Astronomy/ Comets term paper 19761

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Before the seventeenth century, comets were considered portents-warning shots fired at a

sinful Earth from the right hand of an avenging God. However, in the post-Newtonian era, when

their paths were understood to intersect that of the Earth, they were considered actual agents of

destruction. Experts have described comets as the carriers of both life-seeds to the early Earth

and horrific missiles that will one day snuff out life as we know it. At one time or another,

people have blamed comets for war and held responsible for the deaths of men, the birth of good

wine, the London fire of 1666, severely cold weather, etc . . . If one central theme runs

throughout history of comets, it must be the public concern they have commanded.

Comets are ancient objects, formed in the outer reaches of the Solar System from the ice

of gases such as methane, water vapor, and ammonia, combined with dust from primitive rock

compounds. Sometimes comets are described as "dirty snowballs" because they are icy lumps,

or wandering icebergs. Comets are relatively tiny- just a few miles across on average. Their

nuclei are very different from glowing balls of light, with multimillion-mile-long tails. This is

one reason comets occasionally visit the inner Solar System.

Astronomers divide comets into long-period types with orbits of more than 200 years and

short-period types with orbits of less than 200 years (as cited in Branley 1988 p. 43). All comets

begin their journey as long- period types. Gravitational fields of planets then capture long-

period comets. Comets can have orbits at any angle because they can come from any region.

Once comets are captured, they fall into line with the movement of planets, staying close to the

ecliptic, orbiting the sun in the same direction as the planets. One exception is Halley's Comet.

It is a short-period comet with an orbital period of about seventy-six years- known as retrograde

orbits (as cited in Branley 1988 p. 44). Retrograde orbits are simply clockwise orbital motion, as

seen from the north pole of a planet. Most Solar System orbits are counter clockwise.

Like people, comets group too. When several comets with different periods travel in

nearly the same orbit, experts say that they are members of a comet group. One well-known

group includes the spectacular Sun-grazing comet, Ikeya-Seki, of 1965, and seven others having

periods of nearly a thousand years. Brian G. Marsden, an American astronomer, has concluded

that a 1965 comet and the even brighter comet of 1882 split from a parent comet, possibly the

one of 1106 (as cited in Yeomans 1991 p. 184).

One interesting contribution of the comet is the solar effect. The process starts by a

comet approaching the sun. Once the comet approaches the sun, solar heat sublimates, or

evaporates, the ices. This causes the comet to brighten enormously. Sometimes this develops a

brilliant tail, extending millions of kilometers into space. Even as the comet recedes again, the

tail is directed away from the sun.

What are these spectacular comet tails composed of? Comet tails are made up of simple

ionized molecules, including carbon monoxide and dioxide. By action of solar wind, molecules

are blown away, forming a thin stream of hot gases continuously ejected from the solar corona.

In case you do not know the meaning of a solar corona, it is the outermost atmosphere of the

Sun. Amazingly, the thin streams of high gases move at a speed of approximately 400

kilometers (250 miles) per second (as cited in Yeomans 1991 p. 185). In addition, a comet

frequently also displays smaller, curved tails composed of fine dust particles blown from the

coma by the pressure of solar radiation.

Yet, as a comet recedes from the Sun, the loss of gas and other dust particles decrease in

quantity, which contribute to the disappearance of the tail. Some comets with small orbits

contain tails so short that they are practically invisible. However, the tail of at least one comet

has indeed exceeded approximately 320 million kilometers (200 million miles) in length (as cited

in Yeornans 1991 p. 183). Surprisingly, of some 1400 comets on record, fewer than half the tails

were visible to the naked eye, and fewer than 10 percent were conspicuous (as cited in Yeornans

1991 p. 185).

