Few astronomical events have been more mysterious than the appearance
in the sky of comets. What causes these fast-moving objects to suddenly appear
in the sky? Are they omens of future disaster? Does the tail hold deadly disease?
Why do they have a tail and yet appear to barely move during an entire night?
Comets are among the most exciting of astronomical sights, and anyone who was
privileged to see Hyakutake, Hale-Bopp, or Halley (below left to right) will
never forget the thrill.
Above and left is my photograph of Comet McNaught taken on Jaunary
12, 2007 from the bridge going to Wayzata High School. Avove and right is the
same comet a few days later, taken from Australia. Northern hemisphere people
were not privileged to this view. Look up the APOD
site for image comments and the copyright notice.
The image below shows Comet Holmes as it appeared in the sky
October 27, 2007. This comet obrits out near Jupiter and is extremely dim, glowing
at a magnitude of 17. On October 23, it erupted suddenly and brightened to a
naked-eye visible object at magnitude 2.5! This is a change in brightness of
nearly one million times. By clicking on the image, you can watch the brightening
happen. This comet, as of October 27, had grown a coma that went from nothing
to the size of planet Juipiter in just four days! I do not know how long either
of the links will last, but at least the pictures are in here for your enjoyment,
and to let you know that we have seen two naked-eye comets in just the past
10 months! The other two images are mine, taken from the front yard of our home
in Plymouth on October 31,2007. The image below and left of the Jupiter:Holmes
comparison is a view of the comet in the sky, relative to a few stars in Perseus,
and the bottom image is a close-up through a 10" Meade Schmidt-Cassegrain
telescope. It was a really great comet.
Unlike the other
small bodies in the solar system, comets have been known since antiquity. There
are Chinese records of Comet Halley going back to at least 240 BC. The famous
Bayeux Tapestry (left), which commemorates the Norman Conquest of England in
1066, depicts an apparition of Comet Halley (above).
As of 1995, 878 comets have been cataloged and their orbits at least roughly
calculated. Of these 184 are periodic comets (orbital periods less than 200
years); some of the remainder are no doubt periodic as well, but their orbits
have not been determined with sufficient accuracy to tell for sure.
Comets are sometimes called dirty snowballs or "icy mudballs".
They are a mixture of ices (both water and frozen gases) and dust that for some
reason didn't get incorporated into planets when the solar system was formed.
This makes them very interesting as samples of the early history of the solar
When they are near the Sun and active, comets have several distinct parts:
relatively solid and stable, mostly ices (water and carbon dioxide), with a
small amount of dust and other solids;
Coma: dense cloud of water vapor, carbon dioxide gas, and other neutral gases
sublimed from the nucleus;
Hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of
Dust tail: up to 10 million km long composed of smoke-sized dust particles driven
off the nucleus by escaping gases; this is the most prominent part of a comet
to the unaided eye;
Ion tail: as much as several hundred million km long composed of plasma and
laced with rays and streamers caused by interactions with the solar wind.
Comets are invisible except when they are near the Sun. Most comets have highly
eccentric orbits which take them far beyond the orbit of Pluto; these are seen
once and then disappear for millennia. Only the short- and intermediate-period
comets (like Comet Halley), stay within the orbit of Pluto for a significant
fraction of their orbits.
In 1986, several satellites were launched toward Comet Halley. The French
satellite Giotto, got within 600 km of Halley's nucleus. To everyone's surprise,
the nucleus was extremely dark, almost as black as coal. The two images below
are from that satellite. Of interest in the pictures is the process of sublimation
and the venting of gas. The comet orbits the sun at a great distance, and when
it is far away, the comet is cold and unactive. As it approaches the Sun, the
comet material does not melt, but sublimates. This is a process when solid material
turns directly into gas. The venting of the gas is often explosive in nature,
perhaps not quite as much as depicted in the movie "Armageddon," but
enough for the power to cause the comet nucleus to spin, and thus spray the
gas and dust all around. If you think these images of the comet nucleus are
interesting, then try to make
a comet nucleus from household items and some dry ice. It is a really cool
artistic rendering from the University of Michigan website shows a close-up
idea of the venting of comet jets of gas and dust. It is this venting that was
demonstrated in the movie, "Deep Impact" and resulted in one astronaut
being blasted into space. The venting will often cause the comet to tumble wildly
as it nears the sun.
