A
comet is a
small solar system body that has a
coma and/or a
tail and is bigger than a
meteoroid. When close enough to the Sun, a comet
exhibits a visible
coma (fuzzy
"atmosphere"), and sometimes a tail, both because of the effects of
solar radiation upon the
comet's nucleus. Comet nuclei are themselves
loose collections of ice, dust and small rocky particles, ranging
from a
few hundred metres to tens of
kilometres across.
Background
Name and symbol
The word
comet came to the
English language through the
Latin cometes from the
Greek word
komē, meaning "hair of
the head";
Aristotle first used the
derivation
komētēs to depict comets as "stars with hair."
The
astronomical symbol for
comets ( ) accordingly consists of a disc with a hairlike
tail.
Orbits and origin
Comets have a variety of different
orbital periods, ranging from a few years, to
hundreds of thousands of years, while some are believed to pass
only once through the
inner Solar
System before being thrown out into
interstellar space. Short-period comets
are thought to originate in the
Kuiper
Belt, or associated
scattered
disc, which lie beyond the orbit of
Neptune. Long-period comets are believed to
originate in the
Oort cloud, consisting
of debris left over from the
condensation of the
solar nebula, located well beyond the Kuiper
Belt. Comets are thrown from these outer reaches of the Solar
System towards the Sun by gravitational perturbations from the
outer planets (in the case of Kuiper Belt objects) or nearby stars
(in the case of Oort Cloud objects), or as a result of collisions
between objects within these regions.
Comets are distinguished from
asteroids by
the presence of a coma or tail, though very old comets that have
lost all their
volatile
materials may come to resemble asteroids (see
extinct comets). Asteroids are also believed
to have a different origin from comets, having formed in the inner
Solar System rather than the outer Solar System, but recent
findings have somewhat blurred the distinction between asteroids
and comets (see
centaur and
asteroid terminology).
there are a reported 3,648 known comets of which about 1,500 are Kreutz Sungrazers and about 400 are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. The number of comets visible to the naked-eye averages to roughly one per year, though many of these are faint and unspectacular. When a historically bright or notable naked-eye comet is witnessed by many people, it may be termed a Great Comet.
Physical characteristics
Nucleus
Comet nuclei are known to range from about 100 metres to more than
40 kilometres across. They are composed of
rock,
dust,
water ice, and frozen gases such as
carbon monoxide,
carbon dioxide,
methane and
ammonia. Because
of their low mass, comet nuclei do not
become spherical under their own
gravity, and thus have irregular
shapes.
They are often popularly described as "dirty snowballs", though
recent observations have revealed dry dusty or rocky surfaces,
suggesting that the ices are hidden beneath the
crust (see
Debate over comet
composition). Comets also contain a variety of
organic compounds; in addition to the gases
already mentioned, these may include
methanol,
hydrogen
cyanide,
formaldehyde,
ethanol and
ethane, and
perhaps more complex molecules such as long-chain
hydrocarbons and
amino
acids. In 2009, it was confirmed that the amino acid
glycine had been found in the comet dust recovered
by NASA's
Stardust
mission.
Surprisingly, cometary nuclei are among the
darkest objects known to exist in the solar system. The
Giotto probe found that
Comet Halley's nucleus reflects approximately
4% of the light that falls on it, and
Deep
Space 1 discovered that
Comet
Borrelly's surface reflects 2.4–3.0% of the light that falls on
it; by comparison,
asphalt reflects 7% of
the light that falls on it. It is thought that complex organic
compounds are the dark surface material. Solar heating drives off
volatile compounds leaving behind heavy long-chain organics that
tend to be very dark, like
tar or
crude oil. The very darkness of cometary surfaces
allows them to absorb the heat necessary to drive their
outgassing.
Coma and tail
In the
outer solar system, comets
remain frozen and are extremely difficult or impossible to detect
from Earth due to their small size. Statistical detections of
inactive comet nuclei in the
Kuiper belt
have been reported from the
Hubble Space Telescope observations,
but these detections have been questioned, and have not yet been
independently confirmed. As a comet approaches the
inner solar system,
solar radiation causes the volatile
materials within the comet to vaporize and stream out of the
nucleus, carrying dust away with them. The streams of
dust and gas thus released form a huge,
extremely tenuous atmosphere around the comet called the
coma, and the force exerted
on the coma by the Sun's
radiation
pressure and
solar wind cause an
enormous
tail to form, which points away from the
sun.
