What is a Comet?

Composition: A comet is like a large "dirty snowball". They are composed of ice, clay and organic matter, including hydrocarbons in the form of an oil-like tar.⁸

Size: Comets can range in sizes up to several hundreds of miles in diameter. Several comets start out as great behemoths, which break up into smaller comets and eventually particles of dust and vapors over time. The comet Chiron is 120 miles across and so is the comet Pholus. Comet 1993 HA2 is 50 miles across.²² The comet "Shoemaker-Levy 9" that struck Jupiter on 1994 was composed of 21 discernible fragments, several ranging in size from around 0.5 mile to around 2 miles in diameter.⁷ It is estimated that this comet before it broke up was approximately 12 miles in diameter.¹ The comet "Hale-Bopp" is 25 miles in diameter and weighs 10 trillion tons.^{3, 6}

Speed: Comets travel at incredible speeds, around 100,000 miles per hour as they approach Earth.

Mass: It is theorized that the density of comets range from that of a fluffy snowball (0.1 grams per cc) to the density of sheets of ice and dirt (2.0 grams per cc).²²

Origins: Short period comets originate in the outer planetary systems, starting just beyond the orbit of Neptune, 35 to 1000 AU from the Sun.² (One AU is the average distance between the Sun and the Earth – about 93 million miles.) This disc shaped array is referred to as the Edgeworth-Kuiper Belt. Long period comets originate in the Oort Cloud, a huge sphere comprising trillions of comets that surrounds the solar system, 20,000 to 100,000 Astronomical Units (AU) from the Sun. The average short period comet has an orbital period measured from years to hundreds of years. Long period comets will often return after thousands or even millions of years, or not at all.¹⁶ The Oort Cloud is estimated to contain 7 trillion comets. The Kuiper Belt is estimated to contain about 1 billion comets.¹ There are 5 to 10 long period comets that approach the Sun each year.

Appearance: The distinguishing characteristic of a comet is it’s tail. As a comet approaches the sun, various ices on the surface of the comet’s nucleus begin to heat up. Water, ammonia, methane, nitrogen, carbon monoxide and carbon dioxide ice change from a solid to vapors. An extensive cloud of dust and volatiles known as "comae" forms around the comet, which makes them fairly easy to detect. Comets start to brighten markedly when they reach a heliocentric distance of 3 AU from the Sun. These gases are brushed away by the solar winds into a long ion trail. Small grains of dust released as the surface sublimes break free and form a separate dust trail.²³

A virgin comet will be a bright object in deep space. But as the comet makes several close passes around the sun or another star, the outer layer of volatiles will burn off. Large dust particles, too large to be blown away by evaporating ices accumulate on the surface of the nucleus. This dark layer eventually builds up to the point that it becomes an excellent heat shield against solar heating. A crust will form, making the comet very dark and difficult to detect from afar. As this loss of volatiles continues, the comet can reach a stage where the comet will not produce even the characteristic comet tail as the object approaches the sun. In this state, the comet may be impacted by small meteorites that puncture holes in the surface and produce fountaining of gases where sunlight now can penetrate.²²

Both comets and asteroids pose a danger to life on Earth. These objects differ in composition, speed, solar interaction, and orbital paths. Comets represent a greater danger because:

* Comets come into our Solar System from further out, by the time they cross the Earth’s orbit, they are traveling much faster than asteroids. As a result, comets possess significantly greater amounts of kinetic energy.¹

* Asteroids are found in well-established orbits around the Sun and can be monitored over many orbits and the future impact risk can be assessed and mitigated. Comets arrive unpredictably from the Outer parts of the Solar System.¹

* The trajectory of comets are less predictable because they are influenced by the effects of the volatile outgassing, which will produce great jets that will erratically alter the comet’s trajectory.

* Many comets are significantly larger than asteroids and as a result more destructive to mankind. There are significantly greater numbers of large comets than large asteroids. There are approximately 1,500,000 asteroids greater than 1 kilometer (0.6 miles) across in the main asteroid belt.⁵⁴ It is estimated that 1,000,000,000 comets exist in the Kuiper Belt (of which 100,000 are larger than 100 kilometers/60 miles across), and 7,000,000,000,000 comets exist in the Oort Cloud.⁶³ Some of these comets are behemoths, which are several hundred miles across.

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