After the shuttle Columbia broke up over Texas, news reports were filled with startling images and descriptions of its debris strewn over a wide area: a round tank in an empty parking lot, mangled metal objects along roadsides, and the like.

Nearly 84,000 pieces of debris, weighing a total of nearly 85,000 pounds, were found over more than 1 million acres in Texas and Louisiana.

You couldn't help but think: Isn't it a miracle that the falling objects didn't seriously hurt anybody?

Even NASA Administrator Sean O'Keefe, testifying before a Senate committee in May, called it ``amazing.''

The Columbia Accident Investigation Board members wanted to find out just how amazing, so they ordered a study. Their finding: The chance of at least one person on the ground being seriously hurt by the falling debris was 9 percent to 24 percent.

The conclusion was included in Tuesday's report on the cause of the Columbia accident.

The study itself was not released and the researchers were not named. But Bill Ailor of The Aerospace Corporation of El Segundo, Calif., who studies debris from re-entering spacecraft, said the conclusion made sense to him.

``To me it seems like a reasonable number,'' he said. ``Maybe it could be a little lower. I would be surprised if it was much higher.''

Robert Culp, a professor of aerospace engineering sciences at the University of Colorado-Boulder, who also studies space debris, said he considered the estimate much too high.

The board said the study considered factors like where the debris fell, the total weight of all the recovered objects and census figures for the area where shuttle fragments were found.

The area is sparsely populated, with an average of about 85 people per square mile, the board said. It's not clear how many people were outdoors at the time, and the report noted that relatively few pieces of recovered debris posed a threat to people indoors.

The study also concludes that if a similar shuttle breakup occurred over a densely populated area like Houston, ``the most likely outcome would be one or two ground casualties.''

Ailor and Culp said that estimate sounded reasonable.

``The history of U.S. space flight has a flawless public safety record.... It is unlikely that U.S. space flights will produce many, if any, public injuries in the coming years,'' the Columbia report said.

But it noted that the Columbia disaster shows that a shuttle breakup has the potential to harm people on the ground. NASA had not made any documented effort to assess the public risk from shuttle re-entry before the Columbia accident, but it is now working on a national policy for protecting public safety during all operations that involve space vehicles, according to the investigation board.

NASA should be legally responsible for public safety, the accident report says, ``during all phases of shuttle operations, including re-entry.''

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On the Net:

Columbia Accident Investigation Board: www.caib.us

NASA: www.nasa.gov