Sky and Telescope magazine for June 1989 page 592 reports that on March 22-23 there was a near miss which went un notices by strong moonlight but the object was detected 8 days later and cataloged as 1989FC. It was 300 meters in diameter and passed within 690,000 kilometers of the Earth. However, the asteroid was discovered on March 31, 1989 after it had passed the closest point to the Earth. Earth moves 700,000 km in about six hours. The mean distance of Moon from the Earth is 384,404 km.
This event was a wake-up call and stimulated Congressional action on asteroid impact hazards:
In April 1990, AIAA's Space Systems Technical Committee, chaired by E. Tagliaferri, published the position paper, Dealing With the Threat of an Asteroid Striking the Earth. This paper, precipitated by the close passage of asteroid 1989FC - with zero warning time - helped stimulate broad interest and concern in this potential threat to civilization and even to the survival of humanity. Soon afterwards, the U.S. House of Representatives Committee on Science, Space and Technology directed NASA to conduct two workshops related to the asteroid threat as recommended in the position paper, one for the detection and characterization of the threat, including determining the orbits with precision which would allow the accurate prediction of an impact, and another which dealt with issues related to mitigating the threat.
According to the AIAA paper:
On March 23, 1989, an asteroid bigger than an aircraft carrier, traveling at 46,000 miles per hour, passed through Earth's orbit less than 400,000 miles away. Our planet had been at that point only six hours earlier. The asteroid was not detected until after it had passed. Had it struck the Earth, the energy released would have been equivalent to that of 1000 to 2500 megatons of TNT (or 1000-2500 one-megaton hydrogen bombs). In an area of high population density such as the northeast corridor of the U.S., Los Angeles, or Tokyo, millions of people would have died instantly.
The passing of this asteroid, named Apollo Asteroid 1989FC by its discoverers (Henry E. Holt and Norman G. Thomas of the University of Arizona), was not an isolated event. 1989FC is one of a class of objects which periodically cross the orbit of the Earth. The first object of this type was discovered in 1932 by Karl Reinmuth of Heidelberg Observatory.1 It was in an orbit around the Sun that crossed the Earth's orbit, and was named "Apollo," after the Greek Sun god, because of its close approach to the Sun. (Most asteroids orbit the Sun at much greater distances, generally between the orbits of Mars and Jupiter). Subsequent discoveries revealed that a whole class of such objects exists, and that an object the size of the one which just missed Earth in March, 1989, probably comes by undetected once every two or three years.2
Here is a NASA Press Release about the object:
4/19/89: NASA ASTRONOMER DISCOVERS "NEAR-MISS" ASTEROID THAT PASSED EARTH RELEASE: 89-52
An asteroid, a half-mile or more in diameter, passed within a half million miles of the Earth - about twice the distance to the moon - on March 23, the National Aeronautics and Space Administration said today. "On the cosmic scale of things, that was a close call," said Dr. Henry Holt. Holt is a University of Arizona astronomer who discovered the asteroid while working on a U.S. Geological Survey (USGS) project, funded by NASA, to detect and track unknown asteroids that cross the orbit of the Earth. The project is headed by Dr. Eugene Shoemaker, USGS.
Dr. Bevan French, advanced program scientist for NASA's Solar System Exploration Division, Washington, D.C., said that if the asteroid had collided with the Earth, the impact would have been equivalent to the explosion of 20,000 hydrogen bombs creating a crater 5 to 10 miles in diameter - "enough to destroy a good-sized city." Landing in the ocean could have been worse since huge tidal waves could have been created that would sweep over coastal regions, he said.
Although scientists do not know the asteroid's exact size, they believe it to be over a half-mile in diameter. A 6-mile- diameter asteroid hit the Earth about 65 million years ago. It is popularly believed that this caused a global catastrophe that destroyed the dinosaurs. The asteroid, currently designated 1989FC, came closer to Earth than any recorded since Hermes in l937, according to Dr. Brian Marsden, director of the Minor Planets Center at the Smithsonian Astrophysical Observatory, Cambridge, Mass. Hermes passed the Earth at approximately the same distance as 1989FC.
The observatory, which is the international clearinghouse for such discoveries, recorded the discoveries of about 1,800 asteroids in l988. In the designation l989FC, l989 is the year of discovery; F indicates discovery in the sixth half-month of the year (i.e. the end of March); C indicates that the asteroid was the third discovered in that period. If the asteroid is successfully observed on two subsequent approaches to Earth, Holt will be entitled to name it. Holt discovered the asteroid on a series of photographic plates taken March 31 using the 18-inch Schmidt telescope at the California Institute of Technology's Mount Palomar Observatory in California.
The object - estimated to be travelling 46,000 miles an hour - appeared as a trail of light in two photographs of the sky near the constellation Coma Berenices. They were taken an hour apart. The asteroid was detected when the two photographic plates were examained under a stereo microscope. "I knew it was travelling fast by the elliptical spot that it created," said Holt. During the week following the discovery, subsequent observations of l989FC were made by Holt and other astronomers to determine its orbit. Like the Earth, l989FC takes about a year to go around the Sun. But its orbit is highly elliptical and extends past the orbit of Mars and inward past the orbit of Venus. Asteroid 1989FC is now moving rapidly away from the Earth and Sun. It will return, crossing the Earth's orbit again in early October 1989, this time at a greater distance from Earth. Asteroid l989FC is only one of about 30 Earth-crossing asteroids that have been discovered, although there may be many more. Estimates range from several hundred to more than a thousand. Holt and Shoemaker regularly observe the sky during the "dark of the moon," the period just before and just after the new moon.
Copyright 1997 Dr. Sten Odenwald
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