Video: Shuttle Discovery’s Final Flight

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KENNEDY SPACE CENTER, Florida (CNN) - You almost want to rub your eyes when you first see it. It doesn't seem real. It doesn't belong there. But there it is, the space shuttle Discovery mounted on top of a specially modified Boeing 747 airplane.

At first light Tuesday, Discovery flew out from the Kennedy Space Center one last time. In a salute to the past and all those who were part of the shuttle program, the aircraft flew over launchpad A, down the beach and over the space center visitor complex before heading north.

Discovery's next stop will be its last. The oldest of the three orbiters, with more than 148 million miles clocked, is going to the Smithsonian in Washington.

"Bittersweet," said Discovery's last commander, Steve Lindsey, "is an overused word, but it is sad." Lindsey and the five others who flew last February on mission 133 came out to say goodbye.

With every step toward retirement, the shuttle fleet becomes more a part of history. In 30 years of flying there were grand accomplishments and heart-wrenching tragedies. A space flying machine with wings, it was like nothing ever built.

But dwelling in that past would be a mistake, Lindsey said.

"We've got to move on, we've got to make sure that spaceflight doesn't die in this nation," he said. "We still have (the) space station going, but if we don't get ourselves heavy lift, get going with exploration or part of what I'm working on -- the commercial program -- then we risk losing this as a nation, and I don't want to do that."

In some ways, the past is meeting the future here. Just a few miles to the south at Cape Canaveral, Space X is in its final preparations to launch its Dragon spacecraft. It is a hugely critical test scheduled for the end of April. Space X hopes to be the first commercial company to rendezvous and then berth with the international space station.

Next year Space X plans to start ferrying cargo to the station and, in four years, U.S. astronauts.

Alvin Drew, a mission specialist on Discovery, said these companies vying to pick up where the shuttle left off are taking a leap of faith.

"These guys who run the commercial companies will tell you with the money they could have been there in 2015 if the money was there," Drew said. "You tie yourself to government funding, you are making a tough deal, because there's no guarantee the succeeding administrations or congresses are going to continue your funding."

Commercial companies say their new vehicles will be many times safer than the shuttles. It has to be that way now, Drew said.

"We had bigger budgets and a bigger tolerance for failure and loss of life back in the '60s and early '70s than we have in this particular generation," Drew said. "So the shuttle was built for that generation of explorers and I'm not sure it fit well in our current society or current culture. The risks you would take for the shuttle I think are higher than most people are willing to accept in 2012."

When Discovery gets to Washington, it will replace Enterprise, which now sits in the Smithsonian. Enterprise, a test shuttle that never flew in space, will go to New York and eventually into its new home at the Intrepid Museum.

The shuttle Endeavour will, by the end of the year, be heading to the California Science Center in Los Angeles. Atlantis will take up permanent residence at the Kennedy Space Center Visitor Complex. That will be the final exclamation point to the end of an era of space exploration.