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Comets and Meteors

Even though comets were long thought to have supernatural roles, scientists and philosophers tried to understand what comets were and where they came from. The Greek philosopher Aristotle thought that comets were meteors. Much later, Descartes from France thought they were messengers from other worlds. Gradually, though, scientists began to see that comets appear and disappear with regular cycles

Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.

When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice.

When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.

When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.

Each time a comet visits the Sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. For this reason, comets are said to be short-lived, on a cosmological time scale. Many scientists believe that some asteroids are extinct comet nuclei, comets that have lost all of their volatiles.

The following view of Comet West shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles.                           

Comet Hale-BoppDust Tail

Gas Tail

Coma

Where is the sun?

Dying Comet Gives Rare View of SpaceMay 18 2001 @ 12:09

A comet falling toward the sun last summer peeled apart like an onion, giving astronomers a close look at one of the icy balls of dust that may have played a role in forming life on Earth.

Where do Comets come from?

Meteors

The vast majority of meteors are the size of dust-sized particles. Before they hit the Earth they are known as meteoroids, or sometimes asteroids if they are larger. When burning up through the atmosphere they are known as meteors.

A meteor usually appears to observers as a trail of light streaming through the sky, usually disintegrating within a second or two. Those lucky enough to hit the surface of the Earth before burning up are then known as meteorites.

Meteors can be seen frequently in a dark sky any night of the year, but they also come in more concentrated showers. A shower usually comes along when the Earth passes through a trail of dust left along the orbit of a comet.

Because the Earth travels around the Sun once a year and then gets back to where it started from, meteor showers often occur at the same times each year.

Because the Earth is plunging headfirst into a storm of driving debris, the meteor shower seems to emanate from this point in the sky. Since meteors usually come from the radiant and go outwards, be prepared to be lying down with your feet towards the radiant.

Typical Meteorite

Swiss geologists find Mars meteorite in the Sultanate of OmanBern - June 18, 2001Geologists from Bern University and from the Natural History Museum Bern have found more than 180 meteorites in Oman in January/February 2001. The most exciting find is a piece of Mars rock.

Why are so many meteorites found in Oman? Meteorite falls are very rare. Only where the surface of the Earth remains undisturbed and dry for very long times meteorites accumulate over thousands of years. Enrichments of this type were first detected in Antarctica and about 10 years ago, since then deserts are being searched for meteorites systematically.

Known to be from Mars because of gases preserved in glassy material in the rock. These gases matched the atmospheric composition of Mars measured by the Viking mission.