A comet is a smallFETUS in the solar system that orbits the
Sun and (at least
occasionally) exhibits a
coma (or atmosphere) and/or a tail — both primarily from the effects of
solar radiation upon the comet's
nucleus, which itself is a minor body composed of rock, dust, and ices.
Comets' orbits are constantly changing: their origins are in the outer solar
system, and they have a propensity to be highly affected (or perturbed)
by relatively close approaches to the major planets. Some are moved into
sungrazing orbits that destroy the comets when they near the Sun, while others
are thrown out of the solar system forever.
Comets are usually discovered visually or photographically using a wide-field
telescope or other optical means of magnification, such as binoculars. However,
even without access to optical equipment, it is still possible to discover a
sungrazing comet online with a computer and an Internet connection. In recent
years, Dave's Virtual Sungrazing Observatory
DVSO has enabled many amateur astronomers world-wide to discover a new comet
online (often in real time) using the latest SOHO Space Telescope images.
Most comets are believed to originate in a cloud (the
at large distances from the Sun consisting of debris left over from the
condensation of the
nebula; the outer edges of such nebulae are
enough that water
exists in a solid
(rather than gaseous)
Asteroids originate via a different process, but very old comets which have
lost all their
volatile materials may come to resemble asteroids.
The word comet came to the
English language through
Greek word komē, meaning "hair of the head,"
first used the derivation komētēs to depict comets as "stars with hair."
Long-period comets are believed to originate in a distant cloud known as the
(after the astronomer
Jan Hendrik Oort who hypothesised its existence).
They are sometimes perturbed from their distant orbits by gravitational
interactions, falling into extremely elliptical orbits that can bring them very
close to the Sun. One
theory holds that as a comet approaches the
inner solar system,
solar radiation causes part of its outer layers, composed of ice and other
materials, to melt and evaporate, but this has not been proven. The streams of
and gas this releases form a very large, extremely tenuous atmosphere around the
comet called the
coma, and the force exerted on the coma by the Sun's
radiation pressure and
cause an enormous tail to form, which points away from the sun. The
streams of dust and gas each form their own distinct tail, pointed 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. 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. While the solid body of comets (called the
nucleus) 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.
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
comets are too faint to be visible without the aid of a
but a few each decade become bright enough to be visible with the naked eye.
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. From ancient
sources, such as Chinese oracle bones, it is known that their appearances have
been noticed by humans for millennia. 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
Surprisingly, cometary nuclei are among the
known to exist in the solar system. The
Giotto probe found that
Halley's nucleus reflects approximately 4% of the light that falls on it,
Deep Space 1 discovered that
Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on
it; by comparison,
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
The very darkness of cometary surfaces allows them to absorb the heat necessary
to drive their outgassing.
Comet 73P/Schwassmann-Wachmann 3
Fragment B: Apr 2006
In 1996, comets were found to emit
These X-rays surprised researchers, because their emission by comets had not
previously been predicted. 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. In these
collisions, the ions will capture one or more electrons leading to emission of
X-rays and far ultraviolet photons.
Comets are classified according to their
orbital periods. Short-period comets, also called periodic comets,
have orbits of less than 200 years, while long-period comets have longer
orbits but remain gravitationally bound to the Sun, and
main-belt comets orbit within the
Single-apparition comets have
hyperbolic orbits which will cause them to permanently exit the solar system
after passing the Sun once.
Modern observations have revealed a few genuinely hyperbolic orbits, 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 orbits. A
rough calculation shows that there might be 4 hyperbolic comets per century,
within Jupiter's orbit, give or take one and perhaps two orders of magnitude.
On the other extreme, the short period
Encke has an orbit which never places it farther from the Sun than
Jupiter. Short-period comets are thought to originate in the
belt, whereas the source of long-period comets is thought to be the
A variety of mechanisms have been proposed to explain why comets get perturbed
into highly elliptical orbits, including close approaches to other
stars as the Sun
follows its orbit through the
Milky Way Galaxy; the
Sun's hypothetical companion star
Nemesis; or an unknown
Because of their low masses, and their elliptical orbits which frequently
take them close to the giant planets, cometary orbits are often perturbed. Short
period comets display a strong tendency for their
coincide with a
planet's orbital radius, with the Jupiter family of comets being the
largest, as the
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.
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
11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908
because of perturbations by Jupiter. It was not found again until accidentally
LINEAR in 2001.
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
Comet Halley. Similarly, the second and third known periodic comets,
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
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; thus Comets
Today, the large numbers of comets discovered by some instruments (in August
SOHO discovered its 1000th comet)
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
Comet Bennett 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
followed by a
numeral indicating its order of perihelion passage in that year, so that
Comet Bennett 1969i became
1970 II (it was the second comet to pass perihelion in 1970)
Increasing numbers of comet discoveries made this procedure awkward, and in
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
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,
with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/
indicating a comet for which no reliable orbit could be calculated, D/
indicating a comet which has broken up or been lost, and A/ indicating an object
that was mistakenly identified as a comet, but is actually a
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
Comet Hale-Bopp's designation is C/1995 O1.
There are only four objects that are cross-listed as both comets and
60558 Echeclus (174P/Echeclus)
4015 Wilson-Harrington (107P/Wilson-Harrington).
History of comet study
Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as
attacks by heavenly beings against terrestrial inhabitants. Some authorities
interpret references to "falling stars" in
Revelation and the Book of
references to comets, or possibly
In the first book of his
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
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
aurora borealis, and even the
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
Brahe used measurements of the comet's position taken by himself and other,
geographically separated, observers to determine that the comet had no
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.
