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Have you ever looked up in the sky and seen a little ball creeping by?
If so, did you wonder what it was? That little ball is called a comet.
Comets are small, fragile, and irregularly shaped. Most are composed of
frozen gas. However, some are composed of frozen gas and non-volatile
grains. They usually follow very strict paths around the sun. Comets
become most visible when they cross the sun. This also applies to people
who view comets with telescopes. When a comet gets near the sun it
becomes very visible because the sun's radiation starts to sublime its
volatile gases, which, in turn, blow away small bits of the little solid
material the comet has.
Another feature of a comet is a long tail. This is caused by materials
breaking off and expanding. They expand into an enormous escaping
atmosphere called the coma. This becomes at least the size of our
planet. With the comet going so fast, these materials are forced behind
the comet, forming a long tail of dust and gas.
Comets are cold bodies. We see them only because the gases they are
composed of glow in the sunlight. All comets are regular family members
of the solar system family. They are bound by gravity to a strict path
around the solar system. Scientists believe that all comets were formed
of material, originally in the outer part of the solar system, which did
not become incorporated into planets. This material is from when the
planets just started forming. This makes comets an extremely interesting
topic to scientists who are studying the history of the solar system.
In comparison to planets, comets are very small. They can be anywhere
from 750 meters (or less) to 20 kilometers in diameter. However, lately,
scientists have been finding proof that there are comets 300 kilometers
in diameter or greater.
Comets are still compared to the planets, though. Planets usually follow
the shape of a sphere. Most planets are fat at the equator. Comets come
in all different shapes and sizes. Most evidence that science has
revealed says that comets are extremely fragile. A comet is so poorly
structured that it is like a loose snowball--it can be pulled apart with
one’s own bare hands.
Comets have very awkward rotation periods. They are very oblong. When
comets reach their aphelion they are usually near Jupiter or even
sometimes Neptune. Other comets, however, come from even farther out in
the solar system. No matter what, if a comet passes Jupiter, it is
strongly attracted to it. Sometimes Jupiter’s massive gravitational pull
makes comets slam into planets .
Comets' nuclei look like dirty snowballs. They are solid, persisting of
ice and gas. Most nuclei contain rock, actually, small grains of rock
somewhat like rock here on Earth. A nucleus appears to be black in color
because it is made up of carbon compounds and sometimes free carbon.
Since comet nuclei are so small they are difficult to study from Earth.
An interesting feature of a comet that few people know is that even
though a comet appears to have a single tail, it actually has two. One
tail is a dust tail and the other is an ion tail.
Although comets are very old, the oldest comet recorded is Comet Halley.
They are Chinese records of this comet dating as far back as 240 BC Sir
Edmund Halley predicted in 1705 that a comet which had appeared in 1531,
1607, and also 1682 would return in 1758. (unfortunately, the comet
appeared on the day he was born and the day he died, he never got to see
the comet) It was named Comet Halley in honor of him. A sighting of the
comet was confirmed on Christmas day 1758.
Halley predicted the date on which the comet would return using Kepler’s
Third Law which states:
1. All orbits are ellipses with the sun at one focus.
2. A line between a planet and the Sun sweeps out an equal area during
any fixed interval of time (i.e. planets move quickly when they are
close to the sun)
3. (OrbitalPeriod(years))squared = (OrbitalRadius(AU))cubed
A comet that has been discovered more recently is the Hale-Bopp comet.
It is scheduled to appear in April 1997.
Alan Hale is a native New Mexican. Hale is a professional astronomer, he
specializes in studying sunlike stars and searching for other planetary
systems. He has been studying comets since 1970. Here is how he
discovered the comet:
“During my normal study of comets, it is my practice to observe comets
once a week, on the average, and measure their brightnesses. On the
night of July 22--the first clear night here in a week and a half--I
planned to observe two comets. I finished with the first one--Periodic
Comet Clark--shortly before midnight, and had about an hour and a half
to wait before the second one--Periodic Comet D’Arrest--rose high enough
in the east to get a good look at. I decided to pass the time by
observing some deep-sky objects in Sagittarius, and when I turned my
telescope (a Meade DS-16) to M70, I immediately noticed a fuzzy object
in the field that hadn’t been there when I had looked at M70 two seeks
earlier. After verifying that I was indeed looking at M70, and not one
of the many other globular clusters in that part of the sky, I checked
the various deep-sky catalogues, then ran the comment-identification
program at the IAU Central Bureau’s computer in Cambridge,
Massachusetts. I sent an e-mail to Brian Marsden and Dan Green at the
Central Bureau at that time informing them of a possible comet; later,
when I had verified that the object had moved against the background
stars, I sent them an additional e-mail. I continued to follow the comet
for a total of about 3 hours, until it set behind trees in the
southwest, and then was able to e-mail a detailed report, complete with
After he discovered the comet he said “I love this irony -- I’ve spent
over 400 hours of my life looking for comets, and I haven’t found
anything, and now, suddenly, when I’m not looking for one, I get one
dumped right in my lap. I had obtained an observation of P/Clark
earlier, and I needed to wait an hour or so before P/d’ Arrest got high
enough to look at, and I was just passing time til’ then and I decided
to look at some deep-sky objects in Sagittarius. When I turned to M70, I
saw a fuzzy object in the same field, and almost immediately expected a
comet, since I had been looking at M70 last month, and *knew* there
wasn’t any objects there.”
