Comets
How Comets Work
By:
Craig Freudenrich, Ph.D.
We mark the date that we saw a comet that
comes around only once a century (or even once in four centuries), and we
remember the sight for the rest of our lives.
Astronomers find comets fascinating, too.
They're remarkable pieces of our universe's past, and they tell us a great deal
about how the universe was formed.
Comet Image Gallery
Almost every year, we are visited by comets
from the outer reaches our solar system, like ISON or LINEAR.
In this edition of How Stuff Works, we will
examine the fascinating world of comets.
We will find out what comets are, what they
are made of, where they come from and how to observe them.
You can learn where to look for comets and
maybe even discover one yourself.
What
Is a Comet?
Comets are small members of the solar
system, usually a few miles or kilometers in diameter.
They have been described as "dirty
snowballs" by astronomer Fred Whipple and are thought to be made of:
o dust
o ice (water,
ammonia, methane, carbon dioxide)
o some
carbon-containing (organic) materials (e.g., tar)
o a rocky center
(some comets)
Comets are thought to be made from the
earliest materials of the solar system.
When the sun first formed, it blew lighter
material (gases, dust) out into space.
Some of this material (mainly gas)
condensed to form the outer planets (Jupiter, Saturn, Uranus, Neptune) and some
remains in orbit far from the sun in two areas:
o Oort Cloud - a
sphere about 50,000 AUs from the sun; named after the Dutch astronomer Jan Oort
who proposed it
o Kuiper Belt - an
area within the plane of the solar system outside the orbit of Pluto
Astronomical
Unit
The astronomical unit (AU) is a basic unit
of distance used by astronomers. It is the average distance from the Earth to
the sun and is about 93-million miles (150-million km).
The Path of a Comet
Comets are thought to orbit the sun in
either the Oort cloud or Kuiper belt.
When another star passes by the solar
system, its gravity pushes the Oort cloud and/or Kuiper belt and causes comets
to descend toward the sun in a highly elliptical orbit with the sun at one
focus of the ellipse.
Comets can have short period orbits (less
than 200 years such as Halley's comet) or long period orbits (greater than 200
years such as comet Hale-Bopp).
As the comet passes within six AUs of the
sun, the ice begins to go directly from the solid to the gas state
(sublimation) much like the way fog is formed. When the ice sublimes, the gas
and dust particles flow away from the sun to form the comet's tail.
Parts
of a Comet
As a comet approaches the sun, it warms up. During this warming, you can observe several distinct parts:
o nucleus
o coma
o hydrogen envelope
o dust tail
o ion tail
The coma is a halo of evaporated gas (water
vapor, ammonia, carbon dioxide) and dust that surrounds the nucleus.
The coma is made as the comet warms up and
is often 1,000 times larger than the nucleus.
It can even become as big as Jupiter or
Saturn (100,000 kilometers). The coma and nucleus together form the head of the
comet.
Surrounding the coma is an invisible layer
of hydrogen called the hydrogen envelope; the hydrogen may come from water
molecules.
It usually has an irregular shape because
it is distorted by the solar wind. The hydrogen envelope gets bigger as the
comet approaches the sun.
The comet's dust tail always faces away
from the sun.
The tail is made of small (one micron) dust
particles that have evaporated from the nucleus and are pushed away from the
comet by the pressure of sunlight.
The dust tail is the easiest part of the
comet to see because it reflects sunlight and because it is long, several
million kilometers (several degrees of the sky).
The dust tail is often curved because the comet is moving in its orbit at the same speed that the dust is moving away, much as water curves away from the nozzle of a moving hose.
Comets often have a second tail called an
ion tail (also called the plasma or gas tail). The ion tail is made of
electrically charged gas molecules (carbon dioxide, nitrogen, water) that are
pushed away from the nucleus by the solar wind. Sometimes, the gas tail
disappears and later reappears when the comet crosses a boundary where
direction of the sun's magnetic field is reversed.
Comets Can Break Apart
Comet Shoemaker-Levy 9 was broken into 20 pieces, each of which collided with Jupiter in one of the most spectacular examples of interplanetary impacts in recorded history.
Recently, comet LINEAR was also broken into
fragments by the sun's gravity as it passed the sun.
NASA's
Stardust Mission
The spacecraft will fly into the comet's
tail and catch particles in a gel called aerogel, which is mounted on the
panels of the spacecraft.
Once captured, the particles will be
returned to Earth in 2006. By studying the particles, scientists hope to learn
more about comets and the composition of the early solar system.
Observing
Comets
Many comets are actually discovered by
amateur astronomers. To look for comets, here are things to keep in mind:
Go to a place where there are few lights.
o Learn what a comet
looks like (observe as many comets as you can) and what a comet does not look
like (observe other deep sky objects because they also appear as small fuzzy
objects).
o Use binoculars or a
telescope (low magnification, 20-40x).
o Look toward the
east about 30 minutes before sunrise or to the west about 20 minutes after
sunset because comets are often spotted by their tails.
o Sweep the sky
slowly near the horizon.
Comets will appear as small, fuzzy objects.
This type of observing takes discipline, long hours and patience.
On average, comet hunters spend several
hundred hours of observing time to find a new comet.
However, comets are named after their
discoverers, so many people think it is worth the effort.
For a discussion of comet hunting, consult
The Sky: a User's Guide by David H. Levy, who has discovered several comets
including comet Shoemaker-Levy 9 that hit Jupiter.
Comet
FAQ
What
is the most famous comet in history?
Many consider 1P/Halley to be the most
famous comet in history. According to NASA, because of Halley, astronomers
determined that comets make repeat trips around the sun.
What
are the five parts of a comet?
A comet is made up of five parts: the
nucleus, the coma, the hydrogen envelope, the ion tail, and the dust tail.
Which
is the biggest comet?
According to the Royal Astronomical
Society, Comet C/2006 P1, also known as McNaught, is the biggest comet that
astronomers have encountered. It took the Ulysses space probe 18 days to
traverse the solar wind surrounding the comet.
What
does a comet do?
Comets are thought to orbit the sun in
either the Oort cloud or Kuiper belt. When another star passes by the solar
system, its gravity pushes the Oort cloud and/or Kuiper belt. This causes
comets to descend toward the sun in a highly elliptical orbit with the sun at
one focus of the ellipse.
How
many comets are there?
According to NASA, there are 3,701 known comets.
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