Comets, Asteroids, and Meteors Help
Introduction to Comets, Asteriods, and Meteors
Eight known substantial bodies orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Since its discovery, a ninth object, the Pluto-Charon system, also has been considered as a planet. If there are other planets in the Solar System besides these, they have thus far evaded the observation of watchful amateur and professional astronomers.
The planets, along with the Sun and our own Moon, capture most of the visual attention of hobby astronomers. Saturn with its rings, Mars with its ruddy glow, and Jupiter with its visible moons and surface features are perennial favorites. However, insofar as the fate of humanity is concerned, these planets have no direct effect. They stay in their orbits, and we stay in ours. Jupiter, even with its powerful magnetosphere and ferocious radiation belts, does not pose a threat to us here on Earth. Mars, named after the ancient war god, has not sent any living beings to maliciously attack civilizations on Earth (or on any other planet), although there is evidence that rocks have landed here that had their origins on Mars.
The Small Stuff
There are celestial objects that present a threat to life on Earth or, perhaps better stated, have had and will continue to have a profound effect on the way life evolves here. They are pipsqueaks in the Solar System, none of which can be recognized without a telescope and many of which remain unknown to this day. They are comets, asteroids , and meteoroids , remnants of the primordial Solar System. They are leftovers that did not congeal into the planets. Originally, according to the most popular theories, the entire Solar System consisted of objects like them.
Clouds Of Comets
Comets are one of the greatest mysteries in astronomy. They have awed and terrified people since the beginning of recorded history. Even today, some people think comets have supernatural characteristics. Yet comets are, as one astronomer has said, more numerous than fishes in the sea. They are, in the long term, more dangerous too.
A large comet smashing into the Earth would be, for the human race, the equivalent of a shark attack on an individual human being. However, the risk shouldn’t be overblown. It is not necessary to lose sleep worrying about what is going to happen when the next major comet comes down. We can say with confidence that such an event will take place, but we can’t say when, other than to note the fact that the time intervals between massive impacts are measured, on average, in tens of millions of years.
Beyond the orbit of Neptune, the Kuiper belt consists of comets that orbit the Sun at distances from 30 to 50 astronomical units (AU) (Fig. 11-1). The existence of this zone, which circles the Sun in more or less the same plane as the planets, was only a hypothesis until 1992 when the first object there, a large comet, was positively identified. More objects were found after that. Objects found between 1992 and 1994 were all gigantic compared with the comets that have made history, such as Halley’s Comet, which last appeared in 1985–1986. Since 1995, some “normal sized” comets have been found in the Kuiper belt.
Surrounding the Kuiper belt and extending out to a distance of about 1.5 light-years (one-third of the way to the nearest star other than the Sun) is a vast spherical halo of comets called the Oort cloud . The exact dimensions of this cloud are unknown because of its great size and distance. Nevertheless, it is believed to contain millions upon millions of comets.
Normally, the comets in the Kuiper belt and the Oort cloud stay in their parts of the Solar System, the “suburbs” and the “surrounding countryside.” All of them would remain there too if it were not for the fact that our Milky Way galaxy is an unstable and chaotic place. Once in a while, a star comes close enough to the Solar System so that its gravitation has an effect on objects in the Oort cloud, and some of these comets are deflected into paths that take them into the inner Solar System. Some astronomers suspect that there is an as-yet unknown massive planet beyond Pluto that does similar things to comets in the Kuiper belt.
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