WEEKLY WHINE

My coma hurts!

The XFL's got nothing on comet collisions. Who cares about 140 kg linemen colliding with one another when you can bash 3×10¹⁶ kg comets together?

This is the position of Dr Paul Weissman from JPL and Dr Alan Stern from the Southwest Research Institute. They say that in addition to being really cool, comet collisions may be far more important than we thought. They've generated a model that accounts for collisions amongst comets and compared its results to those of previous, noncolliding models. The outcome is, dare I say, explosive.

The existence of a vast comet reservoir was first proposed by Jan Oort of Leiden Observatory in the Netherlands, who realized that the very high eccentricities of many comets implied that they came from orbits a long way away from the Sun - perhaps three light years. The reservoir, he said, would be spherically distributed around the Sun, and passing stars would yank these comets out of their neat orbital holes and send them flying either in toward the Sun or out toward interstellar space.

More recently we've also found out that the Oort Cloud, as it's now called, can be perturbed by passing molecular clouds as well. If the Sun ever had a close encounter with a giant molecular cloud, many comets would be dumped to the solar system at once. There is also a backup comet storage area closer to the Sun, only 50 to 100 AU or so away. Unlike the Oort Cloud, this area is a flat disk, sort of like the asteroid belt. Ken Edgeworth and Gerard Kuiper proposed its existence, but for some reason Kuiper got all the credit when its existence was confirmed in 1988.

So what's the point? Well, the Oort cloud must have taken shape when comets were formed in the outer solar system and then flung out of there by the gravity of the big planetary bullies, Jupiter, Saturn, Uranus, and Neptune. But before this happened, they would have smashed into one another very frequently. Paul and Alan say that this Comet Smackdown would have pulverized comets under 20 km in diameter into bits of dust. The dust would then be sandblasted out of the solar system by radiation pressure from the Sun.

The comet boys say that after all this, only about ten percent of what most modellers think is out there really is out there. At the same time, they say that the comets out there might also be smaller than people thought, with mean diameters fifty percent smaller.

We already knew the primordial solar system was a rough place for planets and satellites. Clearly it was tough on the little guys too.