After 500 or so passes near the Sun off most of a comet's ice and gas is lost
leaving a rocky object very much like an asteroid in appearance. (Perhaps half
of the near-Earth asteroids may be "dead" comets.) A comet whose orbit
takes it near the Sun is also likely to either impact one of the planets or
the Sun or to be ejected out of the solar system by a close encounter (esp.
as the comet approaches the Sun, it encounters the solar wind (above). The comet
nucleus is so small in size and mass that its gravity is minimal. You could
literally jump into orbit! The hydroge, carbon-dioxide, and dust from the comet
almost reaches escape velocity, but is held loosely by the weak gravity of the
nucleus. The resulting comet coma can become very large. The coma of Hale-Bopp
was thought to reach a diameter of over 1 million km. This is why we can see
the comet at night. The coma is large enough that Hale-Bopp in 1998 and Hyakutake
in 1996 were naked-eye visible.
The comet may be speeding toward the Sun anywhere from 50,000-100,000 km/hr,
but the solar wind velocity is well over 1 million km/hr. The solar wind blows
the coma gases and dust away from the nucleus, and opposite the Sun. Whether
the comet is inbound or outbound, the solar wind velocity is always greater
than the comet nucleus velocity, so the tail always points away from the Sun,
as seen in the image to your left.
The length of the the comet tail depends on the proximity to the Sun and to
the comet's velocity. Some tails are only a few million kilometers, while Hyakutake
boasted a tail of over 110 million km!
Comet Wild was imaged by the NASA space satellite "Stardust" on
January 2, 2004, when, at its closest approach was only a few hundred kilometers
from its ancient surface. The purpose of the mission was to gather dust grains
from this comet's tail and return them to Earth. Astronomers were surprised
by the craters, cliffs, and barn-sized boulders on this comet's surface.
Astronomers classify comets into two groups, depending on the length of their
orbits. Short-period comets have visitations of 200 years or less. Long-period
comets have orbits that are greater than 200 years. Short-period comets will
not "live" long due to the repeated subliming of their material on
their regular visits to the inner Solar System. But, they live much longer than
you might think. For instance, if you take a snowball made in the winter and
put it in your freezer, you are likely to find much of it in summer. If you
hang it from the ceiling at room temeperature, it will take almost 45 minutes
to an hour to melt. And it is only the size of your fist. Image a comet whose
nucleus is 10-25 km in diameter. It will take along time for a literal mountain
of ice to melt, so they will live for a long time.
Comets are believed to come from two distant places in the Solar System. The
two images above show the location of the Kupier Belt (left) and the Oort Cloud
(right). Both are named after the astronomers who proposed the existence of
these places, Gerald Kupier and Jan Oort. The Kuiper Belt is an orbitally flattened
region out beyond Neptune and near the orbit of Pluto. Kupier Objects are being
detected at great distances corresponding to the theoretical region, so it may
indeed exist. Way beyond the Kuiper Belt is the spherical Oort Cloud. This is
proposed to be a vast region occupied by small frozen chunks. They are barely
held by the Sun's gravity. It is believed that a passing star may influence
some of these chunks from time to time, causing them to suddenly "fall"
toward the Sun.
For some more detailed notes on comets, please move to Comet
By far the most famous comet is Comet Halley. Below are two images of this
famous comet. To the left is one taken in 1910, and the right from 1986. In
1705 Edmond Halley predicted, using Newton's newly formulated laws of motion,
that the comet seen in 1531, 1607, and 1682 would return in 1758 (which was,
alas, after his death). The comet did indeed return as predicted and was later
named in his honor.
