Comets are sometimes called “dirty snowballs” or “snowy dirtballs.”  There is a lot we don’t know about comets, but here are ten facts we do know:

  1. Comets orbit the Sun, similar to the way planets in our solar system orbit the Sun.
  2. Comets contain dust, ice, carbon dioxide, ammonia and methane originating from the early formation of the solar system (about 4.5 billion years ago).
  3. Comets are  are generally thought to come from two areas – 1) the Oort Cloud and 2) the Kuiper Belt. Both areas are in the outer regions of our solar system. These are areas containing materials left over from the formation of our solar system, which have condensed into icy objects. Although these regions extend beyond the orbits of planets in our solar system, they are still considered part of our solar system.
  4. Comet have elliptical orbits, which brings them close to the sun and takes them far away.
  5. Comets have obits around the Sun that range at the extremes from about 20 years to 200 years. Comets with obits between the extremes are called Halley-type comets.
  6. Comets have three parts:
    1. The nucleus, which is the solid center component made of ice, gas and rocky debris
    2. The coma, the gas and dust atmosphere around the nucleus, which results when the Sun heats the comet’s surface
    3. The tails, which are formed when energy from the Sun turns the coma so that it flows around the nucleus and forms a fanned out tail behind it. Comet tails can extend millions of miles and point away from the Sun, not the direction the comet is moving
  7. We are able to see a comet’s coma and tail when the sunlight reflects off the dust and when it excites some molecules so that they form a bluish tail called an ion tail and a yellow one made of neutral sodium atoms.
  8. Comets range in size from less than 1 km (about 3000 feet) in diameter to as much as 300 km (over 186 miles) in diameter.
  9. A a comet could impact Earth. It is important to understand the nature of comets so we can design better methods to protect ourselves should a large one be on a collision path with Earth.
  10. From NASA’s Deep Impact mission (2005) with the Tempel 1 comet, we now know:
    • The comet’s nucleus is spongy, with holes inside
    • Parts of the surface are fragile and weak
    • The surface of the nucleus is covered with fine dust, like baby powder
    • The surface is composed of carbon-based black material
    • Some parts of the nucleus are smooth and young (likely due to the Sun melting effects), while other areas are cratered and old (likely due to celestial impacts)
    • The nucleus seems to have formed from overlapping layers of different materials, similar to the way the Earth is formed in layers of different materials
    • There is water ice just below the surface and carbon dioxide ice (also known as “dry ice”) farther down
    • The Tempel 1 comet contains materials from the outer, middle, and inner parts of the solar system. We are not sure why or how this occurred

One last point: Comets lose ice and dust each time they come near the sun, leaving behind trails of debris. Eventually, they can lose all their ices, with some turning into fragile, inactive objects similar to asteroids.


1) Comets: “Formation, Discovery and Exploration,” by Charles Q. Choi, Contributor, November 15, 2010



Image: Wikimedia Commons – Comet Holmes (17P/Holmes) in 2007, showing blue ion tail on right