Sneaking up on Earth like a cosmic stealth bomber, a meteorite exploded in a brilliant flash that has blown away scientists' preconceptions about such objects. It also has reinforced concerns about the perils of being a potential target in the cosmic ''shooting gallery'' of asteroids, comets, and meteor streams through which Earth travels.
Actually, meteoric fireballs are commonplace. What made this one special is that it exploded in the atmosphere April 8, 1989 right over the Arecibo (radar and radio telescope) Observatory in Puerto Rico. And it did it during a research program when instruments caught the unexpected event. Now, after detailed analysis of the data, scientists realize they confronted a rare object.
Astrophysicist David Meisel of the State University of New York at Genesco, space scientist John Mathews of Pennsylvania State University at University Park, and several colleagues explain that analysis in the August issue of the journal Icarus. They had expected an ordinary meteorite - dark, dense, and rich in iron. Instead, the fireball showed that the object was low in iron and rich in magnesium silicate. It was a chalky white meteorite of which only three have been found on the ground.
Dr. Meisel notes that ''meteorites usually show signs they formed in intense heat, but this one was made up of pieces stuck together by gravity.'' He adds that it ''likely splattered off a much larger'' object.
Commenting on this, Dr. Mathews says it is interesting scientifically to study a meteorite with such unusual mineral composition. But he emphasizes what he calls the ''more practical fact'' that this was a previously unknown near-Earth object. That means it was in an orbit that repeatedly brought it close to Earth and, finally, right into the atmosphere.
To use Mathew's phrase, the unusual meteorite ''may be indicative'' of other unknown objects in near-Earth orbits. Moreover, the likelihood that it split off a much larger parent body raises the question of whether something really big is lurking nearby.
This is the kind of question that solar system scientists have been considering very seriously in recent years. Responding to a request from the US Congress, they have held several workshops to assess the risk of a damaging meteorite hit on Earth and to explore countermeasures.
A meeting at the United Nations in New York in April reviewed risk assessments. A subsequent meeting at the Lawrence Livermore National Laboratory in California in late May reviewed possible countermeasures.
Several points have emerged from these discussions. First, the threat of damaging impacts must be taken seriously. Earth has been hit many times - sometimes catastrophically. A major impact likely helped do in the dinosaurs 65 million years ago.
Second, while the threat must be taken seriously, there is no reason to lose sleep over it. The more catastrophic a possible impact might be, the less likely it is to happen. But smaller impacts are more frequent and could cause local damage.
Third, the leading conclusion from these assessments is that it would be prudent to mount a concerted global effort to catalog ''what's out there.'' That means identifying all near-Earth objects plus comets or other more distant objects whose orbits intersect that of Earth. Forewarning of a possible impact would likely give enough lead time for countermeasures.
Fourth, a sense of ''fatalism'' is unwarranted. Some people have reacted to the new awareness of impact risk by saying there's little that can be done about it. Yet there are possible countermeasures, as participants in the recent Livermore conference noted. These include such tactics as shooting a strong net-like mesh against a smaller asteroid to rip it into small pieces. Nuclear explosives would be a last resort for the big ones.
For a few million dollars a year, a few dedicated telescopes could build a substantial catalog of possibly Earth-threatening objects. Even in a time of budget restraints, that would be a worthwhile international investment.