The flight of Atlantis will mark the final mission of the 30-year U.S. space shuttle program. It's scheduled to blast off on Friday, although inclement weather may delay the launch a day or two.
When the winged orbiters began flying in 1981, the aim was to make space flight economical and routine. Instead, each mission cost about $1.4-billion in current dollars, and two of the five shuttles were destroyed in catastrophic accidents that cost 14 lives.
Even so, the shuttles have contributed enormously to our understanding of working in space and lifted into orbit tonnes of cargo and hundreds of individuals - including eight Canadian astronauts.
"The space shuttle is an incredible vehicle," said Canada's first man in space, Marc Garneau, pointing to its unique ability to land on a runway like an airplane "This is something that is not going to be duplicated for a very long time because we are going back to conventional rockets"
Why were the shuttles built?
After the United States beat the Soviet Union in the race to put the first man on the moon in 1969, the American government considered what to do for an encore. Richard Nixon, then U.S. president, rejected calls for an even more costly venture to send humans to Mars.
"With the entire future and the entire universe before us, we should not try to do everything at once," Mr. Nixon said in March of 1970, pointing to other pressing priorities on Earth.
He eventually chose a prosaic but practical option, the space shuttle - essentially a reusable space plane that would take off like a rocket but land on a conventional runway. Officials at the U.S. National Aeronautics and Space Administration promised the shuttle would bring down the sky-high cost of getting people and cargo to and from orbit. It was meant to normalize space flight, with launches almost weekly.
"Space flight was going to be an everyday occurrence for average Americans," said Bill Barry, NASA's chief historian. The shuttle was also considered to be an essential "workhorse" for building a space station - another long-term goal of NASA planners. However, the Nixon White House pledged just $5.5-billion to develop the shuttle, far short of the $13-billion requested by NASA.
Why were they so costly?
Building a reusable space shuttle proved far more difficult than first imagined - especially when the job was supposed to be completed for less than half the estimated cost.
One of the most challenging problems scientists had to overcome was protecting the delta-winged orbiter from the enormous heat of friction when it re-entered Earth's atmosphere at 40 times the speed of sound. The solution involved covering much of the 90-tonne space plane with about 24,000 custom-made silica-fibre tiles, which, although extremely light, could withstand temperatures of thousands of degrees. Sorting out these and other problems meant the project slipped years behind schedule.
Trying to create a Cadillac shuttle on a Corolla budget led to compromises that would ultimately raise operating costs and compromise safety. And even though some features were dropped - such as an emergency escape system and reusable liquid fuel tanks - cost overruns still pushed the development price tag to $10-billion. "Costs creep," Mr. Barry said. "That happens whenever you are pushing the edges of new technology."
The shuttles ended up taking longer than expected to prepare for another flight. The turnaround time was supposed to be two weeks; it actually took many months. Additional safety checks instituted after the Challenger accident led to more delays.
Once all the development and operational costs are taken into account, the 134 shuttle launches to date have cost an average of more than $1-billion per mission. Some estimates put the figure as high as $1.4-billion in current dollars, taking inflation into account.
Why were they so dangerous?
Two of the five orbiters met tragic ends - with the loss of both crews. The Challenger blew up shortly after liftoff on Jan. 28, 1986, and Columbia disintegrated during re-entry on Feb 1, 2003.
In safety terms, the space shuttle suffered from a major design flaw: The orbiter was strapped to the side of the rockets. Earlier U.S. space capsules - Mercury, Gemini and Apollo - all sat on top of their rockets. If a rocket blew up, the blast was at least behind the astronauts. With the shuttle, an explosion would surround the spacecraft, minimizing the chances of survival - which is what happened in the Challenger accident.
Furthermore, the side-mounted position of the orbiter increased the likelihood it would be hit by foam insulation falling off the liquid fuel tank during liftoff. In fact, a piece of insulation hit the leading edge of Columbia's left wing, damaging the thermal protection system. When it came time to return to Earth, the inside of the wing super-heated and the spacecraft broke apart.
Why are the shuttles being retired?
In January of 2004, then U.S. president George W. Bush announced his decision to retire the space shuttles as part of a plan to send astronauts back to the moon. They would remain in service only long enough to finish work on the International Space Station. Ending the shuttle program would free up cash so NASA would have the money for the new lunar venture without requiring a big budget increase.
Since that decision was made, however, President Barack Obama has axed Mr. Bush's space plans, which were already running behind schedule and over budget. Mr. Obama did not grant a reprieve to the shuttles. Instead, he has created financial incentives to encourage industry to build new rocket ships that will take American astronauts into orbit. It's hoped the private sector will be able to achieve what NASA failed to do: make space travel an affordable and commonplace experience.
In the meantime, American astronauts will be renting seats aboard Russian Soyuz rockets to get to and from the space station.
Why are they important?
Many space experts note that the shuttles did not live up to their billing. But the astronauts who flew on them cherished the experience - despite the risks.
"It has been a tremendous human invention that has opened the door of space to us," said Canadian astronaut Chris Hadfield. Service missions by shuttle astronauts have kept alive the famed Hubble Space Telescope, which provided dazzling images of the universe. (And don't forget Canada's shuttle contribution, the robotic Canadarm, which made it possible to capture and release the orbiting satellite.)
The shuttles also played an instrumental role in building the $100-billion International Space Station, which showed that many diverse nations - even former adversaries - can work together for a common good. Mr. Hadfield calls the shuttle "the great lifter of humanity."
Almost 70 per cent of the 523 people who have ventured into space since Yuri Gagarin's historic first orbit 50 years ago flew on the shuttle. Much of what has been learned in 30 years of operating the shuttles will prove useful in humankind's outward journey into space.