Meant by NASA to be the workhorses of a new era in space exploration, the space shuttle has been involved in two tragedies and a great leap forward in our understanding of the universe.

Unlike the rockets that brought astronauts to the moon, the shuttles are orbiters that have been designed for repeated use on missions into space. Intended to be used on as many as 100 launches, each shuttle also has the ability to carry satellites and other large payloads into space, and can transport astronauts to and from the International Space Station.

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In all, NASA built four shuttles for space flight - Columbia, Challenger, Discovery, Atlantis, and Endeavour. The first flight was made by Columbia on April 12, 1981.

Three of the shuttles still exist today. The Challenger shuttle exploded after takeoff on its 10th mission in 1986, killing all seven astronauts onboard, including a New Hampshire civilian, Christa McAuliffe. And the shuttle Columbia exploded as it was reentering the atmosphere in 2003, also killing its seven astronauts. After both disasters, review panels issued scathing reports, saying that pressures to complete missions and a culture of complacency had led to an underestimation of risks.

The shuttles consist primarily of three main components: the orbiter vehicle, an external fuel tank and two rocket boosters. After take off, as a shuttle rockets toward space, its tank and boosters are jettisoned back to earth, while the orbiter continues on. According to NASA, the external tank is made of aluminum-lithium and carries more than 500,000 gallons of liquid hydrogen and liquid oxygen. The rocket boosters are about 150 feet long and provide most of the thrust needed for liftoff. The orbiter, the part that actually carries the astronauts, is mostly made of aluminum alloy and has three engines mounted on its fuselage.

Perhaps the greatest triumph of the shuttle came in 1993, when astronauts carried out a complicated repair mission to install corrective optics in the Hubble telescope, whose ability to see far beyond the reach of any earthbound telescope had been clouded by a flaw in its mirror. A final repair mission to the Hubble is planned for 2008.

After decades of use, the shuttles have just about reached the end of their lives, and the Bush administration announced in 2004 that it was bringing the program to a close, and that the shuttle to be replaced by a new space vehicle that could help it achieve its new goal of transporting humans to the Moon by 2020 and, eventually, to Mars. NASA has said that the new vehicle, named Orion, is scheduled to start flying in 2015. NASA's administator, Mike Griffin, said in June 2007 that test flights could begin as early as 2013. (Anahad O'Connor, June 12, 2007)

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