A real comet might not lose mass at the same rate with every passage by the Sun; however, the general idea that a comet has a finite number of passages before it gets used up is clearly correct. But a comet might "turn off" before being reduced to nothingness.
With each passage, ices are preferentially lost and dust is preferentially left behind. Thus, a comet will develop a surface layer that becomes enriched in dust, i. Eventually, such a comet may turn so black and develop such a good insulating surface that it will turn off, continuing in its orbit but no longer looking or behaving like a comet. Clearly, comets can't do this forever.
And if we accept the Copernican Principle, that this time in the history of our solar system is not special, then new comets have been appearing and getting used up for the entire history of the solar system. Assuming the solar system is approximately as old or older than the oldest Earth rocks 3. If there are billion 10 13 comets and each contains 10 14 kg of gas and dust, this is a total mass of. This is a fair amount of stuff! Comets are sources of water.
One could think of comets as possible future sources of water to bring to Earth or to a space station or to a planet like Mars. Alternatively, one could wonder whether the Earth's present abundance of water was deposited here by impacts with comets, early in Earth's history.
The Earth's oceans contain kg of water. This is equivalent to 1 million Comet Halley impacts. Volatiles and solids appear to be fairly well mixed throughout the nucleus of a new comet approaching the Sun for the first time. As a comet ages from many trips close to the Sun, there is evidence that it loses most of its ices, or at least those ices anywhere near the nucleus surface, and becomes just a very fragile old "rock" in appearance, indistinguishable at a distance from an asteroid.
A comet nucleus is small, so its gravitational pull is very weak. You could run and jump completely off of it if you could get traction. As a result, the escaping gases and the small solid particles dust that they drag with them never fall back to the nucleus surface. Radiation pressure, the pressure of sunlight, forces the dust particles back into a dust tail in the direction opposite to the Sun.
A comet's tail can be tens of millions of kilometers in length when seen in the reflected sunlight. The gas molecules are torn apart by solar ultraviolet light, often losing electrons and becoming electrically charged fragments or ions.
The ions interact with the wind of charged particles flowing out from the Sun and are forced back into an ion tail, which again can extend for millions of kilometers in the direction opposite to the Sun. These ions can be seen as they fluoresce in sunlight. It is about spherical with an average diameter of km at 1 AU of the Sun km for Hale-Bopp. Its size varies inversely with the square of the heliocentric distance. Why is there a luminous halo? Sublimation creates a gaseous environment that become excited under the influence of sunlight and of the protons from the solar wind.
The ensuing fluorescence results from a subsequent transition of an electron between two energy levels. Why does the coma vary in size? Comets are very small in size relative to planets. Their average diameters usually range from m or less to about 20 km. Recently, evidence has been found for much larger distant comets, perhaps having diameters of km or more, but these sizes are still small compared to planets. Planets are usually more or less spherical in shape, usually bulging slightly at the equator.
Comets are irregular in shape, with their longest dimension often twice the shortest. The best evidence suggests that comets are very fragile. You could take a big piece of cometary material and simply pull it in two with your bare hands, something like a poorly compacted snowball.
When the nucleus approaches the Sun, the condensed gases evaporate. As the temperature increases, so does the rate of sublimation and new species which have a higher temperature of sublimation are set free.
So, the size of the coma increases when it approaches the Sun. The size of the coma can also vary from one comet to another depending upon the materials it is made of because they have different temperatures of sublimation. Types of Tails Every comet then really has two tails, a dust tail and an ion tail.
If the comet is faint, only one or neither tail may be detectable, and the comet may appear just as a fuzzy blob of light, even in a big telescope.
The density of material in the coma and tails is very low, lower than the best vacuum that can be produced in most laboratories. In the Giotto spacecraft flew right through Comet Halley only a few hundred kilometers from the nucleus. We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun.
However, most comets are too small or too faint to be seen without a telescope. Comets leave a trail of debris behind them that can lead to meteor showers on Earth. For instance, the Perseid meteor shower occurs every year between August 9 and 13 when Earth passes through the orbit of Comet Swift-Tuttle. Astronomers classify comets based on the durations of their orbits around the sun.
Short-period comets need roughly years or less to complete one orbit, long-period comets take more than years, and single-apparition comets are not bound to the sun , on orbits that take them out of the solar system, according to NASA. Recently, scientists have also discovered comets in the main asteroid belt — these main-belt comets might be a key source of water for the inner terrestrial planets.
Scientists think short-period comets, also known as periodic comets , originate from a disk-shaped band of icy objects known as the Kuiper Belt beyond Neptune's orbit, with gravitational interactions with the outer planets dragging these bodies inward, where they become active comets.
Long-period comets are thought to come from the nearly spherical Oort Cloud even further out, which get slung inward by the gravitational pull of passing stars. In , scientists found there may be seven times more big long-period comets than previously thought. Some comets, called sun-grazers, smash right into the sun or get so close that they break up and evaporate.
Some researchers are also concerned that comets may pose a threat to Earth as well. Comets are generally named after their discoverer. For example, comet Shoemaker-Levy 9 got its name because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. In antiquity, comets inspired both awe and alarm, "hairy stars" resembling fiery swords that appeared unpredictably in the sky.
Often, comets seemed to be omens of doom — the most ancient known mythology, the Babylonian " Epic of Gilgamesh ," described fire, brimstone, and flood with the arrival of a comet, and the Roman emperor Nero saved himself from the "curse of the comet" by having all possible successors to his throne executed.
This fear was not just limited to the distant past — in , people in Chicago sealed their windows to protect themselves from what they thought was the comet's poisonous tail.
When frozen, they are the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles. There are likely billions of comets orbiting our Sun in the Kuiper Belt and even more distant Oort Cloud.
The current number of known comets is:. Key Science Targets. Comets orbit the Sun just like planets and asteroids do, except a comet usually has a very elongated orbit. As the comet gets closer to the Sun, some of the ice starts to melt and boil off, along with particles of dust.
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