The space shuttle Discovery made its final voyage today, taking off atop a modified 747 for a trip to Washington, D.C. where tourists will be able to visit the spacecraft at the Air and Space Museum's Virginia annex.

The United States retired its space shuttles last year after finishing construction of the $100 billion International Space Station, a project of 15 countries, to begin work on a new generation of spaceships that can carry astronauts to destinations beyond the station's 240-mile-high (384-km-high) orbit.

Discovery, the fleet leader of NASA's three surviving shuttles, completed its last spaceflight in March 2011. It was promised to the Smithsonian Institution's National Air and Space Museum in Washington, D.C., the nation's official repository for space artifacts.

"It's sad to see this happening," said NASA astronaut Nicole Stott, a member of Discovery's final crew. "But you look at it and you just can't help but be impressed by it. That's my hope now, that every time someone looks at that vehicle they are impressed, that they feel that this is what we can do when we challenge ourselves."

For its last ride, Discovery took off not from its seaside launch pad but atop a modified Boeing 747 carrier jet that taxied down the Kennedy Space Center's runway at dawn. The shuttle's tail was capped with an aerodynamically shaped cone and its windows were covered.

"It's a very emotional, poignant, bittersweet moment," said former astronaut Mike Mullane, a veteran of three space shuttle missions. "When it's all happening you think, 'This will never end,' but we all move on."

After a loop around Washington, D.C., the shuttle carrier plane was scheduled to touch down at Washington Dulles International Airport between 10 a.m. and 11 a.m. EDT. Discovery, which first flew in August 1984, will then be transferred to the Smithsonian's nearby Steven F. Udvar-Hazy Center in Chantilly, Virginia.

Discovery will replace Enterprise, a prototype orbiter on display at the museum that was used for atmospheric test flights in the 1970s.

It is ironic that this would be the last voyage of one of the last shuttles (the other two shuttles are headed for museums in New York city and the space center in Florida).The very first shuttle - Enterprise - was also flown on the back of a 747 in 1977 and was used to test the craft in earth's atmosphere. And before the shuttle runway was built at Kennedy Space Center, all shuttles were ferried from Edwards Air Force Base to Florida, piggybacking on top of its 747 mother ship.

The shuttle stopped flying last year when NASA retired the aging space taxi from service. Then, as now, there are no realistic plans to replace the shuttle and keep Americans in space.

The manned program is a mess. NASA is spending $2 billion a year on a rocket that doesn't have a mission, but that maintains jobs in states where powerful lawmakers keep the gravy train running. The rocket is supposed to be married to a crew capsule that is far behind schedule and may be canceled. Currently, we must depend on the Russians to get Americans to the space station. This will be the case for years to come - at least until private commercial companies can develop the vehicles to ferry Americans back into space.

For that to happen, NASA has to "man rate" the spacecraft that will do the job. A likely candidate is a SpaceX vehicle, the Falcon 9 which has already received funding from NASA to build a prototype man-rated spacecraft capable of taking astronauts to and from the space station. It may be ready as early as 2015 but that may be optimistic.

But as a sign of progress, an unmanned commercial cargo vehicle built by SpaceX is scheduled to make a demonstration launch to the space station on April 30 of this year. The Dragon will be the first commercial rocket to dock at the station and is part of the ongoing COTS program (Commercial Orbital Transportation Services). If successful in a number of such demonstrations, the Dragon will eventually be making regular re-supply trips to the space station.

Other private concerns like Virgin Galactic and XCOR Aerospace are well along in planning for space tourism flights. Bigelow Aerospace is ambitiously planning for a space station of its own having launched a couple of inflatable modules based on an old NASA design. There is no doubt we are on the cusp of a revolution in manned spaceflight that will see private companies able to safely launch men and cargo into space for far less per pound than NASA can manage. What they need is encouragement from NASA and not obstructionism which has been the hallmark of the relationship between public and private space ventures in the past.

There are some analysts who believe the end of the shuttle is necessary for the commercial space industry to find itself. This may be so, but it doesn't change the fact that for the foreseeable future, America will not have its own manned space program for the first time since the 1950's. It's a sad commentary on our government that this state of affairs was allowed to happen. But perhaps we should look at this pause as a means to gather ourselves for the next great leap forward into space - to the moon and beyond.