Anatomy of a Comet Help
Introduction to the Anatomy of a Comet
Before the first physical visit in 1985–1986 when celestial robots swarmed around Halley’s Comet, scientists were not quite certain what comets are made of. People could only observe them from Earth through telescopes and various nonvisual instruments. The observable parts of a comet became known as the nucleus or core , the coma , and the tail (Fig. 11-2).
We now know that comets are solid objects composed of ice, rock, and dust. When they get close enough to the Sun, some of the ice evaporates into space, and some of the dust is blown away by high-speed subatomic particles from the Sun (the solar wind ). The evaporated ice and blown-off dust stream outward from the core of the comet, away from the Sun.
Before humanity got a close look at a comet, there were two theories concerning their anatomy, called the Whipple model and the Lyttleton model . According to astronomer Fred Whipple, comets were believed to consist of a “dirty snowball” core, and the coma and tail were thought to arise as a result of solar heating and the solar wind. Today this model is widely accepted because scientists have actually seen and analyzed some comets close up. They have found objects that look like huge irregular boulders with jets of bright gas spewing out.
According to Raymond Lyttleton, the cores of comets were thought to be swarms of small meteoroids held together by their mutual gravitation. When such swarms ventured close enough to the Sun, some of the material was blown off and away by the solar wind, and this accounted for the coma and tail. Few scientists today accept this as the normal state of comet anatomy.
Before modern astronomy, comets aroused superstition, aggression, and fear. Because they were seen rarely, and because of their ghostly appearance, comets were seen as signs from heaven. Some comets have curved tails that look like sabers or scimitars. This led a few emperors and kings to conclude that the appearance (called an apparition ) of a comet meant war was at hand. In the year 1066, Halley’s Comet became visible just before the Norman French invaded England, an event whose effects have shaped the course of history throughout the world. A famous work of art known as the Bayeaux Tapestry shows people pointing up in fear at the comet while soldiers approach.
The actual size of a comet nucleus can vary from a few meters across to hundreds or thousands of kilometers in diameter. It’s difficult to determine the shape and diameter of any particular comet’s nucleus using a telescope from Earth because by the time the comet gets near enough to us to be visible, the glowing coma obscures the core.
One way to see the comet’s nucleus clearly is to view it when, and if, it passes directly in front of the Sun. Obviously, this opportunity does not present itself very often, but it took place in 1890, and the only thing astronomers could conclude about that comet is that it was either solid but too small to present a substantial image or else it was something so tenuous that it didn’t cast a visible silhouette at all.
Astronomers have now seen some comets at close range, and we know that the cores of these objects are solid. Whipple has been vindicated. Another point of argument in favor of Whipple’s solid-core model is the fact that the major periodic comets (those that return at regular intervals) that come very close to the Sun survive their passage around our parent star. A nucleus comprised of tiny objects would be completely vaporized by the Sun’s heat and by the solar wind as it neared and then passed its perihelion.
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