University Lowbrow Astronomers

How Close?

by Bernard Friberg
Printed in Reflections:  March, 1998.

Last Thursday 3/12 it was reported that an asteroid 1997 XF11 was headed this way in 30 years and may pass within 30,000 miles of earth.  These calculations are based upon only a few data points, so the error in the calculations may be more than 30,000 miles.  This prompted many news reports such as:

Astronomers say a mile-wide asteroid described as “the most dangerous one we’ve found so far” may be on course for a near-miss -- or even a collision -- with Earth in the year 2028.

Some astronomers say the asteroid will come within 30,000 miles of the Earth, and they agree with Dr. Brian Marsden of the International Astronomical Union (IAU) who says, “Chances are it will miss” Earth.  “The chance of an actual collision is small, but one is not entirely out of the question,” says a notice filed by the IAU.

Asteroid 1997 XF11 was discovered December 6 by Jim Scotti of the University of Arizona Spacewatch program, and has been added to a list of 108 asteroids considered to be “potentially hazardous objects.”

Hills said an asteroid the size of 1997 XF11 colliding with the Earth at more than 17,000 miles an hour would explode with an energy of about 320,000 megatons of dynamite.  That equals almost 2 million Hiroshima-sized atomic bombs.  Such an asteroid hitting the ocean, Hills said, would create a tidal wave hundreds of feet high, causing extreme flooding along thousands of miles of coastline.  If it struck land, he said, it would blast a crater 20 miles across and so clog the sky with dust and vapor that the sun would be darkened “for weeks, if not months.”

The thought of a possible collision prompted an immediate effort to refine the calculations and the following day new reports surfaced:  New data shows the asteroid will miss Earth, NASA says.  Just a day after a group of astronomers reported that an asteroid was expected to pass within just 30,000 miles of the Earth’s center and could possibly collide, astronomers at NASA’s Jet Propulsion Laboratory said that their calculations -- based on newly uncovered data -- indicate the asteroid will pass no closer than 600,000 miles away.

“We are saying now that the probability of an impact is zero,” said Donald K. Yeomans of JPL.  Yeomans said he and fellow astronomer Paul W. Chodas dug out some eight-year-old pictures of the heavens taken by the Palomar Observatory telescope and found that the photos contained images of asteroid 1997 XF11, which was then just an unidentified point of light.

With the newfound data, the 1990 pictures, along with the recent observations of the streaking asteroid, Yeomans and Chodas recalculated the orbital path of the asteroid and found that it would miss the Earth by 600,000 miles at its closest approach in October of 2028.  The newly calculated orbital path of the asteroid means it will pass outside the orbit of the moon.

In recent years there have been a handful of close calls by larger asteroids, and some hits by smaller objects.  On March 23, 1989, an asteroid about a half-mile wide crossed the Earth’s orbit about 400,000 miles from Earth.  If the Earth had been in that same spot a mere six hours earlier, a collision would have occurred.  On January 17, 1991, an asteroid estimated to be about 30 feet wide passed within 106,000 miles of Earth.  It was the closest “near miss” ever recorded.  Smaller objects hit the Earth all the time.  Most of them land in oceans or uninhabited areas, unnoticed.  But some make headlines such as a small meteorite hitting a car in 1994.  On October 9, 1992, a meteorite smashed through the rear end of a car in Peekskill, New York.  Nothing was damaged, but the Chevy Malibu.


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