Frozen Visitors From Afar
Comets are really icy planetesimals. This means that they are the ancient leftover building blocks of the quartet of giant gaseous planets inhabiting the outer Solar System: Jupiter, Saturn, Uranus, and Neptune. Many scientists think that comets hold in their frozen hearts the most pristine of primordial elements that went into the construction of our Solar System about 4.56 billion years ago. These very ancient ingredients have been preserved in a kind of “deep freeze” at the outermost fringe of our Solar System where it is both frigid and dark.
In contrast, the primordial rocky planetesimals were similar to the asteroids that dwell in our Solar System today. Asteroids, that are primarily found in the Main Asteroid Belt between Mars and Jupiter, are the relic ancient building blocks of the four inner, solid, and relatively small planets: Mercury, Venus, Earth, and Mars. Both icy and rocky planetesimals collided with one another and merged, thus creating increasingly larger and larger bodies when our Sun and its family were first forming billions of years ago.
The frozen, dusty comets wander into Earth’s warm inner kingdom from the remote Oort cloud, as well as from the Kuiper Belt and Scattered disk. The Kuiper Belt and Scattered Disk revolve around our Star beyond the orbit of Neptune, and they are the source of short-period comets, which are comets that invade the inner Solar System more frequently than every two hundred years. The much more remote Oort cloud is the distant domain of long-period comets which take at least two hundred years to fly into our Solar System’s inner kingdom. Because the Kuiper Belt is so much closer to us than the Oort cloud, short-period comets have played a more important role in our planet’s history than the long-period comets. Nevertheless, Kuiper Belt Objects (KBOs) are sufficiently distant, dim, and small to have been beyond the reach of our technology until 1992. Astronomers have not been able to observe the very remote Oort cloud that is thought to reach at least 10% of the way to the nearest star beyond our own Sun.
Every time a comet comes squealing into the inner Solar System, it loses a small amount of its mass due to sublimation of its surface ices to gas. For example, the well-known Halley’s Comet, is thought to have a lifetime of less than 100,000 years. The comets that we can see today, as they brightly streak into the sky above us, will eventually disappear as a result of their sublimation of ices to gas, only to be replaced by a fresh, new collection of comets, that will come brilliantly soaring inwards toward our Star from their frozen homes in the Oort cloud, Scattered Disk, and Kuiper Belt.
The frozen heart–or core–of a comet is called its nucleus, and it is primarily made up of ice and dust that is coated by a blanket of dark organic material. The ice itself is composed of frozen water, but there are other frozen ingredients existing as well–such as carbon dioxide, carbon monoxide, ammonia, and methane. The nucleus might also contain a small rocky core.
As the comet migrates in the direction of our Sun, the ice on the surface of its nucleus turns to gas, and forms a cloud called a coma. Radiation from the Sun pushes the particles of dust away from the coma, and this creates a flashing, thrashing, dusty tail. Charged particles from our Star change the comet’s gases into ions, thus forming an ion tail. Because the tails of comets are shaped by our Sun’s glare and the solar wind, they invariably point away from our Star.
Most comets have nuclei no larger than 10 miles–or even less. However, some comets possess comas that can be almost 1 million miles wide. Some outstanding comets have tails that are 100 million miles long.
Comets leave behind a trail of debris that can cause meteor showers on Earth. The Perseid meteor shower occurs every year between August 9 and 13 when our planet travels through the orbit of the Swift-Tuttle Comet.
We can observe some comets with the unaided eye when they come screaming inward towards our Sun. This is because their comas and tails reflect sunlight, and sometimes they are bright because of the energy they absorb from our Star. However, most comets are too dim or small to be seen without a telescope. mkv movies download