The average period of Halley's orbit is 76 years but you cannot calculate
the dates of its reappearances by simply subtracting multiples of 76 years from
1986. The gravitational pull of the major planets alters the orbital period
from revolution to revolution. Nongravitational effects (such as the reaction
from gasses boiled off during its passage near the Sun) also play an important,
but smaller, role in altering the orbit. Between the years 239 BC and 1986 AD
the orbital period has varied from 76.0 years (in 1986) to 79.3 years (in 451
and 1066). The closest perihelion passage to the time of Jesus are 11 BC and
66 AD; neither event took place in Jesus' lifetime. It's most famous appearance
was in 1066 when it was seen at the Battle of Hastings, an event commemorated
in the Bayeux Tapestry.
It's next perihelion passage will be in 2061.
Comet Orbit is retrograde and inclined 18 degrees to the ecliptic. And,
like all comets, highly eccentric. For a complete History
of Comet Halley visitations to Earth, check out the website.
Many comets are first discovered by amateur astronomers. Since comets are
brightest when near the Sun, they are usually visible only at sunrise or sunset.
Charts showing the positions in the sky of some comets can be created with a
planetarium program such as Starry Night.
This image of comet West was taken by John Laborde at the Tierra Del Sol Observatory
site in San Diego County. The exposure was 30 minutes with a 135 mm Nikon lens.
(Courtesy John Laborde)
This image was not credited in my websearch, but shows the unusually tight
gas jet. Comet Hyakutake holds the record for the longest tail, at 110 million
This image of comet Hale-Bopp was taken by John Laborde with his home designed
and built, 8.8" f/3.7 Wright Schmidt Camera. The picture was taken at the
Tierra Del Sol Observatory site in San Diego County with a 25 minute exposure
on Kodak PPF400 film. (Courtesy John Laborde)
Comet Hale-Bopp has the largest nucleus ever recorded ... a diameter of 40
km. This comet was simutaneously discovered in the summer of 1996 by Alan Hale
(not the skipper from Gilligan's Island) and Thomas Bopp. They called in their
discovery to Cambridge in Mass., but there were no available large telescopes
available that evening for confirmation. My Uncle Bill Albrecht was contacted
by the Mauna Kea astronomers and asked to look for it with his telescope. He
found the comet the next night and confirmed its existence.
When the comet finally reached its perihelion in 1998, it was bright enough
to be seen with the naked eye and was dubbed "Comet of the Century."
Members of a cult religion in California, "Heaven's Gate," committed
mass suicide in the summer of 1998, believing an alien spacecraft was in the
coma of the comet, coming to take them to their afterlives.
In the summer of 1999, I traveled to New Mexico for an Astronomy conference
and had the opportunity to meet Thomas Bopp.
The view (below) from the Ulysses spacecraft as Comet Hyakutake's ion tail
rushes towards it, at a point 560 million kilometers from the Sun and 550 million
kilometers from the Earth. Travelling at up to 750 kilometers per second, the
tail has taken 8 days to journey from the comet's head to Ulysses, carried by
the solar wind flowing from the Sun. The tail, composed of gases released from
the comet's icy nucleus, can be seen stretching for over 570 million kilometers.
The planets Mercury, Venus and Earth can be seen to the right of the Sun.
Image courtesy of the Particle Physics and Astronomy Research Council.
Artist's impression by David A. Har
A "shooting star" is really ot a star at all, but a piece of space
debris that is falling through the Earth's atmosphere at such a high velocity
that friction causes the debris to become glowing hot. The piece of hot debris
then vaporizes and its trip through space ends. While it is glowing, stargazers
will see the streaking flash of light called a "meteor trail." The
great majority of random meteors are pieces of debris about the size of a grain
of sand, and are rocky in composition. To learn about basic meteors,
please go to their site.