Both the coma and tail are illuminated by the Sun and may become
visible from
Earth when a comet passes through
the inner solar system, the dust reflecting sunlight directly and
the gases glowing from
ionisation. Most comets
are too faint to be visible without the aid of a
telescope, but a few each decade become bright
enough to be visible to the naked eye. Occasionally a comet may
experience a huge and sudden outburst of gas and dust, during which
the size of the coma temporarily greatly increases in size. This
happened in 2007 to
Comet Holmes.
The streams of dust and gas each form their own distinct tail,
pointing in slightly different directions. The tail of dust is left
behind in the comet's orbit in such a manner that it often forms a
curved tail called the
antitail. At the
same time, the ion tail, made of gases, always points directly away
from the Sun, as this gas is more strongly affected by the solar
wind than is dust, following magnetic field lines rather than an
orbital trajectory.
Parallax viewing from
the Earth may sometimes mean the tails appear to point in opposite
directions.
While the solid nucleus of comets is generally less than 50 km
across, the coma may be larger than the Sun, and ion tails have
been observed to extend 1
astronomical
unit (150 million km) or more. The observation of antitails
contributed significantly to the discovery of
solar wind. The ion tail is formed as a result of
the
photoelectric effect of
solar ultra-violet radiation acting on particles in the coma. Once
the particles have been ionised, they attain a net positive
electrical charge which in turn gives rise to an "induced
magnetosphere" around the comet. The comet and
its induced magnetic field form an obstacle to outward flowing
solar wind particles. As the relative orbital speed of the comet
and the solar wind is supersonic a
bow
shock is formed upstream of the comet, in the flow direction of
the solar wind. In this bow shock, large concentrations of cometary
ions (called "pick-up ions") congregate and act to "load" the solar
magnetic field with plasma, such that the field lines "drape"
around the comet forming the ion tail.

Comet Encke loses its tail
If the ion tail loading is sufficient, then the magnetic field
lines are squeezed together to the point where, at some distance
along the ion tail,
magnetic
reconnection occurs. This leads to a "tail disconnection
event". This has been observed on a number of occasions, notable
among which was on April 20, 2007 when the ion tail of
comet Encke was completely severed as the comet
passed through a
coronal mass
ejection. This event was observed by the
STEREO spacecraft.
Comets were found to emit
X-rays in 1996.
This surprised researchers, because X-ray emission is usually
associated with very
high-temperature bodies. The X-rays are
thought to be generated by the interaction between comets and the
solar wind: when highly charged
ions fly
through a cometary atmosphere, they collide with cometary atoms and
molecules, "ripping off" one or more electrons from the comet. This
ripping off leads to the emission of X-rays and
far ultraviolet photons.
Connection to meteor showers
As a result of outgassing, comets leave a trail of solid debris
behind them. If the comet's path crosses
Earth's path, then at that point there will likely be
meteor showers as Earth passes through
the trail of debris. The
Perseid meteor
shower occurs every year between August 9 and
August 13, when Earth passes through the orbit of the
Swift–Tuttle comet.
Halley's comet is the source of the
Orionid shower in October.
Orbital characteristics
Most comets have elongated
elliptical
orbits that take them close to the Sun for a part of their
orbit, and then out into the further reaches of the Solar System
for the remainder. Comets are often classified according to the
length of their
orbital period; the
longer the period the more elongated the ellipse.
- Short-period
comets are generally defined as having orbital periods
of less than 200 years. They usually orbit more-or-less in the
ecliptic plane in the same direction as the
planets. Their orbits typically take them out to the region of the
outer planets (Jupiter and beyond) at aphelion; for example, Comet Halley's aphelion is a little way beyond
the orbit of Neptune. At the shorter
extreme, Comet Encke has an orbit which
never places it farther from the Sun than Jupiter. Short-period comets are further divided
into the Jupiter family (periods less than 20
years) and Halley family (periods between 20 and
200 years).
- Long-period comets have highly eccentric orbits and periods ranging
from 200 years to thousands or even millions of years. (However, by
definition they remain gravitationally bound to the Sun; those
comets that are ejected from the solar system due to close passes
by major planets are no longer properly considered as having
"periods".) Their orbits take them far beyond the outer planets at
aphelia, and the plane of their orbits need not lie near the
ecliptic.
