A Celestial Collision
Those of us old enough to recall the 1951 movie "When Worlds Collide" were astonished by the special effects the movie makers created in depicting the collision of another planet with the earth (while a handful of lucky survivors fled on the first interplanetary rocket ship).
On a smaller scale, and at a safer distance, such a collision might actually have been witnessed by people on the earth at a time not so distant in the past--just before the last Crusade, as a matter of fact.
Early in the evening of June 18, 1178, a group of men near Canterbury, England, stood admiring the sliver of a new moon hanging low in the west. In terms they later described to a monk who recorded their sighting, "Suddenly a flaming torch sprang from the moon, spewing fire, hot coals and sparks." In continuing their description of the event, they reported that "The moon writhed like a wounded snake and finally took on a blackish appearance,"-- claims which today must be viewed with some skepticism.
However, the sighting of an asteroid strike on the moon, with an attendant brilliant flash, is an entirely believable possibility.
At least, planetary scientist Jack Hartung of the State University of New York thought so when he read the 800-year old account. As reported by Terry Dunkle in the March, 1982 issue of Science 82, Hartung gathered enough clues to suggest that a large asteroid (which would be called a meteor if it entered the earth's atmosphere) might have smacked into the moon just over the horizon on the back side.
To test his suspicion, Hartung went to the Lunar and Planetary Institute in Houston, and inspected Russian and American photographs of the moon's back side. Sure enough, in just the right place, he found a remarkably fresh crater, 12 miles across and twice as deep as the Grand Canyon. From it radiated white splatter marks for hundreds of miles.
"To gouge a hole like that," says Dunkle, "would require a boulder as big as the Houston Astrodome slamming into the moon at a speed of 40,000 miles per hour."
Such an impact, reason astrophysicists, would set the moon to ringing like a gong for thousands of years. Since the astronauts left laser-reflecting panels on the moon, it is being found that it does--bulging in and out like a large soap bubble set free in the air.
It is possible to determine the distance to the moon to within four inches by reflecting laser-beams off the panels on the moon and measuring the time of transit back to earth. At Texas' McDonald Observatory, astronomers Odile Calame and J. Derral Mulholland of the University of Texas find that the surface of the moon moves back and forth fully 80 feet!
Such an oscillation clearly implies a collision with something large, sometime within the not-too-distant past, probably within the memory of mankind. The problem is that there is no way to peg the date exactly at 1178.
That is a shame, because if the collision could be definitely dated to the time of the sighting by the men in Canterbury, knowing the size of the crater (thus the energy of the impact) and the present amplitude of the moon's oscillations, a great deal could be learned about the interior of the moon. For instance, it would be possible to tell if it is solid or squishy inside, just as thumping a watermelon gives a clue to its ripeness. This would be a valuable step in understanding the origin of the moon, and that knowledge, in turn, would help us to understand the earth.
