Comets.

Introduction:Comets have been around since the start of our solar system. All comets used to spread fear in people as they were thought to indicate a disaster. Once the telescope was used to observe them people started to see what they were,and started to unlock their secrets. In 1577 Tycho Brahe demonstrated fom parallax meaurements that comets lie beyond the orbit of the Moon and therefore have no connection with the Earths atmosphere. After this comets were seen as members of the solar system,and their orbits were investgated. E.Halley used the law of gravity to postulate that the appearence of a comet in 1682, 1607, and 1531 were in fact the same object. He stated they had an orbit of 76 years, he pradicted that the comet that now bears his name would appear in 1758.It did appear, but he did live to see its reoccurrence.Before the invention of the telescope all comets were only seen by the naked eye and spread fear when they appeared. With the work of Tycho Brahe and E.Halley this stopped the bad omens linked to comets.The invention of the telescope helped find out a lot about comets. But, there is only so much a telescope can see due to the fact that the core of the comet can not be seen because of all the gas around it. Comets also have tails which can be seen using a telescope or naked eye if the comet comes close enough to the Earth.
So what are comets?Comets have been described as dirty snowballs by Fred Whipple, most comets are frozen ice, rock and other materials left over from the formation of this solar system frozen solid,most are less than 10 km's across. The comet is interesting to astronomers because it is untouched since the formation of the solar system. Comets have in the past shaped how life on this planet has developed, it is thought that some have hit the planet so causing most life to be killed off, just like the dinosaurs.It is suspected that the dinosaurs were killed off by an asteroid rather than a comet.But comet have helped to bring vast amounts of water to the planet as well so in a way they have helped this planet evolve into a life bearing planet. It is also known that 3000 years ago comets were seen as a bad omen there are many written notes from Europe and China to support this comment.

The Nucleus:
The nucleus has a black crust and as it moves closer to the Sun its dark colour causes it to absorb heat so causing the frozen water underneath to turn to gas. As the crust cracks under the pressure from gas it is ejected and takes dust along with it. As time goes on more cracks appear and more gas and dust are ejected so stopping us seeing the nucli, but we are blessed with a nice tail that can be seen on the brightest comets without any difficulty. There is a problem with most comet they unfortunately just look like a fuzzy blob even through a telescope.
The Coma:
The Coma is usually a few thousand kilometres across,but it does depend on the size of the nucleus and the distance from the Sun. The most important factor is the size of the nucleus. The side that faces the Sun is heated by the Sun so the bigger the nucleus the greater number of jets will be present. So adding more gas and dust to the Coma. As the coma gets closer to the Sun the force of the solar wind blows the gas and dust present so causing the comet tail to form.

Comet Hyakutake Coma.
The Tail:
As a comet moves closer to the Sun the comet nucleus gets heated forming the Coma (see above) then gas and dust escapes from the comets Coma incresses. It is pushed away from the Coma by the solar winds and is formed into a tail behind the Coma. There are three types of tail.
Dust tail: This is formed as the solar radiation pressure splits the solid particles from the coma, this tail is usually curved, broad without any internal structure.
Ion tail: the Ion tail is formed as the gases from the coma are ionised by the solar radiation, this tail is usually straight and narrow tail. If the comet moves inside the orbit of the Earth then a large tail can be formed and the comet itself becomes bright.
Anti-tail:
 The name given to a tail when it points towards the sun. This is a rare event and typically occurs when the Earth crosses the plane of the comet's orbit when that comet is relatively close to the sun and exhibiting notable tail activity.

Comet Hale-Bopp with it tail.

Well known comets:
There are many comets that are seen over a set period. Halleys comet is just one of the well known comets. Towards the end of this millennium two very nice comets were seen, these were comet Hale-Bopp and comet Hyakutake. These two comets were both naked eye with well developed tails. Below you will find a chart on these two comets as well as other selected comets.
Number & NameOrbital Period Perihelion Date Perihelion Distance Semi-Major Axis Orbital EccentricityOrbital InclinationAbsolute Magnitude
1P Halley 76.1 yrs. 1986-02-09 0.587 AU 17.94 AU 0.967 162.2 deg. 5.5
2P Encke 3.30 yrs. 2003-12-28 0.340 AU 2.21 AU 0.847 11.8 deg. 9.8
6P d'Arrest 6.51 yrs. 2008-08-01 1.346 AU 3.49 AU 0.614 19.5 deg. 8.5
9P Tempel 1 5.51 yrs. 2005-07-07 1.497 AU 3.12 AU 0.519 10.5 deg.Unkown
19P Borrelly 6.86 yrs. 2001-09-14 1.358 AU 3.59 AU 0.624 30.3 deg.Unkown
21P Giacobini-Zinner 6.52 yrs. 1998-11-21 0.996 AU 3.52 AU 0.706 31.8 deg. 9.0
26P Grigg-Skjellerup 5.09 yrs. 1992-07-22 0.989 AU 2.96 AU 0.664 21.1 deg. 12.5
27P Crommelin 27.89 yrs. 1984-09-01 0.743 AU 9.20 AU 0.919 29.0 deg. 12.0
45P Honda-Mrkos -Pajdusakova 5.29 yrs. 1995-12-25 0.581 AU 3.02 AU 0.825 4.3 deg. 13.5
46P Wirtanen 5.46 yrs. 2013-10-21 1.063 AU 3.12 AU 0.652 11.7 deg. 9.0
55P Tempel-Tuttle 32.92 yrs. 1998-02-28 0.982 AU 10.33 AU 0.906 162.5 deg. 9.0
73P Schwassmann -Wachmann 3 5.35 yrs. 2006-06-02 0.933 AU 3.06 AU 0.695 11.4 deg.Unkown
75P Kohoutek 6.24 yrs. 1973-12-28 1.571 AU 3.4 AU 0.537 5.4 deg. Unkown
76P West-Kohoutek -Ikemura 6.46 yrs. 2000-06-01 1.596 AU 3.45 AU 0.540 30.5 deg. Unkown
81P Wild 2 6.39 yrs. 2003-09-25 1.583 AU 3.44 AU 0.540 3.2 deg. 6.5
95P Chiron 50.7 yrs. 1996-02-14 8.46 AU 13.7 AU 0.38 7 deg. Unkown
107P Wilson-Harrington 4.30 yrs. 2001-03-24 1.000 AU 2.64 AU 0.622 2.8 deg. Unkown
Hale-Bopp 4000. yrs. 1997-03-31 0.914 AU 250. AU 0.995 89.4 deg. Unkown
Hyakutake ~40000. yrs. 1996-05-01 0.230 AU ~1165. AU 0.9998 124.9 deg. Unkown