- Single-apparition comets are similar to
long-period comets, but have parabolic or slightly-hyperbolic trajectories, when
in inner Solar System. However, when in outer Solar System, their
orbits are highly-eccentric elliptical. Their aphelions lie in the
Oort Cloud. Gravitational perturbations
from giant planets cause their orbits to change. All comets with
parabolic and slightly-hyperbolic orbits belong to the Solar System
and have certain obital periods, generally hundreds of thousand, or
millions of years. Very few comets have escaped the Solar System
due to close approaches to giant planets. No comets with e>>1
were observed, so there were no observed comets that do not belong
to our Solar System.
- Some authorities use the term periodic comet to refer
to any comet with a periodic orbit (that is, all short-period
comets plus all long-period comets), while others use it to mean
exclusively short-period comets. Similarly, although the literal
meaning of non-periodic comet is the same as
single-apparition comet, some use it to mean all comets
that are not "periodic" in the second sense (that is, to also
include all comets with a period greater than 200 years).
- Recently discovered main-belt comets form a distinct
class, orbiting in more circular orbits within the asteroid belt.
Based on their orbital characteristics, short-period comets are
thought to originate from the
centaurs and the
Kuiper belt/
scattered
disk—a disk of objects in the transneptunian region—whereas the
source of long-period comets is thought to be the far more distant
spherical
Oort cloud (after the Dutch
astronomer
Jan Hendrik Oort who
hypothesised its existence). Vast swarms of comet-like bodies are
believed to orbit the Sun in these distant regions in roughly
circular orbits. Occasionally the gravitational influence of the
outer planets (in the case of Kuiper Belt objects) or nearby stars
(in the case of Oort cloud objects) may throw one of these bodies
into an elliptical orbit that takes it inwards towards the
Sun, to form a visible comet. Unlike the return of
periodic comets whose orbits have been established by previous
observations, the appearance of new comets by this mechanism is
unpredictable.
Since their elliptical orbits frequently take them close to the
giant planets, comets are subject to further gravitational
perturbations. Short period comets display a tendency for their
aphelia to coincide with a
giant planet's orbital radius, with the Jupiter
family of comets being the largest, as the
histogram shows. It is clear that comets coming in
from the Oort cloud often have their orbits strongly influenced by
the gravity of giant planets as a result of a close encounter.
Jupiter is the source of the greatest perturbations, being more
than twice as massive as all the other planets combined, in
addition to being the swiftest of the giant planets. These
perturbations may sometimes deflect long-period comets into shorter
orbital periods (
Halley's Comet being
a possible example).
Early observations have revealed a few genuinely hyperbolic (i.e.
non-periodic) trajectories, but no more than could be accounted for
by perturbations from Jupiter. If comets pervaded interstellar
space, they would be moving with velocities of the same order as
the relative velocities of stars near the Sun (a few tens of
kilometres per second). If such objects entered the solar system,
they would have positive total energies, and would be observed to
have genuinely hyperbolic trajectories. A rough calculation shows
that there might be four hyperbolic comets per century, within
Jupiter's orbit, give or take one and perhaps two orders of
magnitude.
A number of periodic comets discovered in earlier decades or
previous centuries are now "lost." Their orbits were never known
well enough to predict future appearances. However, occasionally a
"new" comet will be discovered and upon calculation of its orbit it
turns out to be an old "lost" comet. An example is Comet
11P/Tempel-Swift-LINEAR, discovered
in 1869 but unobservable after 1908 because of perturbations by
Jupiter.
It was not found again until accidentally
rediscovered by LINEAR
in
2001.
The fate of comets
Departure/ejection from Solar System
If a comet is traveling fast enough, it will enter and leave the
solar system; such is the case for most
non-periodic comets. In
addition, comets can be ejected by interacting with another object
in the solar system (see
Perturbation), such as
Jupiter.
Volatiles exhausted
Jupiter family comets (JFC) and long period comets (LPC) (see
"Orbital characteristics", below) appear to follow very different
fading laws. The JFCs are active over a lifetime of about 10,000
years or ~1,000 revolutions while the LPCs disappear much faster.
Only 10% of the LPCs survive more than 50 passages to small
perihelion, while only 1% of them survive more than 2,000 passages.
Eventually most of the volatile material contained in a comet
nucleus evaporates away, and the comet becomes a small, dark, inert
lump of rock or rubble that can resemble an
asteroid.
