This lesson will require approximately one 30-minute period.
300-23: describe the physical characteristics of components of the solar system - specifically, the sun, planets, moons, comets, asteroids, and meteors
Students will be able to correctly identify the parts of a comet. Students will become familiar with the causes of meteors and meteor showers.
Students will fill in a diagram of a comet after viewing applets and descriptions of comets and meteors.
Nota : This page contains documents for which the access may require a particular software. If the software is not installed, you can download it and follow the instructions for installation.
Begin by asking students what they believe causes meteor showers. Discuss possible ideas.
Introduce the Comets applet, explaining the eliptical orbits of comets, and the result of comets passing through Earth's orbit, leaving behind cometary debris of ice, rock, and dust. Show the Meteor Showers applet to illustrate, making note of the red meteors, which indicate fireballs (especially bright meteors).
Continue the lesson by describing the following while referring to the applets:
A comet can be a beautiful sight and is unlike any other object in the sky. The brightest comets are visible with the naked eye as extended fuzzy patches, although this is rare and most require a telescope to see. They progress across the sky very slowly, remaining in our sky for weeks before fading out of view. Comets are small bodies composed mainly of ice and dust whose orbits take them to remote regions outside the solar system. It is believed that comets originated during the formation of the solar system as condensed chunks of ice and rock, and continue to orbit the Sun in highly elliptical orbits inclined at various angles from the ecliptic.
It was not until 1986 that a cometary nucleus was observed, when spacecraft were sent to take high-resolution photographs of the nucleus of Halleys Comet. Astronomers now know that the nucleus of a comet is composed of ice and dust and typically measures a few kilometres across. As a comet approaches the Sun, solar heat begins to vaporize the ice in the nucleus and it becomes enveloped in a coma, a fuzzy ball of glowing gases which hides the nucleus from our view. The solar wind blows the vaporized gases from the coma, producing a long, luminous tail of gas and dust. A comet does not produce its own energy or light, the gases illuminate because they reflect sunlight. As the comet approaches the Sun, solar radiation becomes more intense, melting more ice and increasing the size and length of the comets tail, which can vary in appearance from fat and short to thin and extended. (Comet Ikeya-Seki in 1965, for instance, had a long, skinny tail which was 150 million kilometres long -- equal to the distance between the Earth and the Sun.) Solar radiation produces two comet tails, although it is not always possible to distinguish both individually. The ion tail is straight and is caused by the solar wind, pointing directly away from the Sun. The dust tail is typically curved slightly and is caused by radiation pressure exerted on the dust particles by photons from the Sun. The tail of a comet will always point away from the Sun, and is not necessarily an indication of the comets direction of travel.
Comets are generally classified in two ways. Some are known as short-period comets and have orbital periods of less than 200 years. They appear at regular and predictable intervals, allowing astronomers to prepare for their return. The most famous short-period comet is Halleys Comet, which has a period of 76 years and is set to return in 2061. The majority of comets, however, are long-period comets that can take up to a million years to orbit the Sun. Most comets discovered each year are of this type, and as such have never been scientifically documented. Because of their long orbital period, there may be millions of long-term comets which have yet to be discovered. It is believed that these comets spend most of their orbital period in the Oort cloud, a cloud of comets some 50,000 AUs from the Sun. This cloud contains millions of comets travelling extremely slowly in their orbits because the Suns gravitational pull is weak at such extreme distances. The best known long-period comets in recent years have been Hyakutake (1996), Hale-Bopp (1997), and Shoemaker-Levy 9, which crashed into Jupiter in 1994.
After familiarizing students with the above content, distribute the lesson's worksheet for students to complete from memory during the last few minutes of class.
Worksheets may be evaluated for accuracy if desired.