An asteroid makes one of the closest known
passes of Earth.
by Vanessa Thomas
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
This illustration represents a view
from a near-Earth asteroid.
Jong, S. Suzuki / JPL /
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.
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.
The small near-Earth asteroid 2003
SQ222 appears as a faint spot inside this green
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
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.
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
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.