Comets are the most numerous and the most mysterious celestial bodies of the Solar System. According to the scientists' assessments, around 1012-1013 comets rotate around the Sun at distances varying from 3 000 to 160 000 astronomical units. There is an especially large concentration of comets in the remote outskirts of the Solar System, known as the Oort cloud. Comets are the longest objects in the Solar System; the length of their tail sometime reaches 1 astronomical unit. When approaching the Sun, the comet assumes a spectacular display. Warmed by the solar heat, the comet's gas and dust fly off its surface forming a bright "tail".
The behavior of the majority of comets is quite unpredictable, although people have attempted to study them since the ancient times. During the Dark Ages, comets heralded wars or epidemics. Thus, the invasion of Anglo-Saxon England by William the Conqueror in 1066 was associated with appearance of Halley comet in the sky. Another comet was associated with the fall of Constantinople to the Ottoman Turks in 1453.
The orbits of the majority of comets are highly stretched ellipses. In 1702, Edmund Halley proved that the comets that were seen in 1531, 1607 and 1682 had the same orbit. Halley proved that it was actually the same comet, which now bears Halley's name.
Most of the time, comets stay far away from the Sun, near the aphelion. The greater is the distance between the comet and the Sun, the lower is the comet's temperature. When the comet's temperature is low, the rate of matter evaporation off the comet's surface decreases, the comet's coma disappears, and it becomes invisible. When near the perihelion, comets move at high velocities, and large tails come into being at that time.
The core of a typical comet is composed of a mixture of frozen water and frozen gases with fragments of refractory stone and metallic particles. The ice of the comet's core begins to evaporate as a comet approaches the Sun. The comet's core masses become bare, and emit large quantities of gases and dust. Gas fountains may even change the orbital path of a comet. A vast luminescent gaseous blanket - the coma - forms around the comet's core. It encompasses the comet's head and the core. As the comet approaches the Sun further, its head assumes an oval shape. The head stretches, and a tail emerges. Usually, the comet's tail is directed away from the Sun, due to the solar light pressure on the molecules of gases and dust that are being emitted from a comet's core.
The core of a comet is actually an aggregate of separate bodies, rather than a solid united body, although sometimes these components are quite large. These bodies (ice, stones, sand particles, and dust) are loosely interconnected. However, periodic comets weaken with each approach to the Sun. Some of them are reasonably "strong," e.g., Halley Comet, which has a period of 76 years, has been observed since 466 BC. Some of the other comets cannot "sustain" more than two or three approaches to the Sun. They disintegrate, and become a swarm of meteorites that continues traveling along the old orbit. When a swarm of such meteorites passes close to the Earth, we come out to the roofs of our houses at night, and watch star showers.
This model's left-side window displays the Sun and the orbit of a comet, as well as Earth's orbit that is given here for the purposes of comparison. Press "Run" button to initiate the animation, press "Stop" button to suspend it, and press "Reset" to return the model to its initial state. Observe the evolution of the comet's tail in the model's right-side window.
You may change such parameters of the comet's orbit as the perihelion velocity and distance to the Sun. A small window in the bottom right corner displays the current parameters of the comet, such as its orbit type, time (in accelerated regime) elapsed since the initiation of the model, and its current distance to the Sun.