The United States has been a space-faring nation for just over 50 years, ever since Alan Shepard's suborbital pop shot aboard Freedom 7 on May 5, 1961. In the following eight years, the US, and mankind, went from being earthbound to making the first lunar landing.

Projects Mercury, Gemini, and Apollo were followed by the longest lull in America's manned space program to date, but starting in 1981 with the maiden flight of Columbia, the space shuttle program became the stalwart backbone of America's manned space flight program. With the final planned shuttle mission only hours from its scheduled launch, we take a moment to look back at over 30 years of history, achievements, and tragic failures.

The Space Shuttle—solid rocket boosters, external tank, and orbiter stack—is one of (if not the most) complicated machine ever built. It is simply an engineering marvel, but the complexity comes at a cost. I recall hearing that the Saturn V moon rocket had over a million separate components; that meant even a 99.9 percent success rate for parts left over 1,000 parts to fail. The shuttle is far more complex than a conventional Saturn V; ponder the perfection needed to keep such a machine flying and spaceworthy.

Early estimates by managers at NASA put the risk of catastrophic failure during a mission at 1 in 100,000; engineers, on the other hand, put the estimate at 1 in 100. A study recently released using all the data available from 30 years of flights reveals that the danger was in fact much higher.

According to the report, the chances of catastrophic loss of craft and crew during the first nine flights was a pucker-inducing 1 in 9. Later flights and safety improvements decreased this risk to 1 in 90. Given these numbers, NASA statistically had a only a six percent chance of completing 25 flights without the loss of an astronaut crew!

Even given those dire odds, astronauts bravely took bold steps beyond the terrestrial confines in order to count themselves among the few in human history who have traveled to outer space. Many of them did so on the shuttle.

The Post-Apollo era

As Eugene Cernan stepped off the ground of the Taurus-Littrow valley and onto the ladder of the lunar module Challenger, he became the last man to step foot on another astronomical body. His mission, Apollo 17, took place in December 1972; 39 years later, we haven't left the confines of low Earth orbit again.

After the cancellation of Apollo, NASA used the leftover Saturn rockets to perform two classes of missions. First was the launch and manning of Skylab, America's first and only space station. Three manned missions were flown to Skylab in 1973 and 1974, allowing astronauts to live and work in the small space station for a total of 171 days. The last flight of Apollo was in July 1975, for the Apollo-Soyuz test project. While the Cold War simmered on earth, US astronauts and Russian cosmonauts met in orbit and shook hands above France.

But after the Apollo-Soyuz Test Project, NASA conducted no manned spaceflights until the first flight of the shuttle program six years later.

Development

People started thinking about the craft that would become the Space Shuttle long before anyone set foot on the Moon. The idea of a spaceplane that would land horizontally has its foundations in the X-15 program of the 1950s, but serious discussion of a space shuttle began in 1969, when the National Aeronautics and Space Council met to discuss the direction of US manned spaceflight in a post-Apollo era. The council seriously considered four choices: a manned mission to Mars, further lunar missions and exploration, low earth orbit infrastructure (Earth-to-orbit shuttle and space station), and the disbanding of manned flight altogether.

The group decided on the construction of a low Earth orbit infrastructure. While this has brought numerous benefits, it has also trapped us in an awkward space-faring adolescence, with nowhere in particular to go. However, this decision left NASA with a number of goals and research avenues to pursue.

The first issue became what the primary focus should be: the construction of an orbital space station or the creation of a new launch platform? Even if the focus were to be a space station, questions remained about how to get the necessary parts to orbit.

One option was developing a new platform and building a space station over the course of many, many flights. An alternative was to continue building and using the existing Saturn V infrastructure—which could get the needed space station components to orbit in relatively few launches. In the end, the decision was made to go with a new, reusable launch platform. It was argued that, given enough launches, a reusable launch system would be the cheaper route.

Since the first days of Robert Goddard's liquid-fueled rockets, manned launch vehicles had followed a similar one-shot design. A powerful rocket, storing all its needed fuel to get to its destination—in one or multiple stages—would be discarded after its usefulness to the flight was over. Often, the only part to return to Earth in one piece was the crew module that kept the astronauts safe, and none of these were ever designed to be reused for spaceflight again. In the design of the space shuttle, the question became one of how reusable was it going to be?

In the end, the Shuttle design was a drastic departure from the multi-stage rockets of the prior NASA missions. The Space Shuttle is not a single vehicle; rather, it's what is referred to as the "stack": an external fuel tank (to keep the main craft's weight down and payload area up), a pair of reusable solid rocket boosters (chosen for their simpler design), and the orbital vehicle itself (a winged, lifting-body glider) powered by three liquid-fueled main engines. In this setup, both of the solid rocket boosters (SRBs) and the entire orbital vehicle were designed to be completely reusable.

In the 30+ years that the Shuttle program has been active, a total of five different spaceworthy orbiters—Columbia, Challenger, Discovery, Atlantis, and Endeavour—have been built and have flown over 130 missions. Those missions have included the construction of the International Space Station (ISS), which has fulfilled the second goal that the National Aeronautics and Space Council had chosen shortly after man first set foot on the Moon.