This is a highly controversial question. There are no documented instances of a meteorite striking an airplane, nor has the Federal Bureau of Investigation released any official statement on the likely effects of such an impact, either in general or in the case of Flight 800. (In a recent interview, FBI assistant director James Kallstrom stated that his agency had looked into the meteorite-impact theory and considered it possible but "highly unlikely" and noted that there is no evidence of such an occurrence.) Even the process of calculating the odds of a meteorite striking an aircraft is filled with uncertainties. For instance, which probability is more relevant: the (minuscule) odds that Flight 800 was struck by a meteorite, or the odds that a meteorite would have struck any airplane since the dawn of commercial aviation? Moreover, a meteorite impact need not be a catastrophic event. Presumably, it would cause a deadly explosion only if it hit a critical part of aircraft, a consideration which adds another unknown into the equation.

With those caveats in mind, here are three responses to the question:

David J. Helfand, chair of the department of astronomy at Columbia University, offers his insight into the question:

"On September 19, 1996, my colleague Charles Hailey and I published a Letter to the Editor of The New York Times in which we established the probability of a meteor striking a commercial aircraft with sufficient kinetic energy to damage the plane (the meteor would not have to be larger than a baseball, we guess). We did this calculation in response to the repeated public claims by officials who stated that the odds of such an event were hopelessly remote. Neither of us is an engineer, so we want to emphasize that we have no real insight into the probability that the plane would explode as a result of such a collision. We have only looked at the likelihood that a collision could take place.

"It is critical to pose this problem properly. It is an a posteriori calculation if we just talk about TWA Flight 800; the odds of that particular flight being hit by a meteor are very small indeed. But the appropriate question to ask in this situation is, 'What are the odds, throughout the history of commercial aviation, of a meteor striking an airplane such that it would penetrate the plane's fuel tanks?' We considered the number of planes in the world, the fraction of the time they spend in the air, their size, and the number of meteors entering the atmosphere per day, etc. Based on those numbers, we derived a probability of about 10 percent.

"We thought this estimate was worth considering (in light of the lack of evidence for other theories), although all attempts to contact officials were deflected. After the publication of our New York Times letter, we received dozens of previous, unpublished letters sent to the Times that offered the same theory, though none of these contained a probability calculation."

Gerrit Verschuur, an astrophysicist affiliated with Rhodes College in Memphis, Tenn., has recently published a book on the effects of meteorite impacts. Here are his thoughts:

"I suggested to the FBI a couple months ago that a meteorite could have struck TWA Flight 800; I also suggested how they might be able to demonstrate that this had occurred. They did not reply.

"Such an impact is extremely unlikely. The probabilities are very difficult to estimate. We do know that four or five automobiles have been struck by meteorites in the last half-dozen years. These events do not always get much notice in the media. On November 22, 1996, a meteorite struck Honduras, leaving the largest impact scar (a 165-foot crater) recorded on our planet this century, but the news did not get out for nearly three weeks. Eventually, the event was reported in British press. Our local media did not report it at all.

"As for evidence, I would re-interview all eyewitnesses to determine whether the so-called fireball reported by some observers before the explosion had a trajectory consistent with a meteorite. Depending on the angle of viewing, and if the object was rushing along the atmosphere from a distance, then one might get the illusion of something fired upward. CBS recently aired a news program about TWA Flight 800. On that program, several people who saw the alleged fireball, including a former Air Force pilot, were adamant that it was real. Their description sounded just like what people say when they see a daylight astronomical fireball, or bolide.

"Despite the incredibly small odds, I think the meteoric hypothesis for the downing of TWA Flight 800 is the best I have heard. Proving it would not be easy, however. If a small object penetrated the fuselage and gas tank of the airplane, it would have left two small holes bent inward. That area of the tank and the fuselage would be blasted apart, however. The small hole might be in the crucial pieces of metal that have not yet been recovered."

Here is another response from one of the leading experts on impact phenomena, David Morrison of the NASA Ames Research Center:

"It is certainly possible for a meteorite to strike a commercial airliner, although the probability is low. We can make a very rough estimate by comparing the area of airliners with the area of cars in the U.S. A typical car has an area on the order of 10 square meters, and there are roughly 100 million cars in the U.S., for a total cross-sectional area of about 1,000 square kilometers. The typical airliner has a cross-sectional area of several hundred square meters, but the number of planes is much smaller than the number of cars, perhaps a few thousand. The total cross-sectional area of airliners is therefore no more than 10 square kilometers, or a factor of at least 100 less than that of cars. Three cars are known to have been struck by meteorites in the U.S. during the past century, so it would appear that the odds are against any airplanes having been hit, but it is not impossible that one might have been.

"To my knowledge, there have been no reports of airplanes being struck, however. If one were hit, it would be more likely to occur on the ground than in the air, because airplanes spend more time overall on the ground. And finally, even if one were hit in flight by a meteorite, it would be unlikely to cause an explosion of the sort that ended the flight of TWA Flight 800 last year."