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Comets
have been around since the beginning of the solar system, and they have been recorded since written history began. Originally,
comets were known as "hairy stars," and were unexplained, unpredictable (although it was thought that they predicted
many things), and brought death and destruction. Very early ideas of what comets were included clouds of fire in the Earth's
atmosphere, or extra bodies moving though the heavens.
As science progressed,
people were better-able to explain comets. In 1577, Tycho Brahe and Mastlin used parallax - the measure of the change of location
in the sky of a body over the course of six months - in order to show that comets were not in the atmosphere, but rather they
were very far away, even further than the moon.
The public and scientists had to wait until
1704 for Edmund Halley to actually fully explain the way comets work. He used the newly developed Newton's Laws in order to
calculate the orbits of comets based upon their motion in the sky. He calculated orbits for many comets, but he also found
patterns in the orbits - some comets seemed to actually have the same orbit as others! He correctly hypothesized that these
were the same comet; it had documented appearances in 1456, 1531, 1607, and 1682. He predicted that this same comet had an
orbital period of 76 years, and would return in 1758. It did, and now bears his name: Halley's Comet. Unfortunately, he did
not live long enough to see it reappear.
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Anatomy:
The center of a comet, the nucleus, is very small compared to the
rest of the comet, and is usually only a few kilometers in diameter. It is the part of the comet that is always there, at
least as long as the comet exists. The nucleus is composed of rocks and ices. As comets age, they lose their ices, and a comet
is considered "dead" when it no longer has any ice, for it can no longer sport any feature other than its nucleus.
The other parts of a comet are only in existence when the comet approaches the
sun - usually once it is closer to the sun than Jupiter (5.2 A.U.). The sun's heat melts some of the ice in the nucleus to
form a huge glowing "head," the coma. The coma can grow to immense proportions, sometimes becoming over 10,000 km
(6,700 miles) across (that's larger than the Earth!). Because the ices of the comet are made of various molecules (such as
carbon, nitrogen, oxygen, and water), and the different molecules weigh different amounts, some can escape the nucleus' weak
gravity more easily. Therefore, another minor feature of the coma is the Hydrogen Cloud, which forms the outer part of the
coma.
Finally, the part of a comet that is most well-known is the tail. Most
comets usually have two tails, and usually one is much brighter than the other. The tails form as a result of the sun's solar
wind - the stream of charged particles that emanate from the sun. The solar wind dislodges gas and dust from the comet and
forces the material into very narrow (relative to their length) tails. The tails always point away from the sun, which is
sometimes counter-intuitive. This is because even when the comet is traveling away from the sun, the tail faces away, so the
comet is, in effect, following its tail.
The gas and dust form separate tails
due to the charge of their constituents. The dust is not highly charged, and so forms a bent tail that slightly lags behind
pointing directly away from the sun; its color is yellow because it reflects the light of the yellow sun. The gas tail is
much more highly charged, and so the solar wind acts to funnel it much more effectively than it does the dust cloud; its color
is blue due to molecules that make it up.
The tails can be several million
miles long each, and the longest have been observed to be over one A.U. (over 93 million miles long). Due to the nature of
how the tails form -- material being blown off of the comet - the tails are how comets loose the bulk of their mass. The material
usually dissipates after several hundred years, but before that happens, the material usually will continue in the orbit of
the comet. If the Earth plows through this, we see a meteor shower.
Impact:
Just like asteroids, comets can hit planets. The most famous impact (and the
only one observed in recent history) is that of comet P/Shoemaker-Levy 9. Discovered in 1993, it impacted Jupiter in July
of 1994. For more information about this event, see the Advanced section's Jupiter page.
Orbit:
Current theory proposes that most
comets originate in the Oort Cloud - the vast cloud of comets that surrounds the solar system out to an estimated distance
of two light-years - or the Kuiper Belt - the belt of comets and asteroids extending from between 30-50 A.U.. Comets usually
stay in the Oort Cloud or Kuiper Belt; however, if a passing object (such as a star) exerts enough gravity, the comet might
be nudged free. Sometimes its new path will expel it from the solar system all together, and it is doomed to roam interstellar
space. The alternate path takes it into the inner solar system, where it can become a short- or long-period comet.
