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That Was Close!
An asteroid makes one of the closest known passes of Earth.
by Vanessa Thomas

This illustration represents a view from a near-Earth asteroid.
E. De Jong, S. Suzuki / JPL / NASA
On Friday, September 27, an asteroid passed less than 55,000 miles (about 88,000 kilometers) from Earth, but astronomers didn't even know the object existed until 11 hours later. Luckily, the asteroid is only a few meters wide and wouldn't have done much damage even if it had hit us rather than merely flying one-fourth the Earth-moon distance past us.

Now called 2003 SQ222, the asteroid was first spotted by Robert Cash of the education-driven Minor Planet Research, Inc. Using detection software called PinPoint, Cash spotted the faint, moving object in images taken on September 28 by the Lowell Observatory Near-Earth-Object Search (LONEOS) 24-inch (60-centimeter) telescope in Flagstaff, Arizona. At the time, the 18th-magnitude object was moving 20 degrees across the sky per day, or twice as far as the moon drifts from day-to-day a tip-off that it was very close.

From the early observations, LONEOS director Edward Bowell was able to calculate an orbit for 2003 SQ222. "The orbit showed clearly that SQ222 had passed within a quarter of the moon's distance to the Earth some eleven hours before being discovered," he notes.

2003 SQ222
The small near-Earth asteroid 2003 SQ222 appears as a faint spot inside this green circle.
At its closest, 2003 SQ222 moved even faster than it did on the 28th traveling 30 degrees per day across the sky. At the time it wasn't visible from the United States, though, because it was in the sky during the daytime.

Assuming it has an albedo typical of near-Earth asteroids, Bowell used 2003 SQ222's distance and brightness to calculate its size. He estimates that the asteroid measures somewhere between 10 and 20 feet (3 to 6 m) across. If it had hit Earth, the small asteroid would've detonated innocently in the upper atmosphere. According to Bowell, objects of this size do so about once per year.

Only ten asteroids have been known to pass within the moon's orbit, but 2003 SQ222 wasn't the closest or smallest one on record. It is, however, the best-observed one to come this close and is probably the smallest asteroid with a well-known orbit.

LONEOS Telescope
The LONEOS telescope
After the asteroid's discovery, other amateur and professional observers tried to find it as well. A few were successful, including British amateur astronomer Peter Birtwhistle, who used a 12-inch (30-cm) telescope to image the asteroid. By the time astronomer Alan Fitzsimmons of Queens University in Belfast, Ireland, observed 2003 SQ222 with the 2.5-meter Isaac Newton Telescope on October 2, the asteroid was already a hundred times fainter than it was when discovered. It is now too faint to be seen, even with the world's best telescopes.

With the few observations made of 2003 SQ222, astronomers can accurately predict its orbit only tentatively into the future. They know it won't come closer to us in the next decade than it did last month, but after that, it could approach about 12,000 miles (20,000 km) from our planet's surface as it makes its 1.85-year circuit around the sun.

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