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Comets


Old men and comets have been reverenced for the same reason, their long beards and pretences to foretell events - Jonathan Swift

Comet Constituents

Hale-Bopp

A comet consist of

  • Nucleus   a few kilometers in size. Composed of water ice and other frozen gases, and 'dust'.

  • Coma   may extend 100,000 kilometers or more outside the nucleus

  • Hydrogen Cloud   surrounds the comet but is invisible in normal light. atoms, yet un-ionized.

  • Ion (Gas) Tail   When a comet moves near the Sun, Ultra Violet radiation breaks the gas into ions. These ions are affected by the solar wind and can stream millions of kilometers away from the Sun into space. This tail therefore always streams away from the Sun, preceding the comet as it moves away from the Sun. The tail is visible because of fluorescence.

  • Dust Tail   Dust given off by the comet is 'pushed' by radiation pressure into a curved path behind the comet's path

It should be noted that many smaller comets do not produce tails.

Probably consist of original material from the time of the birth of the Solar System.

Orbits

A comet's orbit usually has a high eccentricity, i.e. it is very elongated, whereas planetary orbits are much more similar to circular orbits

Planetary orbits are restricted to the plane of the Solar System within a few degrees. Cometary orbits usually have a plane deviating considerably from the plane of the Solar System

Comets typically come close to the Sun at perihelion (usually between Mercury and Mars) and reach the depths of the Solar System (beyond Neptune and Pluto) at aphelion.

The orbital periods of most comets are expected to be of the order of hundreds of years to tens of thousands of years.

Oort Cloud

This cloud is hypothesized to lie at the edge of the Solar System (even the Hubble Telescope cannot detect it, so it is still a hypothesis). It is stated to be the birthplace of long-period comets (like Hale-Bopp, Hyakutake). Gravitational disturbance from a passing star can perturb a member of the cloud and alter its orbit to pass within the inner Solar System (after thousands or millions of years).

The heart of the cloud lies at 50,000 AU and upwards (about one light year). It may hold 100 billion objects.

Jupiter can affect the orbit of a comet if it makes a close approach to the planet.

Kuiper Cloud

Possible source of short-period comets, like Halley's Comet(although this could also have come from the Oort Cloud and become a short-period comet by interaction with the gas giants).

Pluto, Charon and Triton appear to be similar. If this so, then they are likely to be members of a much larger class of objects, rather than a small class with Pluto and Charon meeting by chance and Triton encountering Neptune by chance. Since 1992, over 100 Trans-Neptunian objects have been discovered. It is proposed that this is indicative of the existence of a large number of Kuiper Belt objects of which Pluto, Charon and Triton could form a part.

Halley's Comet

Whereas most comets are long period objects, Halley's Comet is a notable exception - it is the best-known short period comet.

It has a period of between 74.5 and 79 years (the period varies) and its motion is retrograde. Its closest approach to the Sun is 0.59 AU, and it scarcely reaches beyond Neptune at aphelion. It can come as close as 0.15 AU to Earth.

It had been observed in detail in 1607 by Thomas Harriot - these observations were highly useful for Halley in calculating its orbit. Halley observed it in 1682, and noted its similarity to this comet of 1607 and also to that of 1531. It has since returned in 1758 (Halley's prediction being confirmed on Christmas Day by Palitzsch in Deutschland), 1835, 1910, and 1986. Reports have been traced back to 240BC, including the return of 1066 as depicted on the Bayeux Tapestry and the return of 684 as mentioned in the Nurnberg Chronicles .

The discovery of poisonous cyanogen gas in the spectrum of comet tails combined with the realization that in 1910 the Earth was to pass thru Halley's tail caused mass hysteria in some parts.

In 1986, the space probe Giotto was sent to the Comet, about 500 km from the nucleus.

  • The nucleus was found to be 'potato-shaped' and dark black, roughly 8 by 15 km.
  • the comet nucleus to be much darker than we expected. Most of the surface is so black that it reflects only about 4% of the light that hits it less than black velvet reflects! Astronomers hypothesize that this dark material is dust and complex molecules left over when the comet's more volatile ices evaporated as it came close to the Sun.
  • Gases coming off were 80% water vapor.
  • The hydrogen tail seemed to extend to 35 million kilometers.
  • A bow shock between the solar wind and gases released by the comet was detected 400,000 km in front of the comet.
  • there were some craters on the nucleus.
  • The vents from which the material for the coma and tail was escaping when the comet was near the Sun made up only 10% or so of its surface. The vents seem to be active only when exposed to the Sun

For earth-based observers, the return of 1986 was the worst for 2000 years, although it was visible with the naked eye.

There is only one comet that has been seen more than once and also has a longer period than Halley - this is Herschel-Rigollet (seen 1788 and 1939, with period of 156 years).

