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How Do Comets Change as They Approach the Sun?

Comets have very large orbits around the Sun that may take as many as 30 million years to travel. Comets are sometimes referred to as dirty snowballs because they are made up of ice, gas and dust. These materials are remnants left over from the origination of our solar system and can offer a glimpse into the past. When a comet approaches the Sun, the Sun's radiation reacts with the gases and ice, altering the comet's composition and appearance.

Leaving the Oort Cloud and Kuiper Belt
- Comets originate from the Oort Cloud or the Kuiper Belt in the Outer Solar System, a region past all the planets and filled with asteroids and comets. Because it is so far from the Sun, comets stay frozen. They are solid, stable balls of ice, gas and dust. It is only when they enter the Inner Solar System, where the planets are, and approach the Sun that any reaction occurs.
Heating Up in the Inner Solar System and the Coma
- Once a comet's orbit brings it into the Inner Solar System, it begins to heat up and heats at a faster rate as it nears the Sun. The Sun's radiation causes the ice in the nucleus of the comet to sublimate, converting straight from a solid ice to gaseous steam. As the ice sublimates, it pulls carbon dioxide off of the nucleus as well. The evaporated water and carbon dioxide then form a thick cloud around the comet called the coma.
Dust and Ion Tails
- The Sun's radiation then forces the dust from the nucleus of the comet, causing it to drag behind the comet, which creates the dust tail. The dust tail is the most easily visible part of the comet; it looks long and hazy. Solar wind, a fast flow of charged particles from the Sun, interacts with the gases in the nucleus, causing them to become charged, or ionized, by robbing protons or electrons from the atoms of the gases. These ionized gases give comets their blue color. Ionization causes the gases to be stripped from the nucleus and drag behind the comet, forming the ion tail. The ion tail is much longer than the dust tail and can usually be seen with a high-powered telescope.
Other Effects Caused by the Sun
- The Sun's solar wind changes the direction of the comet's tails. The tails are always pointed away from the Sun and change direction once the comet begins to travel away from the Sun. This sometimes causes the dust and ion tails to look like they are in front of the comet. The Sun also causes the comet to leave some of its ice and gas behind each time it travels into the Inner Solar System. Most comets do not complete more than 500 orbits around the Sun before they lose all of their ice and gas and become more like an asteroid.

Astronomy