It all started for Bopp on July 22nd, 1995 on the exact night that Alan
Hale saw the comet. In fact, they both saw the comet within 5 minutes of
each other. Alan Hale was the first person to see it however. Here is
the story of Thomas Bopp.
“On the night of July 22, some friends and I headed out into the desert
for a dark moon observation session. The site, which is west of
Stanfield, Arizona, and a few miles of interstate 8 is about 90 miles
southwest of my home.
My friend Jim Stevens had brought his 17-1/2” Dobsonian. We started the
evening observing some of the messier objects such as the Veil and the
North American Nebulae in Cygnus, when Jim said “Lets look at some of
the globularsin Sagittarius.” We started our tour with M22 and M28,
observing at 50X and then 180X. Around 11:00 local time, we had M70 in
the field when Jim went to the charts to determine the next object of
investigation. I continued watching M70 slowly drift across the field,
when it reached a point 3/4 of the way across an alight glow appeared on
the eastern edge. I repositioned the scope to the center on the new
object but was unable to resolve it. I called to Jim and asked him if he
knew what it might be, after visual inspection he stated he was not
familiar with it but would check the charts. After determining the
general position of the object he was unable to find it on Sky Atlas
2000.0 or Uranometria.
The moment Jim said “we might have something” excitement began to grow
among our group and I breathed a silent prayer thanking God for his
wondrous creation. My friend, Kevin Gill then took a position from his
digital setting circlesand estimated a magnitude.
At 11:15 I said that we needed to check the object for motion and should
watch it for an hour. The group observed it change position against the
star field over that period and at 12:25 I decided to drive home and
report our finding. Arriving at home, initial attempts to send a
telegram were unsuccessful due to an incomplete address I had. After
searching my library I was able to locate the correct address and
confirmation was requested.
At 8:25 A.M. July 23rd, 1995, Daniel Green of the Harvard Smithsonian
Astrophysical Observatory telephoned and said, “Congratulations Tom, I
believe you discovered a new comet.” And that was one of the happiest
moments of my life.”
Thomas Bopp lives in Glendale, Arizona. (Small suburb just barely
outside Phoenix) He is the supervisor for a construction material
company in Phoenix. Bopp is an enthusiastic observer of deep sky
objects. The exact name of the site Bopp saw the comet at is Vekol
Since they discovered the comet within minutes of each other the comet
was named the Hale-Bopp Comet.
Nobody knows the exact orbital period of the comet but it is believed to
be a little over 3000 years. It has passed through our solar system
before (that is, it is not a new comet from the Oort Cloud)
On April 1, 1997, the comet is expected to reach its closed point to the
sun. At this time it will also be most visible because the sun reflects
off the tail of the comet.
It will come .914 astronomical units from the sun. This is not all that
close to the sun considering the fact that some comets have run into the
sun and others have skimmed the surface of it.
Although the comet will be closest to the sun on April 1, it will be
closest to the earth on March 23, 1997. Some people have been saying
that the comet will hit earth and cause human extinction, just like the
dinosaurs. The fact is, however, THE COMET WILL NOT HIT EARTH. The
closest it will come is 120 million miles away from the earth.
Some people are saying that the comet is going to Be huge, and others
say it will be small. We will never know though because we can not see
the nucleus of a comet. The part of the comet we see is the tail. The
tail of a comet can be over 10,000 kilometers long.
In all, comets, the history of comets, and comets waiting to be
discovered is very interesting. I think that one day we will get to see
the nucleus of a comet, and be able to watch comets form in the Oort
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