Comets are relic icy planetsimals. This means that they are what is left of a vast population of primordial building blocks that formed the quartet of giant, gaseous planets dwelling in the outer regions of our Solar System: Jupiter, Saturn, Uranus, and Neptune. On the other hand, rocky planetesimals, similar to the asteroids, are the remnants of the ancient building blocks that went into the construction of the four inner, solid planets: Mercury, Venus, Earth, and Mars. Planetesimals, of both the icy and rocky kind, bumped into one another and frequently merged, forming ever larger and larger objects, when our Solar System was young and first taking shape billions of years ago.
The icy, dusty comets fly, with sparkling tails flashing and thrashing, into Earth’s well-lit and toasty inner domain around our Sun. These strange frozen objects come soaring towards our Sun from their frozen homes located in cold, twilight outer reservoirs that they share with a myriad of others of their frigid kind in our Solar System’s deep freeze. The comets originate in the Kuiper Belt, Scattered Disc, and Oort Cloud. Of these distant domains, that host a multitude of comet nuclei, the Kuiper Belt and Scattered Disc reside beyond the orbit of the outermost major planet, the ice-giant Neptune. The still-hypothetical Oort Cloud is much more remote, and is thought to form a gigantic sphere around our entire Solar System–extending to (at least) 10% of the distance to the nearest star beyond our Sun. Short-period comets originate in the Kuiper Belt and Scattered Disc, and they come screaming into the inner Solar System more frequently than every two hundred years. The Oort Cloud contains the most distant comets–the long-period comets–that invade Earth’s inner kingdom at a minimum of every two hundred years. Because the Kuiper Belt and Scattered Disc are much closer to Earth, short-period comets have played a more important role in our planet’s history than long-period comets.
Every time a migrating comet zips into the warm inner Solar System, it loses some of its mass as a result of sublimation of its ices to gas. This means that these sparkling visitors are doomed. For example, the very well-known Halley’s Comet is predicted to have a life expectancy of less than 100,000 years. The comets that we can see today, as they streak though the sky above us, are destined to evaporate and vanish as a result of sublimation of their ices into gas. However, these ill-fated objects will inevitably be replaced by fresh, new comets that will eventually journey into the melting, merciless heat of the inner Solar System.
The nucleus, or core, of a comet is composed primarily of ice and dust that is trapped within a coating of dark organic material. The ice is mostly frozen water, but other types of ice are trapped within the nucleus, as well–such as methane, carbon dioxide, ammonia, and carbon monoxide ice. As the glittering comet soars inward towards our fiery Sun, the ice that envelopes its nucleus morphs into a gas, and this is what creates a comet’s cloud called a coma. Radiation flowing out from our Star shoves the very small motes of dust away from the coma, and this creates the brilliant, dusty tails that comets are so famous for.
The nucleus of a comet is usually only about 10 miles–or less. However, some comets dramatically display truly awe-inspiring comas, that can be over 1 million miles wide! Some particularly glitzy comets show off amazing tails that extend for 100 million miles.
Leaving a trail of debris behind them, as they make their incredible journey into the inner Solar System, comets have been known to be the source of meteor showers on Earth–such as they Perseid meteor shower that lights up our night sky every year between August 9 and 13. The Perseid meteor shower occurs when our planet passes through the orbit of the Swift-Tuttle comet. mkv movies