A ball of ice and rock, followed by a tail of gas, rockets through the Universe. As it approaches the sun, it burns, glowing with white-hot beauty. It loops around the sun and disappears into space. Civilizations will fade and die before it returns. Depending on the path it took, it may never again be seen by human eyes.
Long Periods and Parabolas
Scientists divide comets into two types: short-period and long-period. Short-period comets start within our solar system. They circle the sun in orbits that take less than 200 years. Halley’s Comet, which passes Earth every 75 years, is an example of a short-period comet.
Long-period comets can take hundreds, thousands, or even millions of years to circle the sun. They start in the Oort Cloud, a sphere of debris that surrounds our solar system. Some astronomers think the Oort Cloud is left over from the formation of the sun and planets. Most of the material in the cloud is in a stable orbit around our solar system. However, sometimes a chunk of rock and ice gets knocked loose and starts heading towards the sun. The path of these long-period comets is best described by a parabola. When a long-period comet approaches Earth, it often puts on a spectacular show for sky watchers.
Sometimes, a comet’s path intercepts a planet. In 1994, a short-period comet known as Comet Shoemaker-Levy 9 crashed into Jupiter, giving astronomers a chance to study what happens when such a collision occurs. Some scientists believe that comets have struck Earth throughout our planet’s history. For instance, a comet’s impact may have suddenly cooled Earth 12,900 years ago, altering the world’s climate for more than 1,000 years.
Watch the following videos about collisions and close encounters with comets.