Breakups/Disintegration
Comets are also known to break up into fragments, as happened with
Comet 73P/Schwassmann-Wachmann
3 starting in 1995.
This breakup may be triggered by tidal gravitational forces from
the Sun or a large planet, by an "explosion" of volatile material,
or for other reasons not fully explained.
Collisions
Some comets meet a more spectacular end—either falling into the
Sun, or smashing into a planet or other body. Collisions between
comets and planets or moons were common in the early Solar System:
some of the many craters on the Earth's
Moon,
for example, may have been caused by comets. A recent collision of
a comet with a planet occurred in July 1994 when
Comet Shoemaker-Levy 9 broke up into
pieces and collided with
Jupiter.
Many comets and asteroids collided into Earth in its early stages.
Many scientists believe that comets bombarding the young Earth
(about 4 billion years ago) brought the vast quantities of water
that now fill the Earth's oceans, or at least a significant
proportion of it. But other researchers have cast doubt on this
theory. The detection of organic molecules in comets has led some
to speculate that comets or
meteorites may
have brought the precursors of life—or even life itself—to Earth.
There are still many near-Earth comets, although a collision with
an asteroid is more likely than with a comet.
It is suspected that comet impacts have, over long timescales, also
delivered significant quantities of water to the Earth's
Moon, some of which may have survived as
lunar ice.
Comet nomenclature
The names given to comets have followed several different
conventions over the past two centuries. Before any systematic
naming convention was adopted, comets were named in a variety of
ways. 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; thus, the
"
Great Comet of 1680" (Kirch's Comet), the
"
Great September Comet of 1882," and the
"
Daylight Comet of
1910" ("Great January Comet of 1910"). After
Edmund Halley demonstrated that the comets of
1531, 1607, and 1682 were the same body and successfully predicted
its return in 1759, that comet became known as
Comet Halley. Similarly, the second and third
known periodic comets,
Comet Encke and
Comet Biela, were named after the
astronomers who calculated their orbits rather than their original
discoverers. Later, periodic comets were usually named after their
discoverers, but comets that had appeared only once continued to be
referred to by the year of their apparition.
In the early 20th century, the convention of naming comets after
their discoverers became common, and this remains so today. A comet
is named after up to three independent discoverers. 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, group of
individuals, or team, the comets' names were distinguished by
adding a numeral to the discoverers' names (but only for periodic
comets); thus Comets
Shoemaker-Levy
1–
9. Today, the large numbers of
comets discovered by some instruments has rendered this system
impractical, and no attempt is made to ensure that each comet has a
unique name. Instead, the comets' systematic designations are used
to avoid confusion.
Until 1994, comets were first given a
provisional designation consisting
of the year of their discovery followed by a lowercase letter
indicating its order of discovery in that year (for example,
Comet 1969i was the 9th comet discovered
in 1969). Once the comet had been observed through perihelion and
its orbit had been established, the comet was given a permanent
designation of the year of its
perihelion, followed by a
Roman numeral indicating its order of
perihelion passage in that year, so that Comet 1969i became
Comet 1970 II (it was the second comet to
pass perihelion in 1970)
Increasing numbers of comet discoveries made this procedure
awkward, and in 1994 the
International Astronomical
Union approved a new naming system. Comets are now designated
by the year of their discovery followed by a letter indicating the
half-month of the discovery and a number indicating the order of
discovery (a system similar to that already used for
asteroids), so that the fourth comet discovered in
the second half of February 2006 would be designated 2006 D4.
Prefixes are also added to indicate the nature of the comet:
- P/ indicates a periodic comet (defined for these purposes as
any comet with an orbital period of less than 200 years or
confirmed observations at more than one perihelion passage);
- C/ indicates a non-periodic comet (defined as any comet that is
not periodic according to the preceding definition);
- X/ indicates a comet for which no reliable orbit could be
calculated (generally, historical comets);
- D/ indicates a comet which has broken up or been lost, referred
to as dark comet;
- A/ indicates an object that was mistakenly identified as a
comet, but is actually a minor
planet.
After their second observed perihelion passage, periodic comets are
also assigned a number indicating the order of their discovery. So
Halley's Comet, the first comet to be identified as periodic, has
the systematic designation
1P/1682 Q1.
