COMETS

Key Concepts

  • Comets are ``dirty snowballs'': ice mixed with dust and carbon compounds.
  • When a comet is close to the Sun, it grows an ion tail and a dust tail.
  • Most comets are in the Kuiper belt and Oort cloud, far from the Sun.

(1) Comets are ``dirty snowballs'': ice mixed with dust and carbon compounds.

When comets are close to the Sun, they sport gaudy tails made of dust and ionized gas. When a comet is particularly close to the Earth, its tail makes it visible to the unaided eye. On average, there's one unaided-eye comet per decade. Aristotle thought that a temporary phenomenon like a comet couldn't be part of the perfect and unchanging heavens; he thought comets were a strange type of cloud in the Earth's atmosphere. In 1577, however, Tycho Brahe attempted to measure the parallax of a comet, and found that it was far beyond the orbit of the Moon. Edmund Halley, a contemporary of Isaac Newton, attempted to calculate the orbits of comets, under the assumption that they were going around the Sun on elliptical orbits. Halley found that a comet that had been observed in 1607 had the same orbit as a comet that he observed in 1682. Halley stated that these were two appearances of the same comet, and that it would reappear in 1758. Sure enough, the comet was spotted on Christmas 1758, and Halley was proved correct (unfortunately, he had died twelve years earlier). The comet was named Halley's comet in his honor.

The most famous of all comets, Halley's comet (a.k.a Comet Halley) last reached perihelion in February 1986, and will next reach perihelion in July 2061. Halley's comet is on an eccentric and inclined orbit. At perihelion, it is only 0.6 AU from the Sun (inside the orbit of Venus); at aphelion, it is 35 AU from the Sun (outside the orbit of Neptune).

Strip away its garish but insubstantial tail, and a comet is nothing more than a dirty snowball several kilometers across. A comet contains very un-exotic materials:

  • frozen water
  • frozen carbon dioxide (dry ice)
  • ammonia
  • dust, and larger chunks of rock
  • carbon
  • complex carbon compounds
(In fact, if you have access to dry ice, you can make a miniature comet yourself. The recipe is given here.)

During its last perihelion passage, in 1986, a small flotilla of spacecraft visited Halley's comet. One of the craft, named Giotto, approached within a few hundred kilometers of the central icy nucleus of the comet. Before being sandblasted to death by the dust cloud surrounding the nucleus, Giotto sent back the picture below:

Halley's comet

The bright patches at the upper right are jets of gas spurted forth by the comet as sunlight heats its surface. The comet's nucleus itself is the dark blob at the lower left. From the information sent back by Giotto, we know that the nucleus (the central dirty snowball) of Halley's comet has the following properties:

  • Irregular in shape: 15 kilometers long, 8 kilometers wide. Craters are visible on the surface of the nucleus.
  • Very dark: reflecting only 3% of the light that strikes it, the nucleus is encrusted with dust, carbon, and dark carbon compounds.
  • Low in density: only 200 kg/m3; the nucleus is a loosely-packed snowball, easily torn apart by tidal forces if it approaches the Sun or a planet within its Roche limit.

(2) When a comet is close to the Sun, it grows an ion tail and a dust tail.

When a comet comes within a few AU of the Sun (within the asteroid belt), the frozen water and carbon dioxide at its surface starts to sublime (turn to gas) as sunlight heats up the nucleus. A comet which approaches the Sun has the following structure.
  • 1) Nucleus: the central dirty snowball. Only a few kilometers across, it contains over 99% of the total mass of the comet.
  • 2) Coma: a cloud of gas and dust surrounding the nucleus. About 100,000 kilometers across (nearly as big as Jupiter), but very low in density.
  • 3 a) Ion Tail: a long streamer of ionized gas. Up to 100,000,000 kilometers long, but even lower in density than the coma.
  • 3 b) Dust Tail: a long streamer of dust particles. Usually slightly shorter than the ion tail.
Thus, a comet has two tails.
The ion tail consists of gas from the coma which has been ionized by photons striking the gas molecules. The ions in the ion tail are pushed away from the Sun by the solar wind, a continuous flow of protons, electrons, and helium nuclei that flows away from the Sun's atmosphere.
The dust tail consists of fine dust particles from the coma. When photons from the Sun strike the tiny dust particles, they transfer momentum to the dust, and push it away from the Sun.

The ion tail is visible because it consists of hot, low-density ionized gas, and thus displays an emission spectrum. The dust tail is visible because the dust grains reflect sunlight. The tails of a comet always point away from the Sun, no matter what direction the comet is traveling in. In the picture of Comet Hale-Bopp shown below, the blue ion tail is seen at the top, and the white dust tail is below it.

Most comets that we see from Earth (that is, comets that venture close enough to the Sun to acquire a coma and tails) have two types of orbit:

  • Long-period (P > 200 years)
    For example, Hale-Bopp (in the picture above) is a long-period comet. There are about 800 known long-period comets. Sometimes the period of a long-period orbit is very long indeed; up to 30 million years, implying an orbital size of nearly 100,000 astronomical units.
  • Intermediate-period (20 years < P < 200 years)
    For example, Halley's comet, with its period of 76 years, is an intermediate-period comet. (A picture of Halley's comet is given below.) There are about 200 known intermediate-period comets.

(There are a few `short-period' comets known, with P < 20 years, but they are extremely rare.)

After a couple hundred passes by the Sun, comets tend to lose all their ice. Thus, all the intermediate-period comets we can see have been growing comas and tails for less than 40,000 years. There must be a repository of comets in the outer solar system which periodically flings new comets to within a few AU of the Sun, where they grow comas and tails.

(3) Most comets are in the Kuiper belt and Oort cloud, far from the Sun.

Short-period comets originate in the Kuiper belt, in the same plane as the planets' orbits, from 30 to 500 AU from the Sun.

Intermediate and long-period comets originate in a much larger, much more distant cloud of comets, called the Oort cloud. The Oort cloud (named after the astronomer who first deduced its existence) is roughly spherical, and stretches from the Kuiper belt to 100,000 AU from the Sun. (That's roughly 4 light-months to 1.6 light-years!) The Oort cloud stretches nearly halfway to Proxima Centauri, the Sun's nearest neighboring star.

It is estimated that there are several trillion comets in the Oort cloud. That would mean the total mass of comets in the Oort cloud is one hundred times the mass of the Earth, or a third the mass of Jupiter. Out of these trillions and trillions of comets, we see only those very rare comets that are on highly elongated orbits, taking them to within a few AU of the Sun. The vast majority of comets never come within 20,000 AU of the Sun. Thus, comets in the Oort cloud are kept in a perpetual deep freeze. In the Oort cloud, the Sun is nothing more than another star, and the comets are kept at the chilly temperature of 3 degrees Kelvin. They are pristine and well-preserved remnants of the initial solar nebula.

updated: