Difference between revisions of "Comet"

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	[[Image:Halley Bayeux.jpg|left|150px]]Comets as a class were known to the ancients, and some of the most prominent and regularly appearing comets observed today appear in ancient records. For example, [[China|Chinese]] records mention a recurrent comet beginning in 240 BC. That comet is most likely Comet Halley, whose discoverer, [[Edmond Halley]], was the first to predict a comet's reappearance in advance. Comet Halley also appeared on April 24, 1066, to astounded onlookers in [[England]] prior to the conquest of England by the Normans, as depicted in the Tapestry of Bayeux.<ref name=Bayeux>Crack, Glen Ray. "[http://www.hastings1066.com/bayhi3a.shtml Bayeux Tapestry Highlights Part 3, Image 1]." <http://www.hastings1066.com/>, January 10, 1998. Retrieved July 20, 2008.</ref>		[[Image:Halley Bayeux.jpg|left|150px]]Comets as a class were known to the ancients, and some of the most prominent and regularly appearing comets observed today appear in ancient records. For example, [[China|Chinese]] records mention a recurrent comet beginning in 240 BC. That comet is most likely Comet Halley, whose discoverer, [[Edmond Halley]], was the first to predict a comet's reappearance in advance. Comet Halley also appeared on April 24, 1066, to astounded onlookers in [[England]] prior to the conquest of England by the Normans, as depicted in the Tapestry of Bayeux.<ref name=Bayeux>Crack, Glen Ray. "[http://www.hastings1066.com/bayhi3a.shtml Bayeux Tapestry Highlights Part 3, Image 1]." <http://www.hastings1066.com/>, January 10, 1998. Retrieved July 20, 2008.</ref>
−	[[Image:Halley1910a.jpg|right|thumb|150px|Halley's Comet on its 1910 appearance. This Yerkes Observatory photograph is highly enlarged; the comet would not have appeared this large or bright to the naked eye.]]In fact, comets have been observed for far longer in history. The ancient [[Greece|Greeks]] called them "evil stars," which in Greek is ''dys'' evil and ''astron'' a star. This is the origin of the modern word ''disaster''. The association of comets with untoward events or prophecies continued as recently as the twentieth century, when Comet Halley made an unusually close rendezvous with [[earth]].<ref name=Newburn>Newburn, Ray. "[http://stardust.jpl.nasa.gov/news/news90.html NASA's Blazing the Trail to Understand Comets]." [[JPL]], [[NASA]], November 21, 2003. Accessed July 20, 2008.</ref>	+	In fact, comets have been observed for far longer in history. The ancient [[Greece|Greeks]] called them "evil stars," which in Greek is ''dys'' evil and ''astron'' a star. This is the origin of the modern word ''disaster''. The association of comets with untoward events or prophecies continued as recently as the twentieth century, when Comet Halley made an unusually close rendezvous with [[earth]].<ref name=Newburn>Newburn, Ray. "[http://stardust.jpl.nasa.gov/news/news90.html NASA's Blazing the Trail to Understand Comets]." [[JPL]], [[NASA]], November 21, 2003. Accessed July 20, 2008.</ref>
	Today, amateur astronomers often discover comets. Recent computerized sky surveys now find most comets as they approach the [[sun]].<ref name=Arnett/>		Today, amateur astronomers often discover comets. Recent computerized sky surveys now find most comets as they approach the [[sun]].<ref name=Arnett/>
	== Parts of a comet ==		== Parts of a comet ==
−	The '''nucleus''' or ''head'' of a comet is a lightweight ball of rock, dust, and ice that resembles a dirty snowball. As a comet approaches the sun at 5 AU or closer, the '''coma''' forms. This is the base of the two ''tails'' of the comet. The ice begins to melt and vaporize, and the solar wind blows the liberated particles away from the sun. The dust forms one tail, and [[ion]]s form another.<ref name=Yeomans/>	+	[[Image:Halley1910a.jpg|right|thumb|300px|Halley's Comet on its 1910 appearance. This Yerkes Observatory photograph is highly enlarged; the comet would not have appeared this large or bright to the naked eye.]]The '''nucleus''' or ''head'' of a comet is a lightweight ball of rock, dust, and ice that resembles a dirty snowball. As a comet approaches the sun at 5 AU or closer, the '''coma''' forms. This is the base of the two ''tails'' of the comet. The ice begins to melt and vaporize, and the solar wind blows the liberated particles away from the sun. The dust forms one tail, and [[ion]]s form another.<ref name=Yeomans/>
	Comet nuclei are currently thought to measure 16 kilometers across or less.<ref name=Yeomans/> A comet nucleus shines with reflected light only. A comet's coma often absorbs ultraviolet radiation and becomes fluorescent. In so doing it can shine more brightly than does the nucleus.<ref name=Hamilton/> A comet's tails can extend for 160 million kilometers<ref name=Yeomans/> and thus appear larger than the constellation of Ursa Major, and occasionally brighter than the [[Milky Way]] itself.		Comet nuclei are currently thought to measure 16 kilometers across or less.<ref name=Yeomans/> A comet nucleus shines with reflected light only. A comet's coma often absorbs ultraviolet radiation and becomes fluorescent. In so doing it can shine more brightly than does the nucleus.<ref name=Hamilton/> A comet's tails can extend for 160 million kilometers<ref name=Yeomans/> and thus appear larger than the constellation of Ursa Major, and occasionally brighter than the [[Milky Way]] itself.