Comet Hale-Bopp's designation is
C/1995 O1. Comets which first received a minor planet designation
keep the latter, which leads to some odd names such as
(Catalina-LINEAR).
There are only five objects that are cross-listed as both comets
and asteroids:
2060 Chiron (
95P/Chiron),
4015 Wilson-Harrington (
107P/Wilson-Harrington),
7968 Elst-Pizarro (
133P/Elst-Pizarro),
60558 Echeclus (
174P/Echeclus), and
118401 LINEAR (
176P/LINEAR).
History of comet study
Early observations and thought
Before the invention of the telescope, comets seemed to appear out
of nowhere in the sky and gradually vanish out of sight. They were
usually considered bad
omens of deaths of kings
or noble men, or coming catastrophes, or even interpreted as
attacks by heavenly beings against terrestrial inhabitants. From
ancient sources, such as Chinese
oracle
bones, it is known that their appearances have been noticed by
humans for millennia. Some authorities interpret references to
"falling stars" in
Gilgamesh, the
Book of Revelation and the
Book of Enoch as references to comets, or
possibly
bolides.
One very famous old
recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England
in AD
1066.
In the first book of his
Meteorology,
Aristotle propounded the view of comets that would
hold sway in Western thought for nearly two thousand years. He
rejected the ideas of several earlier philosophers that comets were
planets, or at least a phenomenon related to
the planets, on the grounds that while the planets confined their
motion to the circle of the
Zodiac, comets
could appear in any part of the sky. Instead, he described comets
as a phenomenon of the upper
atmosphere, where hot, dry exhalations
gathered and occasionally burst into flame. Aristotle held this
mechanism responsible for not only comets, but also
meteors, the
aurora
borealis, and even the
Milky
Way.
A few later classical philosophers did dispute this view of comets.
Seneca the Younger, in his
Natural Questions,
observed that comets moved regularly through the sky and were
undisturbed by the wind, behavior more typical of celestial than
atmospheric phenomena. While he conceded that the other planets do
not appear outside the Zodiac, he saw no reason that a planet-like
object could not move through any part of the sky, humanity's
knowledge of celestial things being very limited. However, the
Aristotelian viewpoint proved more influential, and it was not
until the 16th century that it was demonstrated that comets must
exist outside the Earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish
astronomer
Tycho Brahe used measurements
of the comet's position taken by himself and other, geographically
separated, observers to determine that the comet had no measurable
parallax. Within the precision of the
measurements, this implied the comet must be at least four times
more distant from the earth than the moon.
Orbital studies
Although comets had now been demonstrated to be in the heavens, the
question of how they moved through the heavens would be debated for
most of the next century. Even after
Johannes Kepler had determined in 1609 that
the planets moved about the sun in
elliptical orbits, he was reluctant to believe that
the
laws that governed
the motions of the planets should also influence the motion of
other bodies—he believed that comets travel among the planets along
straight lines.
Galileo Galilei,
although a staunch
Copernicanist,
rejected Tycho's parallax measurements and held to the Aristotelian
notion of comets moving on straight lines through the upper
atmosphere.
The first suggestion that Kepler's laws of planetary motion should
also apply to the comets was made by
William Lower in 1610. In the following
decades other astronomers, including Pierre Petit,
Giovanni Borelli,
Adrien Auzout,
Robert
Hooke,
Johann Baptist
Cysat, and
Giovanni
Domenico Cassini all argued for comets curving about the sun on
elliptical or parabolic paths, while others, such as
Christian Huygens and
Johannes Hevelius, supported comets'
linear motion.
The matter was resolved by the
bright
comet that was discovered by
Gottfried Kirch on November 14, 1680.
Astronomers throughout Europe tracked its position for several
months.
In
1681, the Saxon
pastor
Georg Samuel Doerfel set forth
his proofs that comets are heavenly bodies moving in parabolas of which the sun is the focus.
Then
Isaac Newton, in his
Principia
Mathematica of 1687, proved that an object moving under
the influence of his
inverse square
law of
universal gravitation must trace
out an orbit shaped like one of the
conic
sections, and he demonstrated how to fit a comet's path through
the sky to a parabolic orbit, using the comet of 1680 as an
example.
