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Comets 1
Appearance
The word comet is derived from the Greek word kometes for “long-haired”, suggested by
the appearance of a comet’s tails. Although hundreds of comets pass through the inner
solar system every year, very few are noticed by the general public. About every decade
or so, a comet will become bright enough to be noticed by a casual observer – such
comets are often designated Great Comets. Predicting whether a comet will become a
great comet is notoriously difficult, as many factors may cause a comet’s brightness to
depart drastically from predictions. The late 20th century saw a lengthy gap without the
appearance of any great comets, followed by the arrival of two in quick succession –
Comet Hyakutake in 1996, followed by Hale-Bopp 2. The first great comet of the 21st
century was Comet McNaught 3, which became visible to naked eye observers in January
2007. It was the brightest in over 40 years. Comets move slowly with respect to the
background stars from night to night, and they have a “fuzzy” appearance. The actual
nature of what is being seen in the sky is reflected sunlight and fluorescing gases.
Nomenclature
Prior to the early 20th century, most comets were simply referred to by the year in which
they appeared, sometimes with additional adjectives for particularly bright comets. For
example, the “Great Comet of 1680” (Comet Kirch) or the “Great September Comet of
1882”. In the early 20th century, the convention of naming comets after their discoverers
became common. In recent years, many comets have been discovered by instruments
operated by large teams of astronomers, and in this case, comets may be named for the
instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by
the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the
past, when multiple comets were discovered by the same individual or a team of
individuals, the comets’ names were distinguished by adding a number to the
discoverers’ names. For example, Comet Shoemaker-Levy 1-9. However, the large
numbers of comets discovered today by instruments has rendered this system impractical
(in August 2005, SOHO discovered its 1000th comet), and no attempt is made to ensure
that each comet has a unique name. In 1994, the International Astronomical Union
approved a new naming system. Comets are now designated by the year of their
discovery followed by an alphanumeric code. Prefixes are also added to indicate the
nature of the comet. For example, P/ indicating a periodic comet or C/ indicating a nonperiodic comet. After their second observed perihelion passage, periodic comets are also
assigned a number indicating the order of their discovery. So Comet Halley, the first
comet to be identified as periodic, has the designation 1P/1682 Q1.
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History
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Bad Omens 4 – Comets have been known since antiquity. Written records of comets
go back to at least 1140 B.C. in the Middle East and nearly as far back in China.
Perhaps because the times of their appearances were unpredictable and their forms
varied, comets have always been regarded with apprehension. Often the arrival of a
comet was believed to herald some remarkable event on Earth, usually unfavorable.
For example, Comet Halley was associated with the fall of Jerusalem in 70 A.D., the
defeat of Attila in 451 A.D., and the conquest of England in 1066 A.D., memorialized
on the Bayeux tapestry 5.
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Comet Halley In 1910 6 – Astronomers calculated that the Earth would actually
passed through the tail of Comet Halley during its 1910 appearance. An earlier
study of Comet Morehouse showed that the tails of comets contain cyanogen gas,
which is related to cyanide poison. The media picked up this fact, and despite the
pleas of astronomers, wove sensational tales of mass cyanide poisoning engulfing
the planet. Less than honest entrepreneurs did a brisk business selling gas masks,
and “anti-comet pills” for protection. In reality, the cyanogen gas in a comet’s tail
is so diffuse that there was no chance of it getting into the air and reaching
anywhere near the levels necessary to injure anyone.
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Heaven’s Gate – Heaven’s Gate was the name of a cult co-led by Marshall
Applewhite and Bonnie Nettles (until her death). The cult’s end, coinciding with
the appearance of Comet Hale-Bopp, created a sensation in the United States in
1997. Applewhite convinced 39 followers to commit suicide so that their souls
could take a ride on a spaceship that they believed was hiding behind the comet.
Members of the cult reportedly believed themselves to be extraterrestrials.
Scientific Study – The scientific study of comets can be traced back two thousand
years to the Greeks and Romans. Because of their belief that the celestial realm was
unchanging, most of the classical philosophers thought that comets were located in
the atmosphere, and efforts were made to use them to predict the weather. As late as
the 16th century, the scientists of renaissance Europe were arguing over whether
comets were astronomical or meteorological phenomena.
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Tycho Brahe – A turning point in cometary studies came with the great comet of
1577, which was extensively studied by Tycho Brahe. He carefully measured the
position of the comet against the background of stars with the change in viewing
angle provided by the Earth’s rotation (diurnal parallax). Brahe concluded that
the comet was not within the atmosphere of the Earth, but actually further away
than the Moon.
