What is a Comet?

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Comets are hardly ever seen by the unaided eye so whenever such a rare occurrence takes place, people viewing one up in the sky are often prompted to ask, “What is a comet?”

A comet is a small, irregularly- shaped, icy body that circles the Sun in long, elliptical orbits, which come very close to the Sun, and then go back deep into space, sometimes way past Pluto’s orbit. A comet is made up of a solid nucleus or core, enclosed in a cloudy atmosphere called coma. Together, the two compose a comet’s head, while one or two tails develop as the comet draws nearer to the Sun.

Even with a telescope, a comet is difficult to detect deep in space because of its small size. This is because when far from the Sun, comets start out as just chunks of ice and rocks and the nucleus is frozen cold. In such state, comets are also referred to as “dirty snowballs.”

When it approaches the Sun however, the nucleus’ icy material slowly evaporates, and the melted ice develops into the coma of gas and dust. The coma grows bigger the closer it gets to the Sun, even surpassing the size of planets. Radiation from the sun and solar winds push dust particles away from the coma, forming the comet’s dust tail.

Comets are said to be one of the largest bodies in our Solar System. While a comet’s nucleus only measures from a few hundred meters to tens of kilometers across, some comas have been known to reach diameters of up to 1 million miles or 1.6 million kilometers. In addition, comet tails can extend to distances of more than 100 million kilometers.

While most comets have elliptical orbits, an orbit’s elongation is determined by its orbital period, that is, the longer the orbital period is, the more elongated the ellipse. This is how comets are classified.

Short-period comets are those that have orbital periods of 200 years or less. The orbits of these comets often take them within the regions of the outer planets. The aphelion (farthest point in the elliptical orbit from the Sun) of the well-known Haley’s Comet for instance, is but a short distance away from Neptune’s orbit.

Comets which orbital periods range from 200 years to thousands and even millions of years on the other hand, are classified as long-period comets. These comets have highly eccentric orbits which take them far beyond the planets.

When someone asks, “What is a comet?” we often imagine a mysterious, celestial body that sweeps across the sky from time to time. Simply put though, like the Earth, it’s just another object circling the Sun.

We’ve got some cool articles about comets in space here in Universe Today. Here are two of them:
What Are Comets?
What is the Difference Between Asteroids and Comets
Here are two more from NASA:

Comet
A Bright Comet Is Coming

Tired eyes? Listen to a couple of episodes from Astronomy Cast:

Comets, Our Icy Friends From the Outer Solar System
Asteroid Belt

Source: NASA

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