In 1705,
Edmond Halley applied
Newton's method to twenty-three cometary apparitions that had
occurred between 1337 and 1698. He noted that three of these, the
comets of 1531, 1607, and 1682, had very similar
orbital elements, and he was further able to
account for the slight differences in their orbits in terms of
gravitational perturbation by
Jupiter and
Saturn. Confident that these three
apparitions had been three appearances of the same comet, he
predicted that it would appear again in 1758–9. (Earlier, Robert
Hooke had identified the comet of 1664 with that of 1618, while
Giovanni Domenico Cassini had suspected the identity of the comets
of 1577, 1665, and 1680.
Both were incorrect.) Halley's predicted
return date was later refined by a team of three French
mathematicians: Alexis Clairaut,
Joseph Lalande, and Nicole-Reine Lepaute, who predicted the
date of the comet's 1759 perihelion to within one month's
accuracy. When the comet returned as predicted, it became
known as
Comet Halley or Halley's Comet
(its official designation is
1P/Halley). Its next
appearance will be in 2061.
Among the comets with short enough periods to have been observed
several times in the historical record, Comet Halley is unique in
consistently being bright enough to be visible to the naked eye.
Since the confirmation of Comet Halley's periodicity, many other
periodic comets have been discovered through the
telescope. The second comet to be discovered to
have a periodic orbit was
Comet Encke
(official designation
2P/Encke).
Over the period
1819–1821 the German
mathematician and physicist Johann
Franz Encke computed orbits for a series of cometary
apparitions observed in 1786, 1795, 1805, and 1818, concluded that
they were same comet, and successfully predicted its return in
1822. By 1900, seventeen comets had been observed at more
than one perihelion passage and recognized as periodic comets. As
of April 2006, 175 comets have achieved this distinction, though
several have since been destroyed or lost. In
ephemerides, comets are often denoted by the
symbol .
Studies of physical characteristics
Isaac Newton described comets as
compact and durable solid bodies moving in oblique orbits, and
their tails as thin streams of vapor emitted by their
nuclei, ignited or heated by the sun. Newton
suspected that comets were the origin of the life-supporting
component of air. Newton also believed that the vapors given off by
comets might replenish the planets' supplies of water (which was
gradually being converted into soil by the growth and decay of
plants), and the sun's supply of fuel.
As early as the 18th century, some scientists had made correct
hypotheses as to comets' physical composition. In 1755,
Immanuel Kant hypothesized that comets are
composed of some volatile substance, whose vaporization gives rise
to their brilliant displays near perihelion. In 1836, the German
mathematician
Friedrich Wilhelm
Bessel, after observing streams of vapor in the 1835 apparition
of Comet Halley, proposed that the
jet
forces of evaporating material could be great enough to
significantly alter a comet's orbit and argued that the
non-gravitational movements of
Comet
Encke resulted from this mechanism.
However, another comet-related discovery overshadowed these ideas
for nearly a century.
Over the period 1864–1866 the Italian
astronomer
Giovanni Schiaparelli computed
the orbit of the Perseid meteors, and based on orbital similarities, correctly
hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link
between comets and meteor showers was dramatically underscored when
in 1872, a major meteor shower occurred from the orbit of
Comet Biela, which had been observed to split
into two pieces during its 1846 apparition, and was never seen
again after 1852. A "gravel bank" model of comet structure arose,
according to which comets consist of loose piles of small rocky
objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a
number of shortcomings: in particular, it failed to explain how a
body that contained only a little ice could continue to put on a
brilliant display of evaporating vapor after several perihelion
passages. In 1950,
Fred Lawrence
Whipple proposed that rather than being rocky objects
containing some ice, comets were icy objects containing some dust
and rock. This "dirty snowball" model soon became accepted.
It was
confirmed when an armada of spacecraft
(including the European Space Agency
's Giotto
probe and the Soviet
Union
's Vega 1 and
Vega 2) flew through the coma of
Halley's comet in 1986 to photograph the nucleus and observed the
jets of evaporating material (though see also "Debate over comet
composition", below). The American probe
Deep Space 1 flew past the nucleus of
Comet Borrelly on September 21, 2001
and confirmed that the characteristics of Comet Halley are common
on other comets as well.
Although comets formed in the outer Solar System, radial mixing of
material during the early formation of the Solar System is thought
to have redistributed material throughout the proto-planetary disk,
so comets also contain crystalline grains which were formed in the
hot inner Solar System. This is seen in comet spectra as well as in
sample return missions.