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Edmund Halley – Edmund Halley (a contemporary of Isaac Newton) was the first
to confirm that cometary orbits could be closed ellipses, with a given comet
reappearing at regular intervals. They are referred to as periodic comets. He
reached this conclusion from a study of the great comets of 1531, 1607, and 1682,
all of which had similar orbits. Halley concluded that these were successive
appearances of the same comet, which he predicted would return in 1758.
Although Halley did not live to see his calculations verified, the comet returned
just as he expected, and it was quickly hailed as Halley’s Comet.
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Fred L. Whipple – In 1950, the American astronomer Fred L. Whipple proposed
the “icy conglomerate” hypothesis, later referred to as the “dirty snowball” model.
According to Whipple’s theory, the center (nucleus) of a comet is a lumpy piece
of icy conglomerate wherein silicate dust is cemented by a large amount of ice
(not only water-ice but also ices of more volatile molecules). Moreover, the
amount of ice must be substantial enough to sustain the vaporizations for a large
number of revolutions. Whipple noted that the nuclei of some comets are solid
enough to graze the Sun without experiencing total destruction.
Orbits
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Orbital Classification – There are two primary classifications of comets based of their
orbital periods:
1.
Long-Period Comets – Comets with orbital periods 200 years or greater (often
many thousands of years) are called long-period comets. Most long-period
comets have extremely eccentric orbits, falling in toward the Sun from a great
distance and returning again to the depths of space. Of the comets discovered,
approximately 90% are long-period comets.
2.
Short-Period Comets – Comets with orbital periods shorter than 200 years are
called short-period comets. Their orbits typically take them out to the region of
the outer planets (Jupiter and beyond) at aphelion. Short-period comets are
further divided into the Jupiter family (periods less than 20 years) and Halley
family (periods between 20 and 200 years).
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Comet Halley 7 – Halley’s Comet has an orbital period of 75.3 years. It last
appeared in 1986 and will next appear in 2061.
Orbital Inclinations and Directions – Another key difference between the two
classifications is that long-period comets tend to enter the solar system randomly
from all directions 8. In contrast, most short-period comets have orbits inclined no
more than about 40º to the ecliptic plane (the plane of the Earth’s orbit). As a result,
short-period comets travel around the Sun in the same direction that the Earth and
other planets do, whereas many long-period comets move in the opposite direction
(their inclinations exceed 90º). This difference in the inclinations of long-period and
short-period comets provides an important clue to their different source regions.
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Orbital Changes – The orbit of a comet can be affected in two ways:
1. Perturbation – A comet may pass near enough to some planet (usually Jupiter)
that the planet’s gravity causes a change in the comet’s orbit. This change can
either increase its energy, ejecting the comet completely from the solar system, or
cause it to loose energy, making it a short-period comet 9. The reason most shortperiod comets have aphelia near the orbit of Jupiter is that they were perturbed by
the gravitation of this massive planet.
2. Rocket Effect – Evaporation of ice takes place primarily in jets 10 on the warmest
part of a comet, which corresponds to the “afternoon” side facing the Sun. As the
gas and dust stream away at very high speeds, the reaction creates a force in the
opposite direction, similar to a rocket engine. Acting continuously over a period
of weeks, such a force can have a measurable affect on a comet’s orbit.
Physical Characteristics
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General Description 11 – Comets are the primitive, leftover building blocks of the
outer solar system formation process. Only when they are diverted into the inner
solar system do we see these small bodies and identify them as comets. The visible
part of a comet is its atmosphere, a term used to include both the coma and the tail.
The source of this atmosphere is referred to as the nucleus. The coma and the nucleus
together constitute the head of the comet.
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Nucleus – The center of a comet’s head is called its nucleus. However, telescopic
observations cannot see the nucleus directly, since it is relatively small and is lost
in the glare of the head of an active comet.
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Spacecraft Exploration – In recent years, several spacecraft directly observed
the nuclei of certain comets. In 1986, the European Space Agency’s Giotto
probe and the Soviet Union’s Vega 1 and Vega 2 photograph the nucleus of
Comet Halley 12. In 2002, NASA’s Deep Space 1 probe obtained highresolution images of the surface of Comet Borrelly 13. In 2005, NASA’s Deep
Impact probe blasted a crater on Comet Tempel 1 to study its interior 14. In
2014, European Space Agency’s Rosetta spacecraft went into orbit around
Comet Churyumov–Gerasimenko 15. Followed by Rosetta’s lander, Philae,
setting down on its surface, becoming the first probe to land on a comet
nucleus.
