Comet
From Wikipedia, the free encyclopedia
9P/Tempel collides with Deep Impact's impactor
Comet Holmes (17P/Holmes) in 2007, showing blue
ion tail on right
Comet Wild 2
A
comet is an icy
small Solar System body that, when passing close to the
Sun,
heats up and begins to outgas, displaying a visible atmosphere or coma,
and sometimes also a tail. These phenomena are due to the effects of
solar radiation and the
solar wind
upon the nucleus of the comet. Comet nuclei range from a few hundred
metres to tens of kilometres across and are composed of loose
collections of ice, dust, and small rocky particles. The coma and tail
are much larger and, if sufficiently bright, may be seen from the Earth
without the aid of a telescope. Comets have been observed and recorded
since ancient times by many different cultures.
Comets have a wide range of
orbital periods, ranging from several years to several millions of years. Short-period comets originate in the
Kuiper belt or its associated
scattered disc, which lie beyond the orbit of
Neptune. Longer-period comets are thought to originate in the
Oort cloud,
a spherical cloud of icy bodies extending from outside the Kuiper Belt
to halfway to the next nearest star. Long-period comets are directed
towards the Sun from the Oort cloud by
gravitational perturbations caused by
passing stars and the
galactic tide.
Hyperbolic comets may pass once through the inner Solar System before being flung out to
interstellar space along
hyperbolic trajectories.
Comets are distinguished from
asteroids
by the presence of an extended, gravitationally unbound atmosphere
surrounding their central nucleus. This atmosphere has parts termed the
coma (the central atmosphere immediately surrounding the nucleus) and
the tail (a typically linear section consisting of dust or gas blown out
from the coma by the Sun's light pressure or outstreaming solar wind
plasma). However,
extinct comets that have passed close to the Sun many times have lost nearly all of their
volatile ices and dust and may come to resemble small asteroids.
[1]
Asteroids are thought to have a different origin from comets, having
formed inside the orbit of Jupiter rather than in the outer Solar
System.
[2][3] The discovery of
main-belt comets and active
centaurs has blurred the
distinction between asteroids and comets.
As of August 2014 there are 5,186 known comets,
[4]
a number which is steadily increasing. However, this represents only a
tiny fraction of the total potential comet population, as the reservoir
of comet-like bodies in the outer Solar System may number one trillion.
[5] Roughly one comet per year is visible to the
naked eye, though many of these are faint and unspectacular.
[6] Particularly bright examples are called "
Great Comets".
On 22 January 2014,
ESA scientists reported the detection, for the first definitive time, of
water vapor on the
dwarf planet Ceres, the largest object in the
asteroid belt.
[7] The detection was made by using the
far-infrared abilities of the
Herschel Space Observatory.
[8] The finding is unexpected because comets, not
asteroids,
are typically considered to "sprout jets and plumes". According to one
of the scientists, "The lines are becoming more and more blurred between
comets and asteroids."
[8] On 11 August 2014, astronomers released studies, using the
Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of
HCN,
HNC,
H2CO, and
dust inside the
comae of comets
C/2012 F6 (Lemmon) and
C/2012 S1 (ISON).
[9][10]
Etymology
The word
comet derives from the
Old English cometa from the
Latin comēta or
comētēs. That, in turn, is a
latinisation of the Greek
κομήτης ("wearing long hair"), and the
Oxford English Dictionary notes that the term (
ἀστὴρ)
κομήτης already meant "long-haired star, comet" in Greek.
Κομήτης was derived from
κομᾶν ("to wear the hair long"), which was itself derived from
κόμη ("the hair of the head") and was used to mean "the tail of a comet".
[11][12]
The
astronomical symbol for comets is (
☄), consisting of a small disc with three hairlike extensions.
[13]
Physical characteristics
Nucleus
Nucleus of
Comet 103P/Hartley as imaged during a spacecraft flyby. The nucleus is about 2 km in length.
C/2011 W3 (Lovejoy) heads towards the Sun
Comet Wild 2 exhibits jets on light side and dark side, stark relief, and is dry.
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of
rock, dust,
water ice, and frozen gases such as
carbon dioxide,
carbon monoxide,
methane, and
ammonia.
[14] As such, they are popularly described as "dirty snowballs" after
Fred Whipple's model.
[15] However, some comets may have a higher dust content, leading them to be called "icy dirtballs".