As it travels towards the sun, the gravitational pull of the planets changes its trajectory. About
the time it passes through Jupiter's orbit, the nucleus begins to feel the heat and pressure of the sun, and surface ice starts
to melt, forming the extra features that make it recognizable as a comet. The first encounter with the sun usually changes
its orbit the most.
If the comet's final orbital path takes less than two
hundred years, the comet is classified as a short-period comet. If the orbit takes more than 200 years, then it is called
a long-term comet. Halley's comet is a short period with an orbit of seventy-five years; the recent Hale-Bopp with an orbit
of 4,200 years is a long-term comet.
*Comet Halley, or Halley's Comet, was actually
not discovered by Halley; he was the first person to piece together several comet sightings over the previous several hundred
years, and hypothesized that many sightings every 76 years of comets were actually multiple sightings of the same comet. He
also successfully predicted the next sighting of it, but was unfortunately not able to live to see it. Bright comets of the 20th Century Year/Name | Brightest date / Mag. | Remarks | 1901 Viscara | May 2 / -1.5 | Yellowish;
may have split in two | 1907 Daniel | August 15 / +2.0 | Photographed more than any comet before it. | 1910 Great January Comet | January
17 / -5.0 | Visible in daylight; "Like Venus
with a 10? tail." | 1910 Halley | May 20 / 0.0 | Tail stretched 120? across the sky! | 1911 Beljawsky | October
15 / +1.0 | Golden yellow; simultaneously visible
with Comet Brooks. | 1911 Brooks | October 28 / +2.0 | Distinct bluish tinge. Visible with Beljawsky from Oct. 10-22. | 1927 Skjellerup-Maristany | December 18 / -6.0 | Visible
in daylight; faded rapidly thereafter. | 1941
De Kock- Paraskevopoulos | January 30 / +2.0 | Visible only from Southern Hemisphere | 1947 Great Southern Comet | December
7 / 0.0 | Orange with 25? tail; faded rapidly | 1948 Eclipse Comet | November 1 / -2.0 | Discovered
next to the Sun during a total solar eclipse. | 1957
Arend-Roland | April 21 / 0.0 | Displayed sunward pointing "anti-tail" Apr. 20-May 3 | 1957 Mrkos | August 1 / +1.0 | Visible
both in evening & morning sky. | 1962
Seki-Lines | April 3 / -2.5 | Extremely bright; faded quickly. | 1965 Ikeya-Seki | October 21 / >-15! | Brightest
of 20th century; Kreutz sungrazer, visible in daylight. Split into 3 pieces. | 1970 Bennett | March
19 / 0.0 | "Stunning" Exhibited spiraling
jets of gas in head. | 1970 White-Ortiz-Bolelli | May 18 / +1.0 | Kreutz sungrazer; faded rapidly. | 1973 Kohoutek | December 28 /
-3.0 | Spectacular as seen from Skylab, but then
faded very rapidly; big disappointment! | 1976
West | February 25 / -3.0 | Glimpsed in daylight; split into 4 pieces; had 5 tails! | 1983 IRAS-Iraki-Alcock | May 12 / +1.6 | Passed
within 2.9 million miles of Earth. | 1996
Hyakutake | March 27 / 0.0 | Displayed "immense" tail; 70 to perhaps 100? long. | 1997 Hale-Bopp | April 1 / -0.5 | Visible
to unaided eye from July 1996 thru Oct. 1997, an all-time record. |
Unfortunately, there is no way to exactly predict in advance when another
spectacularly bright comet will appear. There are no fewer than 16 comets that are currently under scrutiny by amateur
and professional astronomers. But the average brightness of these 16 comets is just around 12th magnitude -- or roughly
250 times dimmer than the faintest star that can be seen without any optical aid. That's well out of the reach of most
casual observers.
But every once in a while, a newly discovered comet will appear in our
sky that is so spectacular that it captures the attention of a worldwide audience. Hale-Bopp fell into that category.
When it was at its brightest, in early April of 1997, this comet was readily visible. It was later estimated that the
number of Americans who witnessed Hale-Bopp surpassed those who watched the 1997 Super Bowl.
Comet (Click on photo's
to enlarge)
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