflight launches to the United States. The agency will make the announcement during a news conference from NASA's Kennedy Space Center in Florida."

NASA's Commercial Crew Program calls for the agency to rely on commercial spacecraft by private spaceflight companies to launch American astronauts on trips to and from the International Space Station by 2017. At the moment, four U.S. spaceflight companies — SpaceX, Sierra Nevada, Boeing and Blue Origin — are all in the running for the crew transportation contracts.

All of the companies have received some NASA funding to help develop their spacecraft concepts and technologies, with SpaceX, Sierra Nevada and Boeing splitting a total award of $1.1 billion in 2012 to spur their projects. NASA's next stage of the program is known as the Commercial Crew Transportation Capability.

SpaceX (short for Space Exploration Technologies) is based in Hawthorne, California, and was founded in 2002 by billionaire entrepreneur Elon Musk. The company is developing a manned version of its robotic Dragon space capsule to launch on its own Falcon 9 rockets as its bid for NASA's crew contract. SpaceX's manned Dragon vehicle, called Dragon Version 2, is a seven-person capsule and was unveiled in May.

Boeing, meanwhile, is developing its own capsule: the Commercial Space Transportation 100, or CST-100, capsule. That capsule is a seven-person vehicle designed to launch on expendable Atlas 5 rockets.

The Colorado-based Sierra Nevada Space Systems is building the Dream Chaser space plane, a seven-person spacecraft that will also launch on an Atlas 5 rocket. But instead of choosing the capsule route, Sierra Nevada is building a reusable winged spacecraft that resembles a miniature space shuttle. The project is based on NASA's past research into the HL-20 lifting body concept.

The secretive company Blue Origin, founded by Amazon.com CEO Jeff Bezos, is building a conical spacecraft called Space Vehicle. It received $22 million NASA funding in 2011 to develop its concept and a spacecraft escape system.

Today's announcement could reveal which U.S. firm (or firms) gets to move ahead in NASA's Commercial Crew Program. Although many people expect that the space agency will tap one company, NASA officials could pick more than one to move on.

But to launch American astronauts into space, Russia's Soyuz capsule is the only vehicle currently available. NASA currently pays about $70 million a seat to fly astronauts to the station on the three-person Soyuz capsules.

NASA Administrator Charles Bolden, Kennedy Space Center Director Bob Cabana, Commercial Crew Program Manager Kathy Lueders and astronaut and former space station crewmember Mike Fincke will take part in the NASA briefing from Florida. The space agency will also host a teleconference for members of the press that will air on NASA's news audio webcast service after the initial briefing.

Visit Space.com today at 4 p.m. ET for complete coverage of NASA's U.S. human spaceflight announcement.

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