Asteroid 1997 XF11 set off alarm bells on March 11, 1998 when astronomers concluded the mile-wide rock might very well come within 30,000 miles of the earth in 2028. That's way, way too close for an object that's capable of destroying most of human civilization. Earth would be well inside the margin of error for the flight-path of such an object.
The official warning was issued by Brian G. Marsden of the International Astronomical Union. "The chance of an actual collision is small, but one is not entirely out of the question," the notice said.
"This is the most dangerous [near earth asteroid] we've found so far," Jack G. Hills of Los Alamos National Laboratory.
But little more than one day after the warning, additional data calmed everybody down considerably. It turned out that this asteroid had been photographed in 1990, though not named, by astronomers at the Palomar Observatory. Plotting its position at that time in relation to its position over the last few months (it was identified and named last December 6 by Jim Scotti of the University of Arizona Spacewatch project) gave a much more accurate estimate of its future course. And now, it seems, XF11 will pass by at a healthy remove of 600,000 miles or so on October 26, 2028.
Still, the shake-up provided good motivation to take the problem more seriously, asteroid watchers say. Other big ones are out there. It's just a matter of time before something smacks the earth a mighty blow. As yet, there are very few professional astronomers searching for potential impact objects. And there is no official plan -- not even a tentative hypothesis -- about what to do when a really serious impact threat is identified.
In this century, the biggest probable asteroid impact resulted in the famous Tunguska explosion of 1908. That event leveled several thousand square miles of desolate forest in Siberia and lit up the sky as far away as London. If it had hit Moscow or London or New York, nothing would have been left alive in the target area; not a structure would have remained standing for dozens of miles around. But that event would be nothing compared with a hit from 1997 XF11.
The closest recorded near-miss of a big space rock occurred on May 19, 1996, when asteroid 1996 JA1, measuring a third of a mile across, passed within 279,000 miles of earth, slightly more than the distance to the moon. Of particular concern in this case was the fact that the asteroid was found only a few days before its closest approach to earth. It raised the spectre of sudden, unannounced death from the sky and motivated a considerable increase in sky search programs since then.
A rock the size of 1996 JA1 is at least ten times bigger than the meteor that left the famous mile-wide crater near Winslow, Arizona. But 1997 XF11 is dozens of times more massive than that.
The rock that probably killed the dinosaurs about 65 million years ago is estimated to have been perhaps six miles in diameter. That's dozens of times more massive than 1997 XF11; but then, an estimated 75 to 80 percent of all life on earth, including all the large animals (everything at the top of the food chain), perished as a result.
It's hard to imagine what a direct hit from 1997 XF11 would really do to the earth and the life thereon. As the first announcements of danger began to play in the media, scientists groped for meaningful comparisons. Los Alamos' Jack Hills said the impact would be the equivalent of 320,000 megatons of dynamite, or approximately 2 million Hiroshima-sized bombs. At the peak of the Cold War, all the nuclear weapons on earth had a combined explosive power of perhaps 25,000 megatons, and a very small fraction of those were thought capable of bombing all of human civilization "back to the Stone Age."
Needless to say, 1997 XF11 would do unthinkable damage if it hit. Striking the ocean (a statistical probability), it would send up tsunamis thousands of feet high that would wash miles inland on coasts all over the world, instantly leveling a substantial portion of human civilization. In addition, it would vaporize millions of tons of seawater, sending the vapor skyward. This would have enormous long-range effects on weather, probably setting in motion a year or more of catastrophic storms that would further flood and demolish the land.
If it struck on land, thousands of square miles immediately surrounding the impact would look as though hit by nuclear weapons. Giant earthquakes would shatter many parts of the earth. Firestorms would spread outward from the impact zone, perhaps burning out only when they reached the sea. And cubic miles of earth would be turned to airborn dust, setting in motion the equivalent of nuclear winter, blocking the sunlight with debris for many months, even years. Plankton in the sea -- the base of the ocean food chain -- might entirely perish. Many higher species would starve to extinction. Human society, with some warning, could prepare sufficiently to preserve a viable minority of earth's inhabitants, enough to slowly rebuild. But nothing like modern civilization would remain a year after impact, and half or more of all people could die.
So it is very good news -- to put it mildly -- that asteroid 1997 XF11 is almost certain to pass us by.
Even so, "This whole exercise was a useful reminder of the fact that these things can, in fact, hit the Earth," said Steve Maran, an astronomer with the American Astronomical Society.
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