April 13, 2011 -- NASA’s space shuttle program is ending after two final flights this spring and summer, with no replacement vehicle in the immediate offing. But that doesn't mean NASA hasn't tried.
Here's a look at some of the programs that the United States started and stopped.
The follow-on program to the space shuttle was to be a return to the moon, but the Bush administration and Congress shorted funding for the program.
NASA got one test-flight of the new Ares rocket off the ground before the program was mothballed. Orion, the would-be moonship, could be reborn for future human missions to asteroids and other destinations beyond the space station.
In 1996, NASA pinned its hopes on a next-generation reusable launch vehicle that wouldn't need booster rockets and a disposable external fuel tank to get itself into orbit. But technical hurdles proved too daunting. NASA axed the program, also known as VentureStar in 2001 after spending nearly $1 billion.
In parallel with X-33 development, NASA tried to partner with industry to develop unmanned hypersonic prototypes to test technologies in orbit that would be needed for a fully reusable launch system. It was was to serve as a reusable launch vehicle for small payloads.
But attempting to satisfy both commercial launch needs and technology development proved impossible. The all-composite vehicles were to be launched from an Orbital L-1011 aircraft, which was used for three captive-carry test flights. Powered by a single rocket engine, the X-34 was designed to reach speeds of Mach 8.
The number of people who can live and work aboard the International Space Station is limited by the number of seats on the lifeboats to bring the crew home in case of emergency.
NASA planned for its seven-person X-38 to replace the pair of three-seat Russian Soyuz capsules used today for crew rescue. Johnson Space Center developed the vehicle in-house, conducting 48 test drops of the X-38’s 7,500 square-foot parafoil -- the largest ever flown. The European Space Agency, which was a partner in the X-38 program, was looking at modifying the lifeboat into a capsule.
The launch of a full-scale demonstration vehicle on the space shuttle had been scheduled when NASA, reeling from space station budget overruns, abruptly canceled the program in 2002.
NASA ran out of money for the X-37, also known as the Orbital Space Plane, which was intended to test technologies for a next-generation space shuttle.
The military took over the project in 2004, renamed it X-37B, or Orbital Test Vehicle, and last year launched the first unmanned robotic prototype into orbit. It flew for 224 days.
A sister ship was launched in March and remains in orbit on a classified mission.