Halley's Comet

Encke's Comet

The orbit of this comet lies entirely within Jupiter's, implying encounters with several planets maybe, so possibly little of the original comet is left. It was first seen in 1786, though not actually recognised for what it was. Its return in 1822 was predicted by Encke, hence the name. It has now been viewed on over 50 returns and nowadays it can be tracked with telescopes during its entire orbit. The orbit is becoming smaller and more circular. It is also about two magnitudes fainter than 100 years ago. More information here.

Hale Bopp

Following on from Hyakutake (C/1996 B2, peaking at magnitude zero) only a year before, we had a 'double' not experienced since the first magnitude Arend-Roland and Mrkos of 1957.

Hale-Bopp appeared in early 1997 and one of the most spectacular comets of the 20th Century, although it only came as close as 0.91 AU to the Sun - peaking at a magnitude of -1.

Discovered by Alan Hale and Thomas Bopp.

The period is about 4000 years.

Shoemaker Levy 9

Discovered in 1993 - the ninth comet the team had spotted. It was found that it had split into over 20 pieces and was heading for Jupiter. Between 16 and 22 July 1994, the pieces did indeed strike Jupiter and caused effects on a planetary scale..

Tempel 1

On 4th. July 2005, the 'Impactor' projectile from the US space probe 'Deep Impact' slammed into Tempel 1 at 37,000 km/h, while the comet was at a distance of 133 million kilometers. The resulting crater was similar in size to a football field and scientific analyses are expected from the debris thrown off.

Some Other Comets

Comets are usually named after their discovers and in the case of simulaneous discoveries, a comet can have up to three names attached.

The first five periodic comets for which their returns were predicted and observed were Halley, Encke, Biela, Faye and d'Arrest.

Some historical comets (the earlier ones are sometimes known just by their year)

Comet Discovered Period (years) Comments
1577 observed in detail by Tycho Brahe - hr released a major tract on it in 1588 which was highly influential. He pushed the idea that comets are astronomical bodies, not a feature in the Earth's atmosphere - and this meant that it would be crashing thru the spheres to which the planets were attached, if they actually existed.
Encke 3.3 seen over 50 times
1811 Very large tail just longer than 1 AU By mid-April this comet had become a naked-eye object and it was at its brightest during the autumn of 1811. During October, the tail reached its maximum length of nearly 24. The comet remained a naked-eye object until mid-August 1812. This period of naked-eye visibility was not surpassed until the appearance of Comet Hale-Bopp in 1997.
Biela 1826 (re-discovery of comet seen in 1772 and 1806) 6.7 (originally) Discovered in 1826 by Wilhelm Biela, an officer in the Austrian Army, who worked out its orbit. It split in two in 1846, both returned in 1852, then disappeared
1843 maybe 540 years candidate for the brightest comet of modern times - reaching magnitude -7 and visible to the naked eye in daylight, tail longer than 1.5 AU - the longest tail ever recorded. It came to within about 1.5 million kilometers of the Sun.
Donati's Comet 1858 estimated at about 2000 years
1861 Australia earth passed thru its tail - no effects were noticed
1882 Captured on photographs at just the time when this technique was beginning to prove its worth in Astronomy Last sighting of a 'Giant Comet'
Daylight Comet 1910 4 million years Brightest comet of the 20th Century. It was seen with the naked eye in January and February - and was sometimes confused with Halley of the same year.
Schwassmann-Wachmann 1 1927, Hamburg Observatory comet with least eccentricity (e=0.05) - it has an almost circular orbit between Jupiter and Saturn
Kohoutek 1973, Hamburg 75,000 Did not live up to initial expectations. There is a short-period comet (6.6 years) of the same name
West 1975/6, La Silla, Chile 300,000 Prominent naked-eye object
Hyakutake 1996 about 15,000 years came within 0.1 AU of Earth and 0.23 AU of the Sun
Shoemaker-Levy 9 1994 Hit Jupiter in mid-1994

It was clear and frosty. Above the dirty ill-lit streets, above the black roofs, stretched the dark, starry sky. Only as he gazed up at the heavens did Pierre cease to feel the humilitating pettiness of all earthly things to which his soul had just been raised. As he drove out on to Arbatsky Square his eyes were met with a vast expanse of starry black sky. Almost in the center of this sky, above the Prichistensky Boulevard, surrounded and convoyed on every side by stars but distinguished from them all by its nearness to the Earth, its white light and its long uplifted tail, shone the huge, brilliant comet of the year 1812 - the comet which was said to portend all manners of horrors and the end of the world. But that bright comet with its long, luminous tail aroused no feeling of fear in Pierre's heart. On the contrary, with rapture and his eyes wet with tears, he contemplated the radiant star which, after traveling in its orbit with inconceivable velocity thru infinite space, seemed suddenly - like an arrow piercing the Earth - to remain fast in one chosen spot in the black firmament, vigorously tossing up its tail, shining and playing with its white light amid the countless other scintillating stars. It seemed to Pierre that this comet spoke in full harmony with all that filled his own softened and uplifted soul, now blossoming into a new life.
  