---

Revision as of 17:51, July 24, 2008

A comet is a small solar system body, of irregular shape, composed of water ice, frozen gas and non-volatile dust. They are in highly eccentric orbits around the sun, with apoapsides varying from 4 AU to 30 AU (in the Kuiper belt) and beyond. The relatively short life spans of comets have provoked a still-unsettled debate concerning their origins. This debate is one of the most important of all debates between creationists and non-creationists.^{[2][3][4][5][6][7]}

History of Comet Observation and Exploration

Comets as a class were known to the ancients, and some of the most prominent and regularly appearing comets observed today appear in ancient records. For example, Chinese records mention a recurrent comet beginning in 240 BC. That comet is most likely Comet Halley, whose discoverer, Edmond Halley, was the first to predict a comet's reappearance in advance. Comet Halley also appeared on April 24, 1066, to astounded onlookers in England prior to the conquest of England by the Normans, as depicted in the Tapestry of Bayeux.^[8]

In fact, comets have been observed for far longer in history. The ancient Greeks called them "evil stars," which in Greek is dys evil and astron a star. This is the origin of the modern word disaster. The association of comets with untoward events or prophecies continued as recently as the twentieth century, when Comet Halley made an unusually close rendezvous with earth.^[9]

Today, amateur astronomers often discover comets. Recent computerized sky surveys now find most comets as they approach the sun.^[4]

Parts of a comet

The nucleus or head of a comet is a lightweight ball of rock, dust, and ice that resembles a dirty snowball. As a comet approaches the sun at 5 AU or closer, the coma forms. This is the base of the two tails of the comet. The ice begins to melt and vaporize, and the solar wind blows the liberated particles away from the sun. The dust forms one tail, and ions form another.^[2]

Comet nuclei are currently thought to measure 16 kilometers across or less.^[2] A comet nucleus shines with reflected light only. A comet's coma often absorbs ultraviolet radiation and becomes fluorescent. In so doing it can shine more brightly than does the nucleus.^[3] A comet's tails can extend for 160 million kilometers^[2] and thus appear larger than the constellation of Ursa Major, and occasionally brighter than the Milky Way itself.

Since 1981, satellite-based cameras have repeatedly photographed a number of objects striking earth's atmosphere and vaporizing. These might in fact be small cometary nuclei, each as large as a house. Remarkably, these objects tend to strike more frequently in the early fall than in the early winter. Some critics contend that these objects are mere camera noise, but experiments to replicate house-sized comets have succeeded in duplicating the observed effects.^[7]

Meteor showers often occur with comets, as the earth passes through the dust tail and some of the dust enters the atmosphere. Oddly, such showers almost always occur during late summer or fall.^[4]

Chemical composition

The ice portion of a comet is mostly water but might include other substances. These include ammonia, carbon dioxide, carbon monoxide, methane, and ethane.^[7][2]

In 1998 and 1999, Meier et al. published at least three papers showing that comets are remarkably rich in deuterium or "heavy" hydrogen. This included deuterated water (HDO)^[10][11] and deuterated hydrogen cyanide (DCN)^[12] In fact they have it in twice the concentration of deuterium in the seas of earth and 20 to 100 times the concentration in the rest of the solar system. In 1998 Meier stated flatly that

“

Comets cannot be the only source for the oceans on Earth.

”

In July of 2004, the Stardust mission approached to within 150 miles of Comet Wild 2 and was able to sample its tail and return the samples to earth (January 2006). The returned dust was crystalline and included organic material, water ice, and many terrestrial minerals. These included aluminum, magnesium, calcium, and titanium.^[7]

One year later, on July 4, 2005, the Deep Impact mission conducted the first direct analysis of the rocky portion of a comet nucleus, by launching a projectile into Comet Tempel 1. The projectile liberated much material, including silicates, crystalline silicates, minerals that normally form in liquid water (calcium carbonate forms and clays), an organic material of still undetermined composition, sodium, and a very fine powder.^[7]

Families of comets

By period

Astronomers today divide comets into at least two classes. The official definition of a short-period comet is any comet having a period of 200 years or shorter. Any comet having a longer period is called a long-period comet.^[2][5]

Walt Brown notes that 205 comets have periods of 100 years or shorter, and 659 comets have periods of 700 years or longer. He also counts 50 intermediate-period comets having periods between those two values.

Several very long-period comets, in near-parabolic orbits, are known. No comets have ever been seen in hyperbolic orbits.

By apsis

Roughly 60% of all short-period comets belong to Jupiter's family. These are comets having aphelions varying between 4 and 6 AU. Other short-period comets have aphelia within the Kuiper belt. The long-period comets have calculated aphelia far beyond the Kuiper belt, at about 50,000 AU.^[7]

The perihelia of all comets vary between 1 and 3 AU.

By inclination

Short-period comets tend to lie in or close to the plane of the ecliptic. Long- and intermediate comets can have any inclination from zero to ninety degrees.^[7]

By orbital direction

Short-period comets are almost all in prograde orbits. But more than half of all long-period comets are in retrograde orbits.^[7]

Descriptive table

The following table, adapted from Brown^[7], gives orbital characteristics and composition of the 964 comets now known:

Attribute	Short	Intermediate	Long
Period	< 100 a	100-700 a	> 700 a
Number	205	50	659
Inclination to ecliptic	Usually low	Low to high	Low to high
Prograde portion	93%	70%	47%
Retrograde portion	7%	30%	53%

Life span

Comets have remarkably short life spans. Most periodic comets, particularly in Jupiter's family, have life spans of 10,000^[13] to 12,000 years.^[7] A comet may make a limited number of orbits before all its volatile substances sublimate away, leaving behind an asteroid-like rock. Certain asteroid-like objects, called damocloids, in highly eccentric orbits around the sun might be inactive comets. 21 such objects are known.^[6] Some authorities estimate that half of all near-earth asteroids are in fact cometary remnants.^[4]

Origin

Uniformitarian/evolutionary theories

Brown^[7] details several evolutionary theories of the origins of comets.

Oort cloud

Main article: Oort cloud

Most conventional astronomers invoke the nebula theory and state that comets are debris that failed to accrete into the planets and moons when the solar system formed.^[2][5] The classic Oort cloud theory states that the Oort cloud, a sphere measuring about 50,000 AU in radius, formed at the same time as the solar nebula and occasionally releases comets into the inner solar system as a star (possibly the reputed Nemesis) passes. Jewitt^[6] and others have proposed instead that comets initially formed near or immediately beyond the gas giant planets. Some of these objects persist as Kuiper belt objects. Others passed close enough to the gas giants to gain sufficient energy to launch them into high orbits, where they eventually formed the Oort cloud and, some suggest, continue to resupply it.

Interstellar capture

According to this theory, the sun passes through and perturbs clouds of interstellar dust and gas. In the process it captures large numbers of particles, which accrete into comets.

Meteor stream

Comets form continuously from a stream of meteors in various orbits around the Sun.

Volcanism

Comets are volcanic ejecta, from either the various gas giants or some of their moons.

Exploded planet

A planet originally in the region now occupied by the main asteroid belt exploded about 3,200,000 years ago. The presently observed comets and asteroids are its remnants.

Organic matter

Main article: Panspermia

Many comets contain organic compounds, including methane and ethane. This has led some scientists to speculate that comets brought to earth the initial "seeds" of life.

