Taking away the Sun, planets, satellites, and rings, there is still a substantial amount of material left in the solar system (about half the mass of Jupiter according to Table 6.1). We classify the material as asteroids or comets. Asteroids are rocky and metallic, containing little volatile matter. Comets are a mixture of ices (water, carbon dioxide, and methane ices for example) and solid grains.
Most of the asteroids orbit in a region between Mars and Jupiter called the asteroid belt. The first asteroid was discovered in 1801. It's called Ceres and is the largest asteroid with a diameter of less than 1000km.
It is estimated that there are about a million asteroids of diameter greater than 1km. We have identified less than 10,000 of these, less than 1% of the total. Recall that the asteroid believed responsible for the extinction of the dinosaurs was about 20km in diameter. The object that made Meteor Crater in Arizona was about 50m in diameter, and its impact equivalent to an explosion of 15 million tons of TNT.
The asteroids all revolve around the Sun in the same direction as the planets, and orbit in the same plane as the planets. Most but not all of the asteroids lie in the asteroid belt between the orbits of Mars and Jupiter. While the asteroids are numerous, the area of the asteroid belt is very large. The separation between asteroids is several million kilometers, sufficiently large for our space probes to get through without incident (knock wood).
Skip. Just know that there are different types of asteroids.
Don't worry about too many details here. Several spacecraft have flown by asteroids for close-up measurements. One spacecraft has even landed on an asteroid (see the supplement to the text).
Some asteroids are located outside the asteroid belt. We'll discuss a couple of types of such asteroids.
An interesting group of asteroids lies roughly on the orbit of Jupiter and is called the Trojans. While Jupiter's gravity make it impossible for asteroids to orbit nearby, there are two locations, spaced roughly 60° (see Figure 12.10) from Jupiter where asteroids are stable. There are asteroids there, and the groups are called the Trojans.
In 1990 Trojan-like asteroids were found in association with Mars. It may be that there are Trojan-like asteroids in the orbits of other planets, including Earth.
Public awareness of Near-Earth Objects (NEOs) has increased dramatically in the last decade. Several events have contributed to this phenomenon:
Observations of comets precede written history. The typical comet appears as a fuzzy spot in the sky -- sometimes the tail can be seen with the naked eye. Spectacular comets are more rare, but do occur periodically. You may remember Comet Hyakutake which was visible in March 1996, or Comet Hale-Bopp in March 1997.
Comets themselves are relatively small chunks of icy material. What we see is the atmosphere that forms around the comet as it nears the Sun whose heat melts and evaporates some of the ice. As it gets closer to the Sun, the atmosphere forms a tail.
Comets tend to follow highly elliptic orbits. In such orbits they spend relatively short periods close to the Sun, where they evaporate and become visible. Most of their life is spent far from the Sun in the deep freeze of space. (See Figures 2.5 and 2.10 for a reminder of the nature of orbits.)
Probably the most famous comet is Comet Halley. It has an orbital period of about 76 years. It's last passage near the Sun was in 1986; it will reappear around 2061. The period of its orbit varies due to interactions with the outer planets.
The core of a comet, the chunk of ice, is called the nucleus. When we view a comet in the sky, we see the atmosphere produced by evaporation of gases off the surface of the nucleus. The nucleus is the real comet.
Astronomers picture comets as chunks of ice with dust and silicates mixed in. This is known as the dirty snowball model for comets. Figure 12.16 shows a picture of the nucleus of Comet Halley taken in 1986. It took the coordinated work of two Russian probes (VEGA 1 and VEGA 2) and a European probe (Giotto) to get the picture.
When far from the Sun, a comet's nucleus remains cold and frozen. Near the Sun, the nucleus is heated and evaporates to form an "atmosphere". The atmosphere is the fuzzy object that we see as the "comet". The gravity of the nucleus is insufficient to hold the gases in the atmosphere, so they continuously move away from the core at about 1km/s. The atmosphere of a comet can reach an enormous size, as large as Jupiter.
As a comet approaches the Sun it can develop a tail. The tail is the atmosphere being pushed by sunlight and the solar wind. The tail always points away from the Sun (see Figure 12.19). Sometimes a comet develops two tails, one of neutral particles and one of charged ions (see Figure 12.20).
The Dutch astronomer Jan Oort noticed that new comets have orbits whose aphelia (furthest points from the Sun) are about 50,000AU. He proposed that a reservoir of comets (icy chunks) orbiting at this distance from the Sun, and that periodically one is perturbed by a nearby star such that it drops into a highly elliptic orbit that brings it close to the Sun. This reservoir is called the Oort comet cloud.
Astronomers estimate that there are about a trillion comets in the Oort cloud, and about ten times this number of other objects.
Some comets come from a closer in region that is now called the Kuiper belt. Just recently a new object was discovered in the Kuiper belt. This object is about 800km in diameter and lies a bit further out than Pluto.
A comet can survive indefinitely in the frozen environment of the Oort cloud or Kuiper belt. But once dislodged a number of things can occur on its first pass by the Sun.
Discuss impact of Shoemaker-Levy 9 with Jupiter.