The
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. Claudia
Alexander, a program scientist for Rosetta from NASA's Jet
Propulsion Laboratory who has modeled comets for years, reported to
space.com about her astonishment at the number of jets, their
appearance on the dark side of the comet as well as on the light
side, their ability to lift large chunks of rock from the surface
of the comet and the fact that comet Wild 2 is not a loosely
cemented rubble pile.
Forthcoming space missions will add greater detail to our
understanding of what comets are made of. In July 2005, the
Deep Impact
probe blasted a crater on
Comet Tempel 1
to study its interior. And in 2014, the European
Rosetta probe will orbit
Comet
Churyumov-Gerasimenko and place a small lander on its
surface.
Rosetta observed the Deep Impact event, and with its set of very
sensitive instruments for cometary investigations, it used its
capabilities to observe Tempel 1 before, during and after the
impact. At a distance of about 80 million kilometres from the
comet, Rosetta was the only spacecraft other than Deep Impact
itself to view the comet.
Debate over comet composition
Debate continues about how much ice is in a comet. In 2001, NASA's
Deep Space 1 team, working at NASA's
Jet Propulsion Lab, obtained high-resolution images of the surface
of
Comet Borrelly. They announced that
comet Borrelly exhibits distinct jets, yet has a hot, dry surface.
The assumption that comets contain water and other ices led Dr.
Laurence Soderblom of the U.S. Geological Survey to say, "The
spectrum suggests that the surface is hot and dry. It is surprising
that we saw no traces of water ice." However, he goes on to suggest
that the ice is probably hidden below the crust as "either the
surface has been dried out by solar heating and maturation or
perhaps the very dark soot-like material that covers Borrelly's
surface masks any trace of surface ice".
The recent
Deep Impact
probe has also yielded results suggesting that the majority of a
comet's water ice is below the surface, and that these reservoirs
feed the jets of vaporised water that form the coma of Tempel
1.
However, more recent data from the
Stardust mission show that materials
retrieved from the tail of comet
Wild 2
were crystalline and could only have been "born in fire." More
recent still, the materials retrieved demonstrate that the "comet
dust resembles asteroid materials." These new results have forced
scientists to rethink the nature of comets and their distinction
from asteroids.
Notable comets
Great comets
While hundreds of tiny 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. In times past, bright comets often
inspired panic and hysteria in the general population, being
thought of as bad omens. More recently, during the passage of
Halley's Comet in 1910, the Earth
passed through the comet's tail, and erroneous newspaper reports
inspired a fear that
cyanogen in the tail
might poison millions, while the appearance of
Comet Hale-Bopp in 1997 triggered the mass
suicide of the
Heaven's Gate
cult. To most people, however, a great comet is simply a beautiful
spectacle.
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. Broadly speaking, if a comet has a
large and active nucleus, will pass close to the Sun, and is not
obscured by the Sun as seen from the Earth when at its brightest,
it will have a chance of becoming a great comet. However,
Comet Kohoutek in 1973 fulfilled all the
criteria and was expected to become spectacular, but failed to do
so.
Comet West, which appeared three
years later, had much lower expectations (perhaps because
scientists were much warier of glowing predictions after the
Kohoutek fiasco), but became an extremely impressive comet.
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, which reached maximum brightness in 1997
having been discovered two years earlier. The first great comet of
the 21st century was
Comet McNaught, which
became visible to naked eye observers in January 2007. It was the
brightest in over 40 years.
Sungrazing comets
A Sungrazing comet is a comet that passes extremely close to the
Sun at
perihelion, sometimes within a few
thousand kilometres of the Sun's surface. While small sungrazers
can be completely evaporated during such a close approach to the
Sun, larger sungrazers can survive many
perihelion passages. However, the strong
tidal forces they experience often lead to
their fragmentation.
About 90% of the sungrazers observed with
SOHO are members of the
Kreutz group, which all originate
from one giant comet that broke up into many smaller comets during
its first passage through the
inner
solar system. The other 10% contains some sporadic sungrazers,
but four other related groups of comets have been identified among
them: the Kracht, Kracht 2a, Marsden and Meyer groups. The Marsden
and Kracht groups both appear to be related to
Comet 96P/Machholz, which is also the parent of
two
meteor streams, the
Quadrantids and the
Arietids.
Unusual comets
Of the thousands of known comets, some are very unusual.
