This section addresses, in whole or in part, the following Georgia GPS standard(s):
This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
This section addresses, in whole or in part, the following National Science Education Standards:
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:
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.
Meteor showers are associated with comets. They occur when the Earth passes through the debris of a (burned out) comet.
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?
Slice of an iron metoerite showing the Widmanstatten pattern. Approximately true size.
Slice of a stony iron meteorite. Light color is metal (iron-nickel). Other part is olivine crystals. Approximately twice actual size.
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.
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:
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.
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.
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.
Location of the Asteroid Belt. Image courtesy of NASA.
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,
Page created March 28, 2005
Updated links and email October 1, 2008