Problems for uniformitarian theories

Many comets have life spans less than 10,000 years. According to the nebula theory, comets formed with the rest of the solar system, 4.6 billion years ago. Adherents of the interstellar capture theory try to connect the origin of cometary matter with the big bang, which, they say, happened about 13.7 billion years ago. But in that case, all the short-period comets ought to have disappeared. This is especially true of the Jupiter family. Even if the Kuiper belt is the source of short-period comets, such comets would have to lose much kinetic energy in order to settle into the short-aphelion orbits of Jupiter's family. This begs the question of how and where they lost this energy.^[7]

Brown lists many other problems for various uniformitarian theories posed by comets:

1. Comets form by accretion of water ice and other materials. This explains why comets usually have the consistency of snowballs and not of hard blocks of ice. Accretion from a solar nebula has never been satisfactorily modeled. Accretion requires the sudden release of a stream of matter beyond the sphere of gravitational influence of another body, and into a region in which the matter can form its own, rapidly growing sphere of influence. The putative conditions of the solar nebula do not meet this requirement.
2. Large quantities of water ice have been observed on the poles of the moon, the planet Mercury, and now, most recently, on Mars. Why this ice has not evaporated, particularly from the moon and Mercury, where atmospheric pressures are negligible, has never been explained.
3. Crystalline dust might form from an exploded planet, but it would not be likely to form in any of the other proposed models.
4. If comets formed in the region of the putative Oort cloud, they would not attain near-parabolic orbits.
5. The original Oort cloud model and the interstellar capture model would tend to produce perihelia with predictable alignments. The perihelia of comets are almost completely random.
6. Either of the Oort cloud models would be expected to produce comets with hyperbolic orbits. No such cometary orbit has ever been observed.
7. The observed perihelia, varying from 1 to 3 AU, are incompatible with any origin of comets beyond 3 au. Therefore, comets would have to form in the main asteroid belt or closer to the sun.
8. The Oort cloud, if it exists, cannot be the source of short-period comets. But Gerard P. Kuiper formed the hypothesis of the belt of objects that bears his name to explain this discrepancy.
9. Jupiter's family of comets cannot have come from a planetary explosion at 3 au from the sun, or from any other source described above other than volcanic ejection from Jupiter.
10. If comets formed in the gas giant region and were later launched into the Oort cloud, many of the comets now observed would have destroyed one another long ago.
11. The current composition of comets cannot have resulted from volcanic ejection from a gas giant.
12. The unusual proportion of deuterium in comets, and only in comets, makes their origin from either the solar nebula or an interstellar dust cloud very problematic. The preferential concentration of deuterium is the most difficult observation to explain.
13. The small comets recently observed striking the earth should not be observed if comets originated beyond 3 AU.
14. The observed meteor craters are mostly superficial and thus inconsistent with a formation of comets within the inner solar system millions or billions of years ago. This militates against the exploded-planet and revised Oort cloud models.

For further details, see Brown's table of various comet-origin theories and how well (or poorly) they explain the evidence.

Creation theory

Main article: Hydroplate theory

Brown proposes a radically different theory for the origin of comets: that they originated from matter ejected into space during the global flood. According to his hydroplate theory, the Flood waters broke through a crustal rupture that persists today as the Mid-Oceanic Ridge system. This water was extremely hot and under tremendous pressure. Brown estimates that less than one percent was ejected into space. In the process, the edges of the crust at the rupture crumbled, and much of their material was ejected with the water.

The ejecta easily reached escape speed and soon passed beyond earth's gravitational influence. Some of the rocks abruptly formed spheres of influence of their own and attracted the surrounding water through gravitational accretion. Brown calculates that the ejecta included enough mass for 50,000 comets, far more than have thus far been observed.

Much of the water fell onto the moon and the planets Mercury and Mars, where it condensed and froze in the polar regions. The hydroplate theory also states that many of the rocks fell to the moon and thus created the craters and maria observed today.

Many of the comets were ejected into hyperbolic orbits, never to return to the inner solar system. Others were launched into near-parabolic orbits, either immediately or under the influence of the gas giants.

The high concentrations of deuterium reflect the initial composition of the subcrustal oceans. The organic matter came from earth and was never introduced to earth by the comets.

Problems for the hydroplate theory

The hydroplate theory has two known problems:

1. Near-parabolic comets often have very long periods. Their launch in an event occurring 4400 years ago is difficult to explain, unless they received a gravitational boost from Jupiter or some other gas giant.
2. Humphreys followed up on his model for the creation of planetary magnetic fields^[14] and calculated that the moon suffered two bombardments, neither of which were in time frames compatible with the global flood. According to Humphreys, the maria formed three centuries after the fall of man, and the visible craters formed about a century or so after the flood. If most of the comets, asteroids, and meteoroids now observed were flood ejecta, then they could have subjected the moon (and other moons in the solar system) to a heavy bombardment a century later and formed the craters. But they could not have formed the maria.

These problems, however, are distinctly minor in comparison to the great number of critical difficulties for uniformitarian theories.

References

See Also

• Halley's Comet
• Kuiper belt
• Oort cloud
• Nemesis
• Hydroplate theory