Asteroids miss with astronomers

Close brushes actually fairly common

The only thing that was particularly unusual about two asteroids that zipped past Earth September 8, astronomers say, was that anybody noticed them.

Within hours of each other on September 8, two small asteroids whizzed past Earth within the moon’s orbit. Scientists say such occurrences are probably fairly common. NASA

Such close approaches — one of the asteroids passed within 79,000 kilometers of Earth — actually happen several times a week, according to scientists’ calculations. Yet some media outlets described the close encounter as if it were a brush with Armageddon.

“Quite frankly, I don’t know why they’re making such a fuss about it,” says astronomer Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “This is essentially nothing.”

Astronomers first spotted the two asteroids three days before their close encounter with Earth, using the Catalina Sky telescope near Tucson, Ariz., which routinely scans the skies for near-Earth objects. At the time they estimated the larger asteroid to be 10 to 20 meters in diameter, and the smaller 6 to 14 meters across. But subsequent observations by Richard Binzel and Francesca DeMeo of MIT using NASA’s Infrared Telescope Facility on Hawaii’s Mauna Kea showed that the objects were actually only about half that size.

The discovery of the two space rocks demonstrates that programs like the Catalina survey, designed to find much larger near-Earth asteroids that do have the potential to cause devastating collisions, can also find smaller bodies, Marsden notes.

It might be a little unusual to have two asteroids swing by hours apart. But with small asteroids passing by Earth several times a week, there’s always a chance that two could pass by in the same day, he adds.

Much more intriguing to astronomers was the discovery of a small near-Earth asteroid by the Catalina survey in 2008 just hours before it landed in Sudan, where researchers later recovered the fragments (SN: 4/25/09, p. 13).

“The small fry are interesting, not because of damage,” Binzel says, “but because of their potential for delivering ‘free samples’ to Earth.”

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