[16]
The surface of the nucleus is generally dry, dusty or rocky,
suggesting that the ices are hidden beneath a surface crust several
metres thick. In addition to the gases already mentioned, the nuclei
contain a variety of
organic compounds, which may include
methanol,
hydrogen cyanide,
formaldehyde,
ethanol, and
ethane and perhaps more complex molecules such as long-chain
hydrocarbons and
amino acids.
[17][18] In 2009, it was confirmed that the amino acid
glycine had been found in the comet
dust recovered by NASA's
Stardust mission.
[19] In August 2011, a report, based on
NASA studies of
meteorites found on
Earth, was published suggesting
DNA and
RNA components (
adenine,
guanine, and related
organic molecules) may have been formed on
asteroids and comets.
[20][21]
The outer surfaces of cometary nuclei have a very low
albedo, making them among the least reflective objects found in the Solar System. The
Giotto space probe found that the nucleus of
Halley's Comet reflects about four percent of the light that falls on it,
[22] and
Deep Space 1 discovered that
Comet Borrelly's surface reflects less than 3.0% of the light that falls on it;
[22] by comparison,
asphalt reflects seven percent of the light that falls on it. The dark surface material of the nucleus may consist of complex
organic compounds. Solar heating drives off lighter
volatile compounds, leaving behind larger organic compounds that tend to be very dark, like
tar or
crude oil. The low reflectivity of cometary surfaces enables them to absorb the heat necessary to drive their
outgassing processes.
[23]
Comet nuclei with radii of up to 30 kilometres (19 mi) have been observed,
[30] but ascertaining their exact size is difficult.
[31] The nucleus of
P/2007 R5 is probably only 100–200 metres in diameter.
[32]
A lack of smaller comets being detected despite the increased
sensitivity of instruments has led some to suggest that there is a real
lack of comets smaller than 100 metres (330 ft) across.
[33] Known comets have been estimated to have an average density of 0.6
g/cm
3.
[28] Because of their low mass, comet nuclei do not
become spherical under their own
gravity and therefore have irregular shapes.
[34]
Roughly six percent of the
near-Earth asteroids are thought to be
extinct nuclei of comets that no longer experience outgassing,
[35] including
14827 Hypnos and
3552 Don Quixote.
Coma
The streams of
dust
and gas thus released form a huge and extremely thin 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 pointing away from the Sun.
[37]
The coma is generally made of
H2O and
dust, with water making up to 90% of the
volatiles
that outflow from the nucleus when the comet is within 3 to 4
astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to
370,000,000 mi) of the Sun.
[38] The
H2O parent molecule is destroyed primarily through
photodissociation and to a much smaller extent
photoionization, with the solar wind playing a minor role in the destruction of water compared to
photochemistry.
[38]
Larger dust particles are left along the comet's orbital path whereas
smaller particles are pushed away from the Sun into the comet's tail by
light pressure.
[39]
Although the solid nucleus of comets is generally less than 60
kilometres (37 mi) across, the coma may be thousands or millions of
kilometres across, sometimes becoming larger than the Sun.
[40] For example, about a month after an outburst in October 2007, comet
17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.
[41] The
Great Comet of 1811 also had a coma roughly the diameter of the Sun.
[42] Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of
Mars around 1.5 astronomical units (220,000,000 km; 140,000,000 mi) from the Sun.
[42] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.
[42] Ion tails have been observed to extend one
astronomical unit (150 million km) or more.
[41]
Both the coma and tail are illuminated by the Sun and may become
visible when a comet passes through the inner Solar System, the dust
reflecting sunlight directly and the gases glowing from
ionisation.
[43] 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.
[44]
Occasionally a comet may experience a huge and sudden outburst of gas
and dust, during which the size of the coma greatly increases for a
period of time. This happened in 2007 to
Comet Holmes.
[41]
In 1996, comets were found to emit
X-rays.
[45] This greatly surprised astronomers because X-ray emission is usually associated with very
high-temperature bodies.
The X-rays are generated by the interaction between comets and the
solar wind: when highly charged solar wind ions fly through a cometary
atmosphere, they collide with cometary atoms and molecules, "stealing"
one or more electrons from the atom in a process called "charge
exchange". This exchange or transfer of an electron to the solar wind
ion is followed by its de-excitation into the ground state of the ion,
leading to the emission of X-rays and
far ultraviolet photons.
