With NASA's oldest and most-flown space shuttle now delivered to its museum retirement home, the commercial future of American human spaceflight is taking center stage.

On Thursday, NASA delivered the shuttle Discovery the fleet leader with 39 space missions under its belt to the Smithsonian Institution's National Air and Space Museum in Washington, D.C. Discovery will help teach visitors about the nation's manned spaceflight past, which promises to be much different than its future.

NASA is now looking to the private sector to ferry its astronauts to and from the International Space Station, and it's hoping that at least two companies have orbital taxis up and running by 2017. The hope is that competition can bring costs down, helping open up the final frontier for further exploration and exploitation.

"We look to keep competition for as long as we can, to service (the) space station with crew and cargo as soon as possible," NASA deputy chief Lori Garver told reporters last week. "We want to fly the space station longer than 2020; people have talked about this with our international partners. But we need to get the costs of servicing it and operating it down." [ Top 10 Private Spaceships Headed for Reality ]

Filling the shuttle's shoes NASA retired its space shuttle fleet in July 2011 after 30 years of orbital service. The United States is now dependent upon Russian Soyuz vehicles to ferry its astronauts to and from the station.

But NASA intends this dependence to be temporary. The agency is encouraging a handful of American private spaceflight companies to develop their own manned spaceships. Over the past two years, NASA's Commercial Crew Development program has given a total of $320 million to four American aerospace firms: SpaceX, Blue Origin, Sierra Nevada and Boeing.

SpaceX is using its share of the money to upgrade its Dragon capsule, which NASA also wants to make robotic cargo runs to the space station. In fact, Dragon is slated to launch on an unmanned demonstration mission originally slated for April 30 but now likely pushed back to May 7 during which it will berth with the orbiting lab and offload some cargo a first for a private spaceship.

If the flight goes well, Dragon could begin making bona fide supply runs to the station later in the year, SpaceX officials have said. The company holds a $1.6 billion NASA contract to fly 12 such missions in the coming years.

SpaceX has said that a crewed version of Dragon, which is designed to carry up to seven astronauts, could be operational within three years. Its progress will likely be accelerated by the robotic version's supply missions, according to company founder and Chief Executive Officer Elon Musk.

"Because there are so many similarities between the cargo version of Dragon and the crew version of Dragon, we're learning a great deal about crew transport when we do a cargo mission," Musk told reporters last week.

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NASA now counting on private space taxis