14. Comets

Comet decay and meteor showers

Fig. .. 

Once a comet enters the inner solar system, it returns again and again on a relentless voyage of continual decay and disintegration. Most comets are consumed by their own emissions, blowing themselves away as they sublimate the ice that holds them together. Sooner or later most short-period comets will either fall apart or turn into a dark rocky corpse that looks like an asteroid. It’s just part of the aging process. The only way to avoid this fate is to be thrown out of the solar system through close passage to Jupiter, or to collide with one of the planets or the Sun. Comets are very fragile, with little internal strength and a very low mass density. Some comet nuclei are crumbly, fluffy structures with mass densities less than that of solid ice and far less than solid rock. The central pressure of their nucleus is probably comparable to that under a thick layer of blankets. So it is little wonder that some comets have been observed to break up as the result of tidal forces from either the Sun or Jupiter. They pull on the near side of the comet a little more than the far side, tearing the comet apart. If you could get a piece of a comet in your bare hands, it would most likely fall apart it.

As an example, shortly after its closest approach to the Sun, the Great September Comet (1882 II) divided into four or more pieces stretched along nearly the same orbit like a string of pearls. The nucleus of Comet West also split into pieces when it passed near the Sun in 1976. The pieces of the split nucleus have too little mass to pull themselves together gravitationally. So once a nucleus splits, its pieces remain forever separated. The jets of escaping gas kick them away from each other, and they continue to drift farther apart.

Earth’s cosmic dusting

Fig. .. 

Cosmic dust is everywhere. It is in the air we breathe, the food that we eat, and the water we drink. Some of the dust that has been spawned by comets and asteroids even enters our own hair. The smallest dust particles are so tiny that the air slows them down rather than burning them up. Hundreds of them have been collected from the stratosphere and examined in the terrestrial laboratory, often with the fragile, porous structure expected of comet debris.

The cosmic dust particles that we examine today are time capsules that may date back to the origin of the solar system. They have probably not been significantly altered from the moment of their creation. The delicate primordial comet dust is therefore thought to preserve a record of chemical conditions at the time of planet formation, and it may even contain the ashes of stars that existed before the Sun was born.

Nights of the shooting stars

Fig. .. 

Although meteor showers are commonly called shooting stars, they are not stars, but fragile material from comets. In addition to spewing off small dust particles, which can drift down to the ground, comets also expel larger particles ranging in size from sand grains to pebbles. This debris burns up when it enters our atmosphere, producing visible meteors.

Fig. .. 

When just one of the comet particles rubs against the air, it vaporizes in a streak of light, producing the luminous trail of a meteor. And when many fall into the dark night sky, they produce meteor showers.

Fig. .. 

From the luminous path of a meteor, it is possible to determine the incoming particle’s orbital path around the Sun, and in most cases the orbits are similar to those of comets. A comet ejects the particles along its orbital path as it loops around the Sun, and this material continues to revolve around our star, something like the ice particles that circle Saturn in its ring. The swarm of comet material is called a meteoroid stream. And when the Earth passes though one of these streams, it intercepts some of the orbiting particles that enter our atmosphere and create a meteor shower.

Fig. .. 

When a meteor shower includes large numbers of shooting stars, the trails appear to intersect and emanate from a distant point called the radiant. But meteors that appear to diverge from a point are actually moving on parallel paths, just as parallel railroad tracks seem to come from a point on the distant horizon. Meteor showers are named after the constellation in which their radiant appears.

(page 7 of 8)

Copyright 2010, Professor Kenneth R. Lang, Tufts University