Lecture Notes:

Before the planetarium show "Cosmic Collisions" we reviewed the various objects still lying between the planets. There use to be lots more of this junk, but the planets and the sun have sucked it in like cosmic vacuum sweepers.

• ASTEROIDS: between Mars and Jupiter, consisting of material that was prevented from forming a planet by the gravitational influence (tidal) of Jupiter. There are about 100,000 asteroids, but their total mass is quite small, perhaps only 0.04% of the mass of the earth. Resonances with Jupiter cause gaps in the asteroid belt and also send some asteroids along orbits that cross the earths orbit. Collisions between asteroids also send some of them toward the earth. Asteroids appear on the earth as meteorites.
• COMETS: These appear to be chunks of dirty ice left over from the early days of the solar system. They have variable amounts of carbon (soot-like) covering them, which determines their variably "activity". Those that are totally covered with soot have SHUT OFF; sunlight can not reach the ice and cause it to evaporate. When they fall to the earth, many comets may produce air blasts, similar to that of Tunguska, when the ice become suddenly vaporized. There is a great reservoir of comets in the Oort comet cloud which extends to perhaps 100,000 AU; one-third the distance to the nearest star, Alpha Centauri.
• METEORS: Those meteors that occur in showers such as that of the Perseids of August 12 come from comets and are dropped along the orbits as the ice melts and rock fragments are released. These tiny fragments are vaporized high in the atmosphere. Sporadic meteors which are not associated with showers may be from comets or they may be small asteroids.
• METEORITES: These largely come form the asteroid belt and by definition are large enough to reach the earths surface. Twelve meteorites collect on the Antarctic ice appear to have come from Mars, blasted off its surface from the meteorite impact. Two of those contain evidence for life on Mars.
• INTERPLANETARY DUST: Debris from asteroid collisions and evaporated comets collects as dust between the planets and is concentrated in the ecliptic plane. The dust is visible in the evening or dawn at the Zodiacal light.