Interestingly, amateur astronomer Yuji Hayukutake from Hawaii, discovered Comet

Hayukutake. This discovery was on January 30, 1996 (as cited in rosat-goc-comet) However,

mid -march is at its most visible in the northern hemisphere. This surprise comet has turned out

to be the closest and most spectacular of the century. This comet was the brightest comet to

come near the earth in more than twenty years. Comet Hayukutake gave astronomers a wealth of

details and information.

Not only did Comet Hayukutake bring new discovery with images, but it also renewed

interest in observational astronomy when it passed the Earth in early 1996. The comet improved

perceptions of the constellation it transversed. The appearance of Comet Hayukutake in March of

1996, inspired the same sense of wonder in those who observed it as the ancients who first saw

comets in the sky. Many ancient astronomers have linked comets to dragons breathing fire into

the atmosphere, and viewers can readily see this while observing these celestial spheres (as cited in Vogt1993 p. 25).

Both amateur and professional astronomers relayed heavily on enthusiastic reports of their observations on Comet Hayukutake when it appeared for several days in March of 1996.

The comet provided clear observations of its gas jets, fans and rare disconnection event as it

moved towards the sun. (as cited in rosat-goc-comet).

Did you know Comet Hayukutake, a I O-mile-wide block of space dust and ice, passed

within 9.3 million miles of Earth on March 25, 1996? (as cited in encarta 1994). Although the

Hubble telescope provided many excellent pictures of the comet, amateur astronomers have

aided in tracking the comet. For those of us who were too busy to see this spectacular object,

NASA'S web page provided a graphic picture of the comet, so we too, could get a glimpse of

this spectacular sight of Comet Hayukutake.

When astronomers observed Comet Hayukutake on March 27, 1996, they made the first

ever extreme ultra violet (EUV) image of a comet (as cited in rosat-goc-comet). Interestingly,

this observation of Comet Hayukutake was simultaneous with the X-ray measurements made

from the US, provided High Resolution Imager, reported in April.

Why are these first ever X-ray/EUV images of a comet so remarkable? These images

provided great, and quite unexpected-brightness. Many astronomers discovered large changes in

brightness over a few hours. This very important discovery showed that previous unsuspected

"high energy" processes must have been taking place in the comet. High energy was probably

due to the influence of the Sun's radiation and solar wind.

Astronomers who pointed the German X-ray Roentgen Satellite at Comet Hayukutake

were surprised by a brilliant X-ray emission, especially since no X-ray had ever before been

detected from a comet (as cited in rosat-goc-comet). One theory is that solar X-rays interacting with water molecules in the comet produced the emission. On the other hand, some researchers

believed that some X-rays were generated through the interaction between the Sun's solar wind

and the comet itself. According to astronomer Michael Di Santi and his colleagues, they

identified methane and ethane in Comet Hayukutake as it passed Earth in March of 1996 (as

cited in rosat-goc-comet). Neither compound had ever been confirmed in comets, but the

methane surprised many. This compound was not believed to have been part of the material

from which the solar system arose.

For many, this comet was by far the most interesting thing to have witnessed. A man

describes viewing Comet Hayukutake the night before its closest approach to Earth with a

number of amateur astronomers atop Mount TamalpaisJust north of San Francisco, California.

Many were impressed with Comet Hayukutake. However, the sight was not charming for all.

Comets have been given the title "great" throughout history for being aw-inspiring, for spurring

interest in astronomy and for their high visibility during perigee, the point at which is closest

from the object to the earth. According to urban citizens. Comet Hayukutake did not produce

such an effect on people since its appearance was obscured by light pollution in the cities.

According to all astronomers, the range of phenomena attributed to comets are

extraordinary. Some of it true, much of it nonsense. But all of it adds to their considerable

mystique and perhaps explains the universal interest shown in these, the solar systems' smallest

bodies. Comets are currently thought to be the building blocks of the major planets and sources

for some of Earth's water, volatiles, and organic molecules. Commentary impacts on Earth have

deposited some of the biogenic material from which primitive life may have ultimately formed (as cited inYeomans, 1991 p. 228).

Word Count: 1479

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