War and Peace (Tolstoy).

Meteor Showers/Shooting Stars

These occur when the Earth enters the orbit of a comet, i.e. an orbit strewn with dust emitted by the comet. It is these microscopic particles that are seen burning up in the sky, at a height of about 80 to 100 kilometers and a speed of up to 70 kilometers per second..

These showers appear to be coming from one section of the sky, the radiant. It is this radiant that gives the shower its name.

All other things being equal, more meteors will be seen after midnight than before, because then a location on Earth will be moving into the dust head-on.

Meteor Shower Date of Maximum Comet
Quadrantids January 3 Retains its name despite the constellation of Quadrans no longer existing. It lay in what is now the Northern part of Bootis.
Lyrids April 21 1861I
Eta Aquarids May 4 Halley (maybe) Typical rate in Northern Hemisphere: 20 per hour
Beta Taurids June 30 Encke Daytime shower, detected by radar
Delta Aquarids July 30
Perseids August 12 1862III/Swift-Tuttle (seen again in 1992) Seen froma around 27/7 to 17/8. Consistent and best-known shower.Between 1864-66, Schiaparelli worked out the connection between the shower and Temple-Tuttle.
Orionids October 22 Halley (possibly)
(Alpha)Taurids November 4 Encke 20/10 to 25/11
Leonids November 16 1866I Tempel Tuttle very inconsistent. with superb displays every 33 years or so, for example in 1966, with verified rates of 150,000 meteors per hour in places. It was the Leonid shower of 1833 that led to the identification of specific meteor showers. Other good displays in 1799, 1866 and 1998 (with 1899 and 1933 being poor because the earth missed the main swarm) 33 years is the orbital period of the comet itself.
Geminids December 13
Ursids December 22 Tuttle

Earth crossing Comet's Orbit causing Meteor Showers
The above diagram shows two possible scenarios. On the left the comet debris is evenly spaced out causing regular meteor showers each year of similar intensity. On the right, the debris is spread out much more unevenly, causing irregular intensities of meteor showers.

Sporadics are meteors that are spotted during a storm but do not actually 'belong to' the storm.

The best known shower in the Northern Hemisphere are the Perseids. To observe them in a scientific manner, a group could be spread out in deckchairs like the petals of a flower, with one 'supervisor' reacting to the detection of a meteor, assessing whether it is a Perseid or a sporadic and thus recording all the Perseid meteors.

Tunguska

On 30 June 1908, near the Tunguska River in Russia, a mystery object fell causing widespread damage. Trees were felled over thousands of square kilometers. Since no crater was formed, a comet is a possible candidate for the incident (apparently, no expedition visited the site until 1927).

Historical Overview

It is appropriate to include some information about history, purely for background purposes. For about 2000 years, Chinese astronomers appear to have made systematic studies of comets. Tycho Brahe in 1577 appears to have been in the forefront of signalling the start of European activity. Tycho noted that other observers in distant locations placed the comet in roughly the same position of the Sky as himself, whereas nearby objects have different bearings to different observers. I say 'roughly' given the precision at the time, but nevertheless this was enough to deduce that the Comet was far away, it was definitely not the atmospheric phenomenum that appeared to be the accepted wisdom at the time.

The topic received a great boost from the work of Messier and Mechain in the second half of the eightenth century. With the help of Mechain, Messier compiled his famous list of 'peculiar' objects which were not comets, as an aid to hunting for the real thing.

Twelve comets are named after Messier and seven after Mechain (including one which is jointly named). Messier also independently discovered the return of Halley's Comet in January 1759. The last comet with Messier's name is from 1798.

The principle of naming comets after their dicoverer(s) appears to have been due to Messier, and used from 1759 onwards.

Other famous comet-hunters of this period include Olbers of Deutschland and the German astronomer Caroline Herschel who worked originally from Bath, Britain and discovered eight comets before 1800. The most famous of all comet-hunters appears to have been Jean-Louis Pons, who discovered his first comet in July 1801, although Messier discovered it independently 24 hours later. 80% of the comets discovered in the first 27 years of the 19th century were by Pons. His full total was 37 comets. Until 1825, he was based in Marseille (where apparently he had first starting working as a caretaker in 1789) then moved to Florence, dying in 1831.

Wilhelm von Biela was a captain in the Austrian army who discovered three comets. As seen in the table above, Comet Biela split in two in 1845-6

Tempel from Deutschland but mainly observed from France and Italy.

USA : amateurs Barnard, Brooks, Swift, professionals Perrine, Tuttle, Carolyn Shoemaker

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Crab Nebula

Big Red Spot on Jupiter