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What Are Comets?

Comets are small celestial bodies that orbit the Sun. Primarily made of dust and ice, they are thought to be remnants of the formation of the Solar System.

A comet glowing in the dark winter sky.

Comet PanSTARRS was visible in early 2013.

Comet PanSTARRS C/2011 L4 was visible to observers on Earth in early 2013.


Where Do Comets Come From?

Comets are thought to come from 2 places in the Solar System:

  • The Oort Cloud is a region almost a light year from the Sun. Oort Cloud comets have very long orbital periods, spanning several million years, and are known as long period comets.
  • The Kuiper Belt is a region beyond the orbit of the planet Neptune. Kuiper Belt comets tend to have a short orbital period, usually around 200 years, and are therefore also known as short period comets.

They Have a Coma

One of the distinguishing features of a comet is that most of them develop a tail, known as a coma, when they come close to the Sun.

Away from the Sun, comets are frozen celestial bodies that are hard to detect. However, as a comet comes closer to the Sun, the Sun’s heat and radiation vaporize the ice and dust of the comet. These vaporized gasses collect dust and stream out of the center of the comet like a tail. This tail can be thousands of miles long.

Difficult to See

While most comets passing by the Sun are hard to observe from Earth without specialized equipment, some comets are bright enough to be seen by the naked eye. The brightness of the comet is due to sunlight reflecting and refracting off the dust in the tail.

Comets usually have 2 tails, which point in different directions. The dust in the comet is responsible for one tail. This tail, also called the dust tail, tends to be broad and curved. The gasses in the comet make the second tail, called the plasma or the ion tail. This tail is thin and straight and tends to point directly away from the Sun.

Did You Know?

A light year (light-year or lightyear) is a unit of distance and is defined by the International Astronomical Union as the distance traveled by light in a vacuum during a Julian year. It is approximately equal to 10 trillion kilometers (6 trillion miles).

Topics: Astronomy, Comets, Asteroids



  1. What Are Comets?
  2. Dec 2018: Comet 46P/Wirtanen
  3. Nov 2013: Comet Encke (2P/Encke)
  4. Nov 2013: Comet ISON (C/2012 S1)
  5. Nov 2013: Comet Lovejoy (C/2013 R1)

More about comets

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