When the orbit of Earth passes through the tail of a comet, or the orbital
path of a comet, lots and lots of comet pieces will suddenly fall into the Earth's
atmosphere and create light shows called "meteor showers," or even
the more spectacular "meteor storms." These events are the subject
of the next section of this page, and are included in the comet page because
showers and storms of meteors are related to comets.
A meteor shower sometimes occur
when the Earth passes thru the orbit of a comet. While the subject of meteors
is associated with the Asteroids Page, a meteor shower is related to comets
and is a predictable event. As seen in the image below, when the Earth passes
though the debris trail of a regular short-period comet's orbit, that debris
will rain down on the Earth, generating some spectular displays. Some occur
with great regularity: the Perseid meteor shower occurs every year between August
9 and 13 when the Earth passes thru the orbit of Comet Swift-Tuttle. Comet Halley
is the source of the Orionid shower in October. The Leonids occur in November.
Now, if the conditions are just right, an unusually large number of meteors
will strike the atmosphere, giving rise to a Meteor Storm, and these are the
most spectacular. The most recent event took place November 4, 2001, and I enjoyed
watching over 300 in just one hour from my backyard in Plymouth, MN. We camped
as a family between the garages of our house and that of my neighbor. While
it was cloudy for most of the night, we continued to peek out of the tent in
the hopes of clearing. Finally, around 3:00 AM, the cloud cover broke for a
little bit, revealing some clear areas above. We saw 10-12 meteors in about
15 minutes before it clouded over again. I reawoke at 4:30 AM, and the sky was
brilliantly clear. My wife and daughters did not want to look, but I stuck by
head out in awe at the sight of many meteors. I saw two at a time on several
occasions, some running parallel and others going opposite of each other. At
one time I even saw three meteros simultaneously. It was so fantastic. I admit
that the Perseid Meteor Shower of August, 2000 was cool because of the brilliant
Northern Lights, but to see so many meteors in the morning of November 4, 2002
was something I may never witness again.
Below are three images of famous meteor showers. To the right is an etching
of the famous Leonid storm in 1833 when upwards of 100,000 meteors per hour
were visible. The middle image is from the 1995 Lyrid meteor shower Aprin 22-23,
1995 and shows how a shower looks if the radiant (point of Earth entry into
the debris tail) is overhead. To the left is an image of the August 9-12, 1996
Perseid shower. Please check out the article from the
November 2002 Astronomy Magazine regarding the 2002
potential Meteor Storm.
If you think that meteor showers or meteor storms are cool, imagine what it
would be like if a large chunk of comet, or perhaps even the entire comet itself
were to strike the Earth. This is an event that few even considered a possibility
just 10 years ago, but now has become a potential disaster worth preparing for.
Please move over to a webpage that looks at Impact
possibilities from large space objects.
Some other interesting websites are listed below for your perusal:
LINEAR - This is a page for the
Lincoln Near Earth Asteroid Research, that is a project to telescopically find
potentially dangerous rocks in space that may cross the Earth's orbit and strike
our planet. LINEAR is responsible for the discovery of many comets annually.
Stardust Mission A spacecraft,
depicted above, is designed to fly into a comet tail during Jannary, 2004, collect
pieces of primordial solar system material in aerogel, and return the stuff
to Earth for analysis. I am sure that Green Peace and other environmental groups
will panic if we bring foreign material from deep space to Earth and risk exposing
the human population to some dormant virus.
Small Comets This site
proposes an idea that small, house-sized comets are "raining" down
on the Earth all the time, and may be the source of our oceans.
You have finished looking at the coldest and most distant objects in the Solar
System. Time now to come back toward Earth for the final
sets of objects that occupy the attention of our Sun ... The
Asteroids. Or, you can return to Ice
World Introduction, or the Planet
Introduction, or the Syllabus.
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