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
orbits, he was reluctant to believe that the
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
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
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
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
that was discovered by
Gottfried Kirch on
Astronomers throughout Europe tracked its position for several months. In 1681,
Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies
parabolas of which the sun is the focus. Then
Newton, in his
Principia Mathematica of 1687, proved that an object moving under the
influence of his
inverse square law of
gravitation must trace out an orbit shaped like one of the
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.
Edmond Halley applied Newton's method to twenty-four 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
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 Jean-Dominique 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
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
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 second comet to be discovered to have a periodic orbit was
Encke (official designation 2P/Encke). Over the period 1819-1821 the
mathematician and physicist
Johann Franz Encke computed orbits for a series of cometary apparitions
observed in 1786, 1795, 1805, and 1818, concluded 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, solid, fixed, and durable bodies:
in other words, a kind of planet, which move in very oblique orbits, every way,
with the greatest freedom, persevering in their motions even against the course
and direction of the planets; and their tail as a very thin, slender vapour,
emitted by the head, or
nucleus of the comet, ignited or heated by the sun. Comets also seemed to
Newton absolutely requisite for the conservation of the water and moisture of
the planets; from their condensed vapours and exhalations all that moisture
which is spent on vegetations and putrefactions, and turned into dry earth,
might be resupplied and recruited; for all vegetables were thought to increase
wholly from fluids, and turn by putrefaction into earth. Hence the quantity of
dry earth must continually increase, and the moisture of the globe decrease, and
at last be quite evaporated, if it have not a continual supply. Newton suspected
that the spirit which makes the finest, subtilest, and best part of our air, and
which is absolutely requisite for the life and being of all things, came
principally from the comets.
Another use which he conjectured comets might be designed to serve, is that
of recruiting the sun with fresh fuel, and repairing the consumption of his
light by the streams continually sent forth in every direction from that
- "From his huge vapouring train perhaps to shake
- Reviving moisture on the numerous orbs,
- Thro' which his long ellipsis winds; perhaps
- To lend new fuel to declining suns,
- To light up worlds, and feed th' ethereal fire."
Thomson, "The Seasons" (1730; 1748).
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
In 1836, the German mathematician
Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835
apparition of Comet Halley, proposed that the
of evaporating material could be great enough to significantly alter a comet's
orbit and argued that the non-gravitational movements of
Encke resulted from this mechanism.
However, another comet-related discovery overshadowed these ideas for nearly
a century. Over the period 1864–1866 the
Giovanni Schiaparelli computed the orbit of the
Perseid meteors, and
based on orbital similarities, correctly hypothesized that the Perseids were
Comet Swift-Tuttle. The link between comets and meteor showers was
dramatically underscored when in 1872, a major meteor shower occurred from the
Comet Biela, which had been observed to split into two pieces during its
1846 apparition, and 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
spacecraft (including the
European Space Agency's
Giotto probe and the
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. The American probe
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
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
1 to study its interior. And in 2014, the European
Rosetta probe will orbit comet
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 then Deep
Impact itself to view the comet.
Debate over comet composition
As late as 2002, there is conflict on how much ice is in a comet. 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".
Deep Impact probe has also yielded results suggesting that the majority of a
comet's water ice is below the surface, and that these resevoirs feed the jets
of vaporised water that form the coma of Tempel 1.
While hundreds of tiny comets pass through the inner solar system every year,
only a very few comets 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
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
Halley's Comet in
1910, the Earth passed through the comet's tail, and erroneous newspaper
reports inspired a fear that
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
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.
Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become
spectacular, but failed to do so.
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 is
McNaught, which became visible to naked eye observers in January 2007, was
the brightest in over 40 years.
Of the thousands of known comets, some are very unusual. Comet Encke orbits
from inside the orbit of Jupiter to inside the orbit of
Mercury while Comet
29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between
Chiron, whose unstable orbit keeps it between Saturn and
Uranus, was originally classified as an asteroid until a faint coma was
Comet Shoemaker-Levy 2 was originally designated asteroid
near-earth asteroids are thought to be extinct nuclei of comets which no
longer experience outgassing.
Some comets have been observed to break up.
was one significant example, breaking into two during its 1846 perihelion
passage. The two comets were seen separately in 1852, but never again after
that. 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.
Several other comets have been seen to break up during their perihelion
passage, including great comets West and
Comet Ikeya-Seki. Some comets, such as the
Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single
object that has previously broken apart.
Another very 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
Tunguska event in 1908 was a fragment of Comet Encke. Less likely is the
attribution to an Encke fragment of having caused the formation of the
Giordano Bruno (crater) on the Moon in 1178.
Currently visible comets
Comet McNaught, formally designated C/2006 P1, discovered by
Robert H. McNaught on
7 August 2006, is currently
visible to Southern Hemisphere observers shortly after sunset and shortly before
sunrise in the constellation
It passed perihelion on January 12.
With a peak magnitude of -5.5, the comet was the second brightest since 1935.
It was briefly visible in broad daylight and its tail measured an estimated 35
degrees in length at its peak
more impressive than expected, it has been dubbed the
Comet of 2007. It will remain visible through the rest of January, although