The vagabonds: comets, asteroids, and meteorites.

The planets are not the only objects within our solar system. Besides the planets, there are asteroids, comets, meteors, and meteorites. In addition there is Pluto. Recently, there has been some debate on whether Pluto is a planet or not. For the sake of this class, we shall consider Pluto a planet, but we leave its discussion for this section.

Below is the ninth planet in the solar system. We separate it because it does not fall into the terrestrial category, nor does it fall into the Jovian category. Pluto is the smallest planet in the solar system. In addition, the orbital plane of Pluto is tilted more than any other planetary orbit. Pluto also has the distinction of possessing the largest eccentricity of any orbit amongst the planets. In fact, the orbit of Pluto brings it to a position closer to the sun than the orbit of Neptune.

Observations of Pluto are very difficult due to its distance from the sun. Even 10 years ago data on the planet was generally accompanied by a question mark because we weren't sure of the numbers we were giving. We will assume today's numbers to be more accurate and better known. One distinguishing characteristic of Pluto is that it has one Moon. This moon is not too much smaller than Pluto itself. It is given name Charon. One interesting characteristic of the Pluto-Charon system is that the two objects are mutually synchronous. That is to say, the orbit of Charon and the rotation of Charon and Pluto are such that the same side of Charon always faces Pluto and the same side of Pluto always faces Charon.

Earlier we discussed Bode's law which is a mathematical formula used to predict the distances to the planets from the sun. Up until Neptune the planets obey this law fairly accurately. However, Bode's law predicts that a planet should exist between the orbit of Mars and Jupiter. Given this gap astronomers sought to find the planet that should exist between Mars and Jupiter. In 1801 an astronomer named Piazzi discovered an object whose orbit is in the prescribed location. This object was named Ceres. A year later in 1802 another object was discovered at roughly the same distance from the sun but in a different region of the sky. This object is called Pallas. Thinking the planet which should be in this location had been destroyed, astronomers began hunting this region of the sky for various objects.

Today we know these objects to be asteroids. There are estimated to be 100,000 asteroids in orbit around the sun between the orbits of Mars and Jupiter. Given this large mass of asteroids, the region between Mars and Jupiter is referred to as the asteroid belt. Astronomers believe there are some 10,000 of these asteroids visible from the Earth. If all the asteroids were put together it would form an object not large enough to be considered a planet.

Although the orbit of the asteroids lie between the orbit of Mars and Jupiter, they are not confined to this region. Some asteroids have orbits which are so eccentric that they come within the orbit of Mars and even the orbit of Earth. The orbit of some asteroids is greatly affected by the planet Jupiter. The gravitational pull of Jupiter has created gaps within the asteroid belt referred to as Kirkwood gaps. These are regions in the asteroid belt devoid of asteroids. The gravitational pull of Jupiter is harmonic in these regions and therefore pulls the asteroids out of these regions with regularity. This is similar to the way the moon Mimas sweeps out the Cassini division in the rings of Saturn. In addition, Jupiter's gravity has captured a group of asteroids called the Trojan asteroids. The asteroids are trapped in the orbit of Jupiter in regions called Lagrange points.

Asteroids were likely formed at the very beginning of the solar system formation. They formed quickly in their current state and they are therefore excellent indicators of conditions at the beginning of the solar system formation.

On occasion these asteroids, or pieces of the asteroid, come too close to the gravitational pull of a planet. When this happens the asteroid plummets to the planets surface. If the planet has a significant atmosphere, the debris may be destroyed before striking the surface of the planet. The destruction is generally accompanied by a fiery trail due to the frictional heating of the debris. This fireball is referred to as a meteor. If the objects survives this fiery destruction and impacts with the surface it is referred to as a meteorite. Extremely large meteorites will leave a large crater upon impact. One such crater exists in Arizona. It is referred to as the Barringer crater.

Meteorites are classified into three categories. The first category is stony meteorites. These meteorites look like common rock and are very difficult to distinguish as meteorites. Although they comprise about 95 percent of all meteorites they are the most difficult to discover because they blend in with common stones. The second category is stony iron meteorites. They contain a roughly equal mix of rock and iron. The last category is iron meteorites. They are mostly iron but may be 10 to 20 percent nickel.

The three classifications of meteorites is probably indicative of their origins. Stony meteorites likely come from the exterior portion of asteroids. Iron meteorites, if properly treated, show crystal patterns referred to as Widmanstatten patterns. These crystal patterns can only exist if the material cooled slowly. This would indicate that they originate in the centers of asteroids. These centers cooled slowly much like a hot potato's insides cool more slowly than the outsides. The stony iron meteorites probably originate from a layer between the inside and the outside of the asteroid.

Casper College's Tate Museum has an excellent display of meteorites. I highly encourage all students to visit this display at their earliest convenience.

The last group of leftovers we wish to discuss are comets. Comets also formed early in the lifetime of the solar system. They are composed predominantly of dirt and ice, and they are sometimes referred to as dirty snowballs.

Comets generally have very eccentric orbits or orbits which are hyperbolic in shape. These hyperbolic orbits bring the comets around the sun only one time. They are referred to as open orbits. Since these comets truly are part of our solar system, they must have come from the very edge of the solar system. Comets which take more than a few hundred years to orbit the sun are referred to as long period comets.

Short period comets are probably following orbits that they did not originally have. Each time a comet orbits the sun portions of it evaporates. Therefore, it can only orbit the sun a finite number of times. Given this information, a comet which goes around the sun every 100 years would have made more than 40,000,000 trips around the sun if it had that orbit from the beginning of the solar system. It's unlikely a comet could last 40,000,000 trips around the sun. So how do these comets follow paths of less than 100 years? The orbital path of these comets was likely changed by a close encounter with Jupiter or some other large solar system body. The gravitational pull of this body would have changed its orbital path and given it a new orbital path with the period of less than 100 years.

Comets become spectacular in appearance as they approach the sun. As a comet approaches the sun the ice of which it is composed begins to evaporate. As these ices evaporate they create a coma surrounding the comet which is very reflective of sunlight. As the comet gets closer to the sun this coma becomes larger and brighter. In addition, the sunlight begins to push away the vaporized gases of the comet. These pushed gases will form the tails of the comet.

Comets will often have several tails. But the tails come in two types. The first type is referred to as the dust tail. As the name implies, it is formed from dust pushed away by the light of the sun. This type of tail is generally curved in such a shape that it points away from the sun and behind the path of the comet. The second type of tail is called an ion tail. It is formed from the charged particles produced by the comet evaporation. These charged particles are pushed away by the solar wind. The ion tail is generally straight and pointing away from the sun. As the comet approaches the sun the tails get longer. And as the comet recedes from the sun the tails will get shorter.

Evaporation of the comets as they approach the sun causes loose particles to exist in space. These particles will orbit the sun along the same general path the comet follows. On occasion, the Earth will pass through this orbital path. And when it does so, the particles fall to the Earth in a meteor shower. Since the Earth will always pass through that comet's orbit at the same time of years every year, these meteor showers are regular and predictable.

This page was last updated on 06/13/01.