Earth image Comets, Asteroids, and Meteors

Dr. Pamela Gore
Georgia Perimeter College


  1. Distinguish between comets, asteroids, meteors, and meteorites.
  2. Discuss the three main types of meteorites.
  3. Describe the orbit of a comet, and the orientation of the comet's tail.
  4. Describe the orbit of asteroids.
  5. Explain the significance of Apollo objects.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E1f. Describe the characteristics of comets, asteroids, and meteors.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Large numbers of chunks of rock orbit the sun. Some of those that the earth meets in its yearly orbit around the sun glow and disintegrate from friction as they plunge through the atmosphere-and sometimes impact the ground. Other chunks of rocks mixed with ice have long, off-center orbits that carry them close to the sun, where the sun's radiation (of light and particles) boils off frozen material from their surfaces and pushes it into a long, illuminated tail.

This section addresses, in whole or in part, the following National Science Education Standards:
  • The earth is the third planet from the sun in a system that includes the moon, the sun, eight other planets and their moons, and smaller objects, such as asteroids and comets. The sun, an average star, is the central and largest body in the solar system.[


Comet. Image courtesy of NASA.

Many comets exist. Approximately 12 are discovered each year, mainly by amateurs. Several hundred billion are estimated to exist.

Comets have very elliptical (elongated, not round) orbits. It takes comets varying lengths of time to orbit the sun. Some have fairly short periodicities (3 years) and others have periodicities of hundreds of thousands of years. For example, Halley's comet returns every 76 years. It was first seen in 240 BC by the Chinese.

Millions of comets are believed to form a roughly spherical cloud (called the oort cloud) beyond the orbit of Pluto, at a distance of about 100,000 AU, nearly half the distance to the nearest star. (Pluto's orbit is at about 40 AU, varying from 29.7 to 49.2 AU.)

Comets tend to be small (several km in diameter). For example Halley's comet is 16 x 8 km.

Essentially large dirty snowballs, composed of a porous mixture of frozen gases, rocky and metallic material:

  1. water ice (Halley's comet is 80% water)
  2. carbon dioxide (dry ice)
  3. ammonia
  4. methane

Note: You can make a comet in the classroom with dry ice, ammonia, dirt, etc.

As a comet nears the sun, solar energy vaporizes frozen gases, forming a glowing "head" called a coma.

The comet also develops a tail of ionized gases and dust that may be millions of km long. The tail always points away from the sun, (due to radiation pressure and solar wind) and only follows behind the comet as the comet approaches the sun. As the comet moves away from the sun, the tail preceeds the head.

The orbit of a comet. Image courtesy NASA.

The orbit of a comet. Image courtesy NASA.

Comets have no light of their own, but reflect sunlight.

Meteor showers are associated with comets. They occur when the Earth passes through the debris of a (burned out) comet.

Meteors, meteoroids, and meteorites

Meteor = shooting star.
Glow of small particles being heated as they enter the atmosphere.
Can see 6 - 60 per hour.

Meteorites = rocks that reach the earth's surface from space.

Meteoroid = the object before it enters the Earth's atmosphere. Most are destroyed by about 80 km above the earth's surface.

Fireballs = very bright meteors with trains that last up to 30 minutes.

Bolide = Fireball that breaks up on atmospheric entry.
Possible source of CO2?

Types of meteorites

  1. Irons
    Composed of iron and nickel (like Earth's core)
    Internal metallic crystalline structure that looks "criss-crossed". Called Widmanstatten pattern. About 5% of all meteorites

    Slice of an iron metoerite showing the Widmanstatten pattern. Approximately true size.

  2. Stony irons
    Composed partly of Fe, Ni and partly of silicate minerals, including olivine (like Earth's mantle).
    About 1% of all metoerites.

    Slice of a stony iron meteorite. Light color is metal (iron-nickel). Other part is olivine crystals. Approximately twice actual size.

  3. Stony
    Silicate-rich (Si and O) with 10-15% Fe and Ni (like Earth's crust)
    Light gray colored. About 94% of all meteorites.
    1. Ordinary chondrites
      Have light-colored spherical structures called chondrules.
      May be droplets of once molten material. About 80% of all meteorites. Most abundant type of meteorite.

