On the Possibility of Instantaneous Shifts of the Poles (cont.)
By Flavio Barbiero
The Apollo Objects
Severe consequences can follow when the Earth is hit by a comet or an asteroid. We know for certain that this happened many times in the past and can happen again at any moment in the future.
If the Earth didn't have oceans and atmosphere its surface would be marked with huge numbers of craters like the Moon and Mercury . On our planet, instead, erosion and sedimentary processes very quickly erase the traces of collisions with asteroids, meteorites and comets. Only where recent ice sheets have scraped the surface, thus uncovering the traces of ancient collisions as in parts of Canada, is it possible to count the craters accurately. Based on this approach G.W. Wetherill (see: G.W. Wetherill, "The Apollo Objects", Scientific American May 79) has estimated that in the last 600 million years our planet has been hit by at least 1500 objects with a diameter larger than one kilometre.
The majority of these collisions is caused by a class of celestial bodies named by astronomers "Apollo objects", that is a class of asteroids whose perihelion lies inside the orbit of the Earth. The first of these objects was discovered by Reinmuth on 1932 and named Apollo, which gave the name to the class. At present, more than one hundred Apollos of a diameter of at least one kilometre are known. The largest discovered so far, Hephaistos, has a diameter of ten kilometres. The total number of Apollo objects with a diameter of one kilometre or more is estimated to be between 1,000 and 2,000.
As the perihelion of the Apollos lies inside the orbit of the Earth, it follows that periodically they have the chance to collide with it. The probability of such an event is estimated at 5. 10-9 per year per single Apollo. Therefore we have a probability of at least 4 collisions each million year period with objects as large as one kilometre or more. As the size of the objects becomes smaller, this probability grows exponentially to reach one impact every few centuries for objects of 100 to 200 metres diameter.
The direct effects of a collision with an Apollo-like object are devastating. Gehrels (see:Tom Gehrels, "Collision with comets and asteroids", Scientific American, March 96) estimates that a one-kilometre-wide object, colliding with the Earth at a speed of 20 kilometres per second, would liberate an energy equivalent to ten billion Hiroshima-type nuclear bombs.