Officials worried about the safety of landing a space shuttle in the early days of the program, but their concerns decreased as mission after mission returned to Earth safely, a space historian said.

In fact, until the Columbia disaster Saturday, there had never been an accident during descent or landing in 42 years of U.S. human space flight.

The early concern about the shuttle was that, during its hour-long descent from orbit, it has no power.

"It's a glider and it has all the aerodynamic capability of a brick," said Roger Launius, chair of the space history division at the National Air and Space Museum.

"It comes down real fast and real hard."

While the vehicle can maneuver a bit to line up with the runway, the pilot can't turn on power and return to the sky for another attempt if something goes wrong, as an airplane pilot can, Launius said.

"Once you come down, you're not going back up," said Launius, who also is the former chief historian for the National Aeronautics and Space Administration.

In the 1960s and 1970s -- as the shuttle was developed -- NASA considered adding special engines so it could fly like a jet toward touchdown, but that was nixed because it made the vehicle too heavy, Launius said. And aerodynamic tests showed "they could make this thing work as a glider," he said.

One advantage of that is a reduced risk of explosion, since there are no jet engines blazing, he said. That's "strikingly different than launch, where you're sitting on a very powerful bomb (undergoing) controlled release," he said.

There was concern about the landing of the very first shuttle flight, in 1981. The Columbia was protected by thousands of tiles to shield it from the intense heat that builds up from air friction during re-entry. Some were damaged or lost before Columbia returned to Earth, and there were fears that the heat could enter through unprotected parts of the shuttle and cause a catastrophe, Launius said. But that didn't happen.

Over the years, landings had moved down NASA's priority list of concerns, Launius said.

That's apparently true among the public as well.

Half a dozen teachers training at a space center were in disbelief Saturday morning after seeing reports about the Columbia disaster.

The teachers were in the middle of a space simulation at the Challenger Learning Center in Radcliff, Ky., when they heard that contact had been lost with the shuttle.

"I'm just astounded, it's completely shocked me," said Danny Allen, a science teacher at Cumberland County High School in Burkesville. "Nobody worries about the landing."