Oort Cloud

• A comet is basically a ball of ice, frozen gas, rock and dust, ranging in diameter from half a mile to 60 miles. This ball is called the nucleus, and at its heart it often hides a solid rocky core. Just beyond Neptune's orbit, so-called short-period comets circle the sun in the Kuiper belt. Occasionally one of them is nudged into an orbit that will take it closer to the sun, and because these orbits take only 200 years to complete, the reappearance of these comets is often predictable. Other long-period comets circle the sun in the much more distant Oort Cloud; the distance between the sun and the Oort Cloud is 100,000 times greater than the distance between the sun and Earth. These long-period comets may take as long as 30 million years to complete their orbits, so their behavior is more difficult for Earthbound astronomers to predict.
  
  

Approach

• As a comet approaches the sun, the heat vaporizes the ice and frozen gases on its surface, creating a diffuse atmosphere called a coma. That atmosphere can be as many as 63,000 miles across. As the ice melts and vaporizes, dust grains are released, giving the coma a hazy appearance. The "head" of the comet you see in the sky is actually the glowing coma of the comet with the nucleus hidden at its core.
  
  

Tail

• Charged particles ejected from the sun's atmosphere flow outward along paths through the sun's magnetic field; this flow of particles is called the solar wind. This wind sweeps gas and dust from the comet's nucleus outward to form two tails. One of these tails is largely dust, while the smaller plasma tail is made of hot ionized gas. Often the plasma tail is more difficult to see. Both tails always point away from the sun.
  
  

Effects

• Most comets have highly elliptical orbits where the sun is much closer to one end of the ellipse than to the other. The closest point to the sun on its orbit is called its perihelion. As the comet approaches the sun, it forms a coma and tails, losing a substantial amount of material in the process. Once it leaves the inner solar system, the comet will gradually cool down, and both coma and tail will die away. Sometimes comets may lose all of their volatile material, in which case they no longer form coma and tails; at this point they are little different from large asteroids on highly eccentric orbits.