The final flight of the space shuttle marks the end of an era, leaving a vacuous trail to an uncertain future. Its absence will bring into focus potentially wrenching decisions ahead for the future of human spaceflight.

As an engineering achievement, the space shuttle is without peer, designed and built by the geniuses that sprang from the Apollo program. The sight, sound and feel of a space shuttle launch can only be understood by direct experience. The space shuttle is not just a rocket. It is a complex, reusable space transportation system--part rocket, part plane and part mini space station--that has enabled a crew of up to seven astronauts to conduct missions to learn how to live and work in space for weeks at a time, performing scientific experiments, deploying and repairing satellites of every conceivable type, assembling and supporting space stations and then returning back to Earth. The space shuttle has unique capabilities--such as the ability to grapple and service large satellites, perform major repairs on the space station, and return significant-sized cargo back to Earth--that will never be replaced and will probably be sorely missed someday. Although versions of these features could be built into future systems, it will come at a cost, and they're not likely to match the capability and versatility of the space shuttle. The shuttle's main task over the past decade has been to support the assembly of the International Space Station (ISS), which is now complete.

The space shuttle, though, is also a cultural icon. Just look around and take note every time you see a reminder of the space shuttle. Whether it's on TV, in the movies, on license plates, or on a kid's toy--it's just about everywhere. Symbols are important, and the space shuttle is an important symbol. To me, it symbolizes American ingenuity and technological prowess and a core belief that we are explorers and risk-takers looking to the next frontier. The space shuttle has also inspired countless young students to study engineering and science, probably its greatest legacy.

But the space shuttle program has fallen far short of its original goal of providing routine, low-cost access to space. The space shuttle was born in the waning days of Apollo. After planting flags on the moon to win the space race, NASA set its sights on making transportation to space routine, affordable and accessible. While these goals were laudable, in practice they were unachievable. The total cost of the space shuttle program since its inception in the early 1970s is estimated to be $174 billion. That's an average of about $4 billion each year, or about $1.3 billion for each of the 135 flights. While the space shuttle's unique capabilities were indeed impressive, the shuttle's costs were prohibitive, and the rationale for sending humans into space sometimes seemed conjured up simply to justify the program's existence. For the past decade or so, the quip was that we have the space shuttle to support the space station and we have the space station because we need somewhere to go with the space shuttle.