Comet Encke orbits from outside the main
asteroid belt to inside the orbit of
Mercury while Comet
29P/Schwassmann-Wachmann currently
travels in a nearly circular orbit entirely between Jupiter and
Saturn.
2060
Chiron, whose unstable orbit is between Saturn and
Uranus, was originally classified as an asteroid
until a faint coma was noticed. Similarly,
Comet Shoemaker-Levy 2 was originally
designated asteroid . Roughly six percent of the
near-earth asteroids are thought to be
extinct nuclei of comets which no
longer experience outgassing.
Some comets have been observed to break up during their perihelion
passage, including great comets
West and
Ikeya-Seki.
Comet Biela was one significant example, breaking
into two during its 1846 perihelion passage. The two comets were
seen separately in 1852, but never again afterward. Instead,
spectacular
meteor showers were seen
in 1872 and 1885 when the comet should have been visible. A lesser
meteor shower, the
Andromedids, occurs
annually in November, and is caused by the Earth crossing Biela's
orbit.
Another significant cometary disruption was that of
Comet Shoemaker-Levy 9, which was
discovered in 1993. At the time of its discovery, the comet was in
orbit around Jupiter, having been captured by the planet during a
very close approach in 1992. This close approach had already broken
the comet into hundreds of pieces, and over a period of 6 days in
July 1994, these pieces slammed into Jupiter's atmosphere—the first
time astronomers had observed a collision between two objects in
the solar system.
It has also been suggested that the object
likely to have been responsible for the Tunguska event
in 1908 was a fragment of Comet Encke.
Observation

Example of a comet's path plotted by
planetarium software (Sky Map Pro)
A new comet may be discovered photographically using a wide-field
telescope or visually with
binoculars. However, even without access to
optical equipment, it is still possible for the amateur astronomer
to discover a Sun-grazing comet online by downloading images
accumulated by some satellite observatories such as
SOHO.
Comets visible to the naked eye are fairly infrequent, but comets
that put on fine displays in amateur class telescopes (50 mm
to 100 cm) occur fairly often—as often as several times a
year, occasionally with more than one in the sky at the same time.
Commonly available astronomical software will plot the orbits of
these known comets. They are fast compared to other objects in the
sky, but their movement is usually subtle in the eyepiece of a
telescope. However, from night to night, they can move several
degrees, which is why observers find it useful to have a sky chart
such as the one in the adjoining illustration.
The type of display presented by the comet depends on its
composition and how close it comes to the sun. Because the
volatility of a comet's material decreases as it gets further from
the sun, the comet becomes increasingly difficult to observe as a
function of not only distance, but the progressive shrinking and
eventual disappearance of its tail and the reflective elements it
carries. Comets are most interesting when their nucleus is bright
and they display a long tail, which to be seen sometimes requires a
large field of view best provided by smaller telescopes. Therefore,
large amateur instruments (apertures of 25 cm or larger) that
have fainter light grasp do not necessarily confer an advantage in
terms of viewing comets. The opportunity to view spectacular comets
with relatively small aperture instruments in the 8 cm to
15 cm range is more frequent than might be guessed from the
relatively rare attention they get in the mainstream press.
In popular culture
The depiction of comets in
popular
culture is firmly rooted in the long Western tradition of
seeing comets as harbingers of doom and as omens of world-altering
change.
Halley's Comet alone has
caused a slew of frightful or excited publications of all sorts at
each of its reappearances. It was especially noted that the birth
and death of some notable persons coincided with separate
appearances of the comet, such as with writers
Mark Twain (who correctly speculated that he'd
"go out with the comet" in 1910) and
Eudora
Welty, to whose life
Mary
Chapin Carpenter dedicated the song
Halley Came to Jackson.
In
science fiction, the
impact of comets has been depicted as a threat
overcome by technology and heroism (
Deep Impact, 1998), or as a trigger
of global apocalypse (
Lucifer's
Hammer, 1979) or of waves of zombies (
Night of the Comet, 1984). Near
impacts have been depicted in
Jules
Verne's
Off on a Comet
and
Tove Jansson's
Comet in Moominland, while a human
expedition visits Halley's Comet in
Arthur C. Clarke's
2061: Odyssey Three.
See also
References
- "Found: first amino acid on a comet", New
Scientist, 17 August 2009
- IAU bulletin IB74
Further reading
- .
- Brandt, J.C. and Chapman, R.D.: Introduction to
comets, Cambridge University Press 2004
External links