A small asteroid, perhaps 3 to 6 meters in diameter-the
size of a room or house-came within 88,000 km of Earth late on Friday,
September 27. Less than a quarter of the distance to the Moon, this is
the closest well-documented Earth encounter of an asteroid that has not
struck our atmosphere.
“In a good month, we find five to 10 near-Earth asteroids, but usually,
the ones we discover are as big as mountains, or at least football
stadiums, so this one was unique for us,” said Edward Bowell, Director
of Lowell Observatory’s Near-Earth-Object Search (LONEOS).
Known as 2003 SQ222, the asteroid was imaged a few hours after close
approach by Michael Van Ness, a graduate student at Northern Arizona
University, Flagstaff.
LONEOS (http://asteroid.lowell.edu/asteroid/loneos/loneos.html) is one
of five teams funded by NASA’s Near Earth Objects Observations program
to look for asteroids and comets that could come close to or strike our
planet. LONEOS is the third leading discoverer of asteroids.
The first images of SQ222 were made on a series of CCD-camera frames
(charge-coupled device) taken for Minor Planet Research
(http://www.minorplanetresearch.org), an organization collaborating
with LONEOS on a project with an aim of having high school students make
asteroid discoveries at the Challenger Learning Center in Peoria,
Arizona. Robert Cash, of MPR, used automatic moving-object detection
software to find three trailed images of an object moving at 20 degrees
per day, almost twice as fast as the Moon, across the sky. Cash relayed
his discovery back to Lowell Observatory and to the international
clearinghouse for asteroid and comet observations, the Minor
Planet Center, in Cambridge Massachusetts
(http://cfa-ww.harvard.edu/cfa/ps/mpc.html).
Predicted positions were posted on the MPC’s Near-Earth Object
Confirmation Page so observers worldwide could follow the object.
Meanwhile, Bowell noticed that it was possible to compute a fairly
reliable orbit. “The orbit showed clearly that SQ222 had passed within a
quarter of the Moon’s distance to the Earth, some 11 hours before being
discovered,” said Bowell. “So, I e-mailed our results to the Minor
Planet Mailing List to which hundreds of amateur and professional
astronomers subscribe, with a request for further observations.”
Brian Skiff, LONEOS’ chief observer, acquired fresh CCD frames on
September 29, but the LONEOS team was unable to locate the asteroid’s
images. Once again, Bob Cash found the by then very faint images of the
asteroid after visually searching the frames for more than three hours
in the wee hours of September 30th. Two sequences of images are visible
at:
http://asteroid.lowell.edu/asteroid/loneos/sq222.html.
Independently, British amateur astronomer Peter Birtwhistle, using a
30-cm telescope west of London, was able to image the asteroid. “It is
remarkable that Birtwhistle was able to detect the asteroid using such a
small telescope,” said Bowell. “He did so by tracking the motion of the
asteroid and by aligning and co-adding (or stacking) the frames to bring
out the faint asteroid images.”
“The essential rapid teamwork between Lowell Observatory and keen
amateur astronomers made it possible to confirm and image this
fast-moving, small asteroid as it shot past us,” said Bowell.
SQ222’s known brightness and distance allow calculation of its size.
Most asteroids have either coal-black surfaces or are about four times
more reflective. Bowell estimates the asteroid to be just 3 to 6 meters
in diameter, most likely making it the smallest asteroid for which we
have a reliable orbit. (Smaller and closer asteroids have been seen in
space, especially by the Spacewatch team at the University of Arizona
(http://spacewatch.lpl.arizona.edu), but it has not been possible to
follow them long enough to secure good orbits.)
Perhaps the final detection of SQ222 was made by British astronomer Alan
Fitzsimmons (Queen’s University Belfast) on October 2. Fitzsimmons,
working through thin cloud, managed to detect the asteroid using the
2.5-m Isaac Newton Telescope at La Palma in the Canary Islands. By then,
SQ222, receding rapidly from Earth, was about 100 times fainter than at
discovery.
After Fitzsimmons’ observations, the orbit of SQ222 was good enough to
compute a reliable value of what astronomers call the minimum orbital
intersection distance, (MOID).
This is the minimum distance between the orbit of the asteroid and that
of the Earth. Bowell calculated the MOID to be a little over 4 Earth
radii (about 27,000 km).
“This distance is, roughly speaking, the very closest the asteroid could
have come to the center of the Earth during its fly-by,” said Bowell.
“Therefore, SQ222 could not possibly have struck the Earth.” Even if it
could have, it would have exploded harmlessly in the upper atmosphere,
with an energy comparable to that of a small atomic bomb, as friction
with the air vaporized its surface, added Bowell.
“Objects the size of SQ222 actually do burn up in Earth’s atmosphere
every year or so, producing a spectacular light show,” said Bowell.
In what is most likely a coincidence, an intense shower of meteorites
was reported in India about 10 hours before SQ222’s closest approach to
Earth. Could the asteroid and the meteorites be fragments of a larger
asteroid that was broken apart by a collision with another asteroid or
by tidal disruption during a previous very close Earth approach? It
seems very unlikely, but work is ongoing to test the plausibility of the
idea.
Will SQ222 make another close pass by Earth? It is hard to say, as the
orbit is not accurate enough to make reliable predictions for more than
a few years into the future. Certainly, there seems no possibility of it
returning within the next decade. Also, SQ222 will be too faint to see
in the foreseeable future, even using the most powerful telescopes.
-END-
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www.lowell.edu .
For more information:
The MPR detection software used to find SQ222, called PinPoint, was
developed by Robert Denny, and is commercially available
(http://www.appliedinsight.com/index.html).
Peter Birtwhistle’s images may be seen at
http://www.birtwhi.demon.co.uk/Gallery2003SQ222.htm.
At discovery, the asteroid was about 18th magnitude in the R band.
It is estimated that there are about 500 million near-Earth asteroids of
SQ222’s size or larger. Alan W. Harris, of the Space Science Institute,
calculates that about 3,000 such objects per year come closer than the
Moon, and about 100 of them come closer than SQ222 each year. Moving at
an average speed of 17 km/s, any one of them stays within the Moon’s
orbit for 8 hours or less. Thus, it is likely that there are a couple of
asteroids like SQ222 within the Moon’s distance right now, and two more
will come close to us tomorrow. Why don’t we see more of them? Usually,
they are moving too fast for our CCD cameras to register. Harris points
out that “these things are completely harmless. They are no more than
interplanetary tourist attractions.”
2003 SQ222 was moving across the sky at more than 30 degrees per hour at
its closest approach. It would not have been observable from the U.S.
as it was in the daylight sky. It could have been seen, with difficulty,
from Europe. The closest approach is about twice the distance of
geosynchronous communications satellites. Aside from the occasional
fireball, an asteroid that explodes in the upper atmosphere, only 10
asteroids are known to have approached the Earth closer than the Moon’s
distance.
For documentation of SQ222’s discovery, visit:
http://groups.yahoo.com/group/mpml/message/10707
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