On April 12, timed to coincide with the 30th anniversary of the first Space Shuttle launch and the 50th anniversary of Yuri Gagarin's historic 1961 spaceflight, NASA will announce where the three remaining orbiters will be displayed in their retirement. After 135 launches, the program will come to an end. The Obama administration and the NASA Authorization Act of October 2010 have cut a chunk of NASA's funding in an attempt to shift the development of space vehicles to private corporations. Until another U.S. launch vehicle is built, NASA crews will visit the International Space Station using Russia's Soyuz spacecraft.

Conceived in the early 1950s, before the Apollo missions of the 1960s, the Space Shuttle program was in the works for decades before the first orbiter launched from Florida's Kennedy Space Center. The goal was to build an orbital spacecraft -- the first -- that could withstand repeated use. Mission accomplished. Over the past three decades, Space Shuttle crews have been responsible for launching numerous satellites and interplanetary probes, servicing and constructing space stations and conducting various space science experiments.

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A Space Shuttle is made up of a number of large components, including solid rocket boosters and an external tank known as the stack. But the most iconic piece is the Orbital Vehicle, the large white bird that carries the astronauts and payload and detaches from the tank after reaching speeds of 17,500 mph. Over the years, six Orbital Vehicles were designed and built: Enterprise, Columbia, Challenger, Discovery, Atlantis and Endeavour. Enterprise, the first of the six, was specifically constructed for test flights in the atmosphere. Without a functional heat shield and engines, Enterprise is not built for spaceflight. Original plans called for the orbiter to be modified after test flights were completed so that it could withstand spaceflight, but design changes in the program made that impractical.

Today, Enterprise sits inside of the Smithsonian Institution's National Air and Space Museum's second facility, the Steven F. Udvar-Hazy Center at Dulles International Airport in Chantilly, Virginia. Last year, engineers evaluated the Enterprise and determined that it is safe to fly on the Shuttle Carrier Aircraft, a heavily modified Boeing 747 that NASA uses to transport orbiters. It could take flight once again as the National Air and Space Museum is all but guaranteed Discovery and, once that announcement is made, it's expected that the museum will begin preparations to loan out Enterprise to other institutions.

That leaves two orbiters unaccounted for -- Challenger was destroyed during a launch failure and Columbia on its voyage home -- Endeavour and Atlantis. They're in demand. In December 2008, NASA released a "request for information" to museums, educational institutions and other relevant organizations to gauge who might be interested in acquiring one of the retiring space shuttles. At the time, NASA estimated that "it would cost about $42 million to prepare [each] vehicle and deliver it via a modified 747 Boeing aircraft carrier," according to the Houston Chronicle. While the final figure is expected to be closer to $28.8 million to cover transportation, each interested group was asked to submit a proposal that included guarantees to cover additional costs: the orbiters must be kept and displayed indoors and funds must be provided for the ongoing care and upkeep.

That's a big price tag to obtain one of NASA's prized shuttles, but it could pay for itself rather quickly. It's been estimated by a Houston group that the decision could be worth more than $45 million in annual economic impact and could create about 750 jobs, according to Development Counsellors International, a marketing firm working with the Johnson Space Center to bring one of the orbiters to Texas.

Twenty-one different institutions were quick to submit a proposal in an attempt to win one of the two orbiters. (Some reports claim there are now 29 competing institutions, but fewer than that are serious contenders. We've collected them in the gallery above.) Only three will be selected, but they might not see an orbiter for another year or so.

Called "down processing," the act of preparing an orbiter for display in a museum is a long, complicated and expensive ordeal. It involves making sure that any toxic residue left behind by the hydrazine rocket propellant, is removed. "You don't want a situation where, 10, 20 or 30 years from now, hazardous materials start outgassing, and tourists start passing out," Robert Pearlman, editor of collectSPACE.com, told SPACE.com. It also involves sending in technicians to remove many of the orbiter's key components. NASA will keep the original engines, for example, replacing them during down processing with replicas. These, and the other components that NASA's technicians excise from the orbiter's bodies will be put into storage and used for future programs. Many of these pieces contain ammonia, monomethylhydrazine, nitrogen tetroxide and other chemicals that no museum would want in their display rooms.

After the winning institutions are announced on April 12, there's not much that NASA will be able to do to appease those who won't be receiving an orbiter. Most of the other pieces of the program have already been promised to various institutions and organizations. The Adler Planetarium in Chicago will receive the Mission Simulation and Training Facility's fixed simulator, for example. The Training Facility's motion simulator will go to Texas A&M's Aerospace Engineering Department; the full fuselage mock-up from the Space Shuttle program will be housed at Seattle's Museum of Flight; and other parts of the program will be divvied up between the National Air and Space Museum, the National Museum of the U.S. Air Force, Florida's Wings of Dreams Aviation Museum and the Virginia Air and Space Center.

For the 18 institutions that won't be awarded one of the orbiters, there's always a NASA tile. The agency is making about 7,000 tiles from the Space Shuttle's thermal protection system available to institutions -- but also all interested schools and universities around the world -- at a cost of $23.40 each. But even those are in demand: There's a limit of one tile per organization.

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