Where do comets come from?:
The passage of stars and other huge intersellar molecular clouds and the influence of the galastic disk itself, disturbs the orbits of the cometary nuclei in the Oort cloud and occasionally material is thrown out of this Oort cloud, this then moves into the inner solar system.Over the years, comets actually became rarer within our solar system. They no longer fill the skies as they did over the past 4 billion years, and today a prominent naked-eye comet can be expected only about once a decade when they appear they are a vert breath taking sight.
CometPerihelionQ(AU)H.Brightest Magnitude Tail/Comments
Viscara. 1901 Apr 24.0.2455.9 -1 15° & 45°
Daylight. 1910 Jan 17.0.1295.0-5 45°
Skjellerup-Maristany. 1927 Dec 18.0.1765.2-6 35°
Southern. 1947 Dec 2.0.1106.0 -5 25°
Eclipse. 1948 Oct 27.0.1355.5 -4 30°
Arend-Roland. 1957 Apr 8.0.3165.0 0 20° Anti Tail
Mrkos. 1957 Aug 1.0.3554.2 1 3°-4°
Ikeya-Seki. 1965 Oct 21.0.0086.2 -10 45° Kreutz Sungrazer
Bennett. 1970 Mar 20.0.5384.0 1 20°
Kohoutek. 1973 Dec 28.0.1426.0 1 10°
West. 1976 Feb 25.0.1975.0 -2 30° Multi Dust Tail
Iras-Araki-Alcock. 1983 May 21.0.9919.0 1 Perigee 0.03 AU
Hyakutake. 1996 May 1.0.2305.3 0 60°-107°
Hale-Bopp. 1997 Apr 1.0.914-0.65 -0.5 15°
Some of the brightest comets seen over the last century.
Astronomers with powerful telescopes can see many more comets, but even in this case it is still rare for as many as 15 or 20 comets to be detectable in the sky at any one time.Most comets are very faint at a magnitude of 6 or below. A low magnitude comet shows very little detail, the tail will also be very hard to see if it is at all visible in most small telescopes.

Comet Mullera.

Today, most comets are located outside our solar system in part of the original cloud of dust and gas that has remained virtually untouched for billions of years. These regions are referred to as the Oort Cloud and the Kuiper Belt.
The Oort Cloud: Was first theorized by the Dutch astronomer Jan Oort in 1950. His investigation of the orbits of comets with very long orbital periods brought him to conclude that a large "cloud" of comets existed far outside the solar system, possibly within the range of 5-8 trillion kilometres (or more) from the sun. The total number of comets within this belt was estimated as a trillion. It is thought that objects within this cloud are occasionally ejected either by collision with one another, or by the gravitational forces of stars. Many of the ejected objects probably never cross the paths of the planets, and still more do not come close enough to be seen with even the largest telescopes. However, a few do manage to travel into the inner solar system and are subsequently seen from Earth. This cloud remains a theory only, as it has never been directly detected.
The Kuiper Belt: A region first theorized by the Dutch-American astronomer Gerard Kuiper in 1951. Seeing that Oort's cloud of comets did not adequately account for the population of comets with short orbital periods (making complete orbits around the sun in less than 200 years), Kuiper conjectured that a belt of comets probably existed outside the orbit of Neptune within the range of 30 to 50 astronomical units (2.8 to 4.6 billion miles) from the sun. Collisions and perturbations by the planets of our solar system are believed to be the reasons for the ejection of bodies from this belt. Around 1988, astronomers from the University of Hawaii and University of California at Berkeley began searching for members of the Kuiper belt using modern electronic cameras attached to a large telescope on Mauna Kea, Hawaii. The equipment was capable of detecting extremely faint objects. After nearly 5 years of systematic searching they found a distinct image on 1992 August 30, which was subsequently designated 1992 QB1. The object was moving very slowly, and calculations eventually revealed the object took 291 years to orbit the sun at an average distance of 43 AU. Since, the discovery of that object over three dozen additional objects had been found as of the end of 1996.
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