    2. Carbonaceous chondrites
      Similar to ordinary chondrites, but contain hydrous (water-bearing) minerals, volatile trace elements, and about 5% organic compounds, including inorganically-produced amino acids
      (Amino acids are the building blocks of proteins). Perhaps these meteorites are linked to the origin of life on Earth.
      Minerals were formed at low temperatures.
      Contain chondrules and irregular pieces of crystalline material.
      Overall, chemically similar to the sun, but without the H and He.
      About 5% of all meteorites.

      Carbonaceous chondrite meteorite that fell over Pueblo de Allende, Chihuahua, Mexico, February 1969. Chondrules (light colored circles) formed at least 4.5 billion years ago. About four times actual size.

    3. Achondrites
      Without chondrules.
      Similar to basalt. Similar to the Earth's oceanic crust.
      About 10% of all meteorites.

Facts about meteorites

Age of all known meteorites is between 4.4 and 4.6 billion years.

Meteorites represent primitive solar system material.

Several hundred tons of meteoritic matter fall to Earth each DAY! Most is microscopic.

Meteorites are a significant addition to the amount of carbon on the Earth.

Meteorites were probably derived from a differentiated body which has been dirsupted by collisions. They contain no high pressure minerals, hence they are not derived from a large parent body. Probably come from once-molten asteroids.

Largest known meteorites:

In 1970 about 2300 were known to exist. Now, more than 5000 have been found.

Some are derived from Mars (as indicated by composition of gases contained in them), and some are derived from the moon.

Did a Mars-sized meteorite hit the Earth to form the Moon?

Only two documented casulties. One killed a dog in Egypt in 1911, and one hit Mrs. Hewlett Hodges of Sylacauga, Alabama when it crashed through her roof and severely bruised her arm on November 30, 1954.

Other recent impacts:
In Wethersfield, Connecticut in 1971, a 284 gram (0.6 pound) meteorite crashed through the roof of a house.

In Wethersfield, Connecticut in 1982, another meteorite struck a different house. This one crashed through the roof and into the living room, bounced into the dining room, and landed under a table.

Meteor showers occur at specific times during the year. Some are the result of the earth passing through the tail of a comet.


Asteroids. The one at the top is Gaspra, in the middle is Ida, and on the bottom is Dactyl (which is actually a tiny "moon" of Ida). All these pictures were taken by the spacecraft Galileo. Image courtesy of NASA.

"Asteroid" means "star-like" (to distinguish them from the major planets)

The first (and largest) asteroid, Ceres, was discovered in 1801.
Only a few 100 km across (< 1000 km)
Smaller than the jovian moons.
Highly inclined orbits.

Second discovered in 1802 (Pallas).

About 9000 now known.

Irregular shapes.

If all were put together, they would form a body smaller than Pluto, and smaller than the moon.

Many were discovered by amateurs.
Many probably still undiscovered.

Compostion of Asteroids

Three major types:
  1. C-Asteroids
    Carbon-rich. Dark colored. Carbon and silicates. Most abundant type.
  2. S-Asteroids
    Silicate-rich. Lighter colored.
  3. M-Asteroids
    Metallic. Spectrum indicates that they are metal-rich minerals. Least abundant type.
Densities vary from 2.3 to 3.6 g/cm3

Location of Asteroids

Asteroid belt is mostly between Mars and Jupiter. But not all lie within the asteroid belt. Most lie between 2.2 and 3.2 A.U.

Location of the Asteroid Belt. Image courtesy of NASA.

Their orbits have large eccentricities and inclinations.
Unlike planets, some asteroids share the same orbit. There are about 100 different "families" of asteroids with similar orbital characteristics. Why? Pieces of one or more larger asteroids?

About 30 asteroids are known to cross Earth's orbit. These are called the Apollo asteroids or Apollo objects.
Most of these are a few km in diameter; largest is 8 km.
There is a potential for collisions with the Earth. A 1 km asteroid would produce a crater 10 - 50 km across. Energy release would be approximately equal to 30,000 one-megaton nuclear bombs.

Meteor Crater, Arizona is 1 km across. Created by a 30 meter meteorite.

Meteor Crater, Arizona, also known as Barringer Crater. Image courtesy of D. Roddy, NASA.

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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 28, 2005
Updated links and email October 1, 2008