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Composition – The data obtained by spacecraft confirmed the basic aspects of
Whipple’s model of the nature of cometary nuclei. They are composed of a
mixture of rock, dust, water ice, and frozen gases such as carbon dioxide,
carbon monoxide, methane, and ammonia. In addition, the nuclei contain a
variety of organic compounds. The surfaces of comet nuclei are very dark.
Solar heating drives off volatile compounds (ice), leaving behind the organic
compounds that tend to be dark, like tar or crude oil.
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Coma 17 – When a comet is within 3 to 5 AU of the Sun, ultraviolet sunlight
causes its icy matrix to sublimate vigorously, creating a spherical envelope of gas
and dust (50,000 miles or more in diameter) around the nucleus. This is called the
coma of the comet. The coma is illuminated by the dust reflecting sunlight and
the gas glowing due to fluorescence (see below). The inner portion of the coma is
the brightest part of a comet. Sometimes this inner coma is symmetric about the
nucleus, but more often it is brightest in the direction of the Sun. Spectroscopic
analysis of inner comae indicates the existence of complex organic molecules. A
number of these molecules are thought to be the building blocks of life. In fact,
some scientists believe that the organic material raining down on the primeval
Earth from comet infall may have led to the origin of life on our planet.
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Structure – Comet nuclei range from few hundred feet to tens of miles across.
Because of their low mass, comet nuclei do not become spherical under their
own gravity and therefore have irregular shapes. Uneven heating can cause
newly generated gases to break out of weak spots on the surface of comet’s
nucleus, like a geyser. These streams of gas and dust can cause the nucleus to
spin 16, and even split apart.
NASA’s Stardust spacecraft (launched in February 1999) collected particles
from the coma of Comet Wild 2 in January 2004, and returned the samples to
Earth in a capsule in January 2006.
Tails – As the comet moves still closer to the Sun (about 2 AU), it comes into the
sphere of influence of two solar forces, solar wind and radiation pressure. These
forces push material away from the coma to form a comet’s tails 18. For this
reason, the tails always point away from the Sun and may actually precede the
comet, depending upon its position in its orbit 19.
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Plasma Tail 20 – The tenuous gases that make up the comet’s atmosphere are
not gravitationally bound to the nucleus. As they expand, these gases are
ionized, that is, they lose one or more electrons. When the solar wind – a
stream of charged particles (mostly protons and electrons) ejected from the
Sun – comes in contact with these ionized gases in the coma of a comet, they
are accelerated away from the comet’s head forming the plasma tail. Plasma
tails are straight, extend tens of millions of miles, and are primarily observed
by the blue glow of ionized carbon monoxide stimulated to fluorescence by
absorption of ultraviolet light from the Sun.
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Fluorescence – The emission of light (and other radiation) by an object
after it has absorbed radiation of a different wavelength, usually ultraviolet
light. Specifically, the energy source “kicks” an electron of an atom from
a lower energy state into a higher energy state, then the electron releases
the energy in the form of light when it falls back to a lower energy state.
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Dust tail 20 – Most comets have two tails, the plasma tail and the dust tail.
Radiation pressure – the pressure exerted by the electromagnetic radiation
(light) emitted by the Sun – is extremely effective in pushing small particulate
matter (dust) out of the coma. Although generally shorter than plasma tails
(usually less than 5 million miles long), dust tails can be as bright or even
brighter. Reflecting sunlight, they appear yellow-white in color and are
curved.
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Tail Orientation – Dust tails are curved because radiation pressure pushes
solid grains into higher orbits, with lower velocities and longer periods. In
these new orbits, they lag behind the head of the comet. This is in contrast to
the ions in plasma tails, which are low enough in mass that gravitational
forces on them are negligible compared to the solar wind.
Hydrogen Envelope – As a comet absorbs ultraviolet light, chemical processes
release hydrogen (photochemistry), which escapes the comet’s gravity and forms
a huge hydrogen envelope, spanning millions of miles. This envelope cannot be
seen from Earth because its light is absorbed by Earth’s atmosphere, but it has
been detected by spacecraft.
