October 3, 2003 | On
Saturday, September 27th, a very small asteroid plunged past
Earth well inside the Moon's orbit. Unseen, it passed just
78,000 kilometers (a fifth the Moon's distance) above Earth's
surface before barreling back into interplanetary space.
Judging by its faintness — 18th magnitude when first picked up
the next day — it can't be any larger than 3 to 6 meters
across. That's "SUV or room size," notes Edward L. Bowell,
principal investigator for the Lowell Observatory Near-Earth
Object Search (LONEOS) at Anderson Mesa, Arizona, where the
first images were taken.
LONEOS collaborates with Minor Planet Research, Inc., where
Robert A. Cash used the PinPoint detection software to discover
the object's faint trails on three LONEOS images. He
immediately sent his measurements to the Minor Planet Center
in Cambridge, Massachusetts, which alerted astrometric
observers around the world.
More images of the rapidly receding, fading object were
acquired on the 29th by LONEOS, and also by amateur astronomer
Peter Birtwhistle in Berkshire, England, using a Meade 12-inch
Schmidt-Cassegrain telescope. The Minor Planet Center
announced the find on October 1st, dubbing it 2003 SQ222. Brian G. Marsden's orbital elements,
refined on October 3rd, indicate that the tiny planetoid is
traveling in a low-inclination orbit that takes it to well out
beyond Mars's distance from the Sun, then inward as close as
Venus, in a period of 1 year 10 months.
If it ever hits Earth it should break up in the upper
atmosphere, causing virtually no harm — much like the slightly
smaller Park Forest meteorite that dropped
fragments on a Chicago suburb last March.
Asteroid 2003 SQ222 now tops the Minor
Planet Center's list of the closest known approaches by
asteroids outside the Earth's atmosphere. But larger objects
have come even closer. Meteor Crater near Flagstaff, Arizona,
was produced by the prehistoric impact of an asteroid perhaps
1,000 times more massive than 2003 SQ222.
The meteoroid that exploded over Tunguska, Siberia, in 1908
may have been 30 times wider than 2003 SQ222. When hundreds of tourists saw the great
Grand Teton National Park fireball of August 10, 1972, they
were witnessing the atmospheric graze of an object about twice
the size of 2003 SQ222 before it skipped
back into space.