[46]
Tails
Diagram of a comet showing the dust trail, the dust tail (or
antitail) and the ion gas tail, which is formed by the solar wind flow.
In the outer
Solar System,
comets remain frozen and inactive 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 observations by the
Hubble Space Telescope[47][48] but these detections have been questioned.
[49][50] 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 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 type II or dust tail.
[43]
At the same time, the ion or type I tail, made of gases, always points
directly away from the Sun because this gas is more strongly affected by
the solar wind than is dust, following magnetic field lines rather than
an orbital trajectory.
[51]
On occasions - such as when the Earth passes through a comet's orbital
plane, and we see the track of the comet edge-on, a tail pointing in the
opposite direction to the ion and dust tails may be seen – the
antitail.
[52] (The dust tail of the comet prior to its rounding of the sun, is collinear with the dust tail post the rounding of the sun).
The observation of antitails contributed significantly to the discovery of solar wind.
[53]
The ion tail is formed as a result of the ionisation by solar
ultra-violet radiation of particles in the coma. Once the particles have
been ionized, 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. Because 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.
[54]
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".
[54] This has been observed on a number of occasions, one notable event being recorded on April 20, 2007, when the ion tail of
Encke's Comet was completely severed while the comet passed through a
coronal mass ejection. This event was observed by the
STEREO space probe.
[55]
In 2013
ESA scientists reported that the
ionosphere of the planet
Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."
[56][57]
Jets
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.
[58] These streams of gas and dust can cause the nucleus to spin, and even split apart.
[58] In 2010 it was revealed
dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.
[59]
This is known because a spacecraft got so close that it could see where
the jets were coming out, then measure the infrared spectrum at that
point which shows what some of the materials are.
[60]
Orbital characteristics
Most comets are
small Solar System bodies with 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.
[61] Comets are often classified according to the length of their
orbital periods: The longer the period the more elongated the ellipse.
Short period
Periodic comets or short-period comets are generally defined as having orbital periods of less than 200 years.
[62] They usually orbit more-or-less in the
ecliptic plane in the same direction as the planets.
[63] Their orbits typically take them out to the region of the outer planets (
Jupiter and beyond) at
aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of
Neptune.
Comets whose aphelia are near a major planet's orbit are called its "family".
[64] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.
[65]
At the shorter extreme,
Encke's Comet
has an orbit that does not reach the orbit of Jupiter, and is known as
an Encke-type comet. Short-period comets with orbital periods shorter
than 20 years and low inclinations (up to 30 degrees) are called
"Jupiter-family comets".
[66][67]
Those like Halley, with orbital periods of between 20 and 200 years and
inclinations extending from zero to more than 90 degrees, are called
"Halley-type comets".
[68][69] As of 2014, only 74 Halley-type comets have been observed, compared with 492 identified Jupiter-family comets.
[70]
Recently discovered
main-belt comets form a distinct class, orbiting in more circular orbits within the
asteroid belt.
[71]
Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further
gravitational perturbations.
[72] Short-period comets display a tendency for their
aphelia to coincide with a
gas giant's orbital radius, with the Jupiter family of comets being the largest.
[67]
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. These perturbations can deflect long-period comets into
shorter orbital periods.
[73][74]
Based on their orbital characteristics, short-period comets are thought to originate from the
centaurs and the Kuiper belt/
scattered disc[75]
—a disk of objects in the trans-Neptunian 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).
[76]
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
toward 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.
[77]
Long period
Orbits of the
Kohoutek Comet (red) and the Earth (blue), illustrating the high
eccentricity of its orbit and its rapid motion when close to the
Sun.
Hyperbolic
comet discoveries[78] |
Year |
# |
2013 |
8 |
2012 |
10 |
2011 |
12 |
2010 |
4 |
2009 |
8 |
2008 |
7 |
2007 |
12 |
Long-period comets have highly
eccentric orbits and periods ranging from 200 years to thousands of years.
[79] An eccentricity greater than 1 when near
perihelion does not necessarily mean that a comet will leave the Solar System.
[80] For example,
Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage
epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the
eccentricity drops below 1 as it moves further from the Sun. The future orbit of a long-period comet is properly obtained when the
osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the
center of mass of the Solar System.
By definition long-period comets 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". The orbits of long-period comets take them far beyond the
outer planets at aphelia, and the plane of their orbits need not lie
near the ecliptic. Long-period comets such as
Comet West and