Origin
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Oort Cloud 21 – The first satisfactory theory for the origin of comets was proposed by
the Dutch astronomer Jan Oort in 1950. He noted that in all cases where the orbits of
long-period comets had been carefully determined, the aphelion distances measured
about 50,000 AU. Oort hypothesized the existence of a “cloud” of comets associated
with the Sun, but very far beyond the known planets. Since the orbits of long-period
comets are not confined to the ecliptic but show a full range of inclinations, Oort
concluded that this cloud was roughly spherical. This distant reservoir of comet
nuclei is now called the Oort cloud. At the distance of the Oort cloud (now estimated
to extend 3 light years from the Sun) cometary nuclei are so far from the Sun they
cannot be observed, and their ices are preserved indefinitely in the cold of outer
space. However, periodically, they are perturbed by the passage of a nearby star 22 or
by the galactic tide (the tidal force exerted by the Milky Way galaxy). At these times,
some of their orbits are changed so that they fall towards the inner solar system. Only
then will they develop the coma and tails associated with comets. To account for the
several new comets discovered each year, Oort calculated that the cloud contained
about 100 billion comets. More recent calculations suggest that the actual number is
ten times greater (1 trillion comets).
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Formation – The Oort cloud is thought to be a remnant of the original nebula that
collapsed to form the Sun and planets about 4.6 billion years ago. The most
widely-accepted hypothesis of its formation is that the Oort cloud’s objects
initially formed much closer to the Sun as part of the same process that formed
the planets and asteroids, but the gravitational interaction with the young Jovian
planets ejected them into extremely long elliptical orbits. This process also
served to scatter the objects out of the ecliptic plane, explaining the cloud’s
spherical distribution. While on the distant outer regions of these orbits,
gravitational interaction with nearby stars and the galactic tide further modified
their orbits to make them more circular. However, recent research suggests that a
large number of Oort cloud objects are the product of an exchange of materials
between the Sun and its sibling stars as they formed and drifted apart, and it is
suggested that many (possibly the majority) of Oort cloud objects were not
formed in close proximity to the Sun. Furthermore, computer simulations
indicate that the Sun captured comets from other stars while it was in its “birth
cluster”, implying that a substantial fraction of the Oort cloud comets (perhaps
exceeding 90%) are from the protoplanetary discs of other stars.
Scattered Disk – Because of its unstable nature, astronomers now consider the
scattered disc to be the place of origin for most short-period comets observed in the
Solar System, with the centaurs being the intermediate stage in an object’s migration
from the disc to the inner Solar System. Eventually, perturbations from the Jovian
planets send such objects towards the Sun, transforming them into short-period
comets. While the nearest distance to the Sun approached by scattered objects is
about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered
objects among the most distant and cold objects in the Solar System. The innermost
portion of the scattered disc overlaps the Kuiper belt, but its outer limits reach much
farther away from the Sun and farther above and below the ecliptic than the Kuiper
belt.
Fate
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Comets are transient by nature; they come and go. Most comets that venture into the
inner solar system make but a single pass before returning to the Oort cloud. Those
that linger in short-period orbits have varying fates (see below). Comets that do not
go violently suffer a slow decline into oblivion. Sooner or later, all the ices sublimate
and are lost to space, leaving behind a mass of dust. If the comet initially had a rock
core, the end result becomes a member of an extinct-comet class of asteroids. In fact,
it is estimated that roughly one-third to one-half of all asteroids that cross or approach
Earth’s orbit are extinct comets.
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Ejected – Comets can be gravitationally ejected out of the solar system after
passing near a massive planet.
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Breakup – Comets are known to break up into fragments, as happened with
Comet 73P/Schwassmann-Wachmann 3 starting in 1995 23. This breakup may be
triggered by tidal gravitational forces from the Sun or a large planet, by an
explosion of volatile material, impact, or for other reasons not yet understood.
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Collisions – A comet can crash into a planet or satellite, or pass so close to the
Sun 24 that it vanishes in a cloud of dust.
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Comet Shoemaker-Levy 9 25 – Torn apart by Jupiter’s tidal forces during a
close encounter in 1992, Comet Shoemaker-Levy 9 collided with the planet in
1994. At least 20 fragments (the largest measuring over 1 mile in diameter)
impacted the planet at about 40 miles per second. Some of these fragments
created large fireballs that rose thousands of miles high and left giant dark
marks in the atmosphere that lasted for months after the collision 26. The
presence of craters on the surfaces of planets and satellites in the outer solar
system is evidence that comets sometimes impact planets, but never before
has such a collision been witnessed.
Exocomets
Exocomets (or extrasolar comets) are comets that reside outside of our solar system.
They include comets that orbit stars other than the Sun and interstellar comets (located in
interstellar space, and therefore not gravitationally bound to a star). The first exocomets
were detected in 1987 around the star, Beta Pictoris. There are now a total of 11 stars
around which the presence of exocomets have been observed or suspected. However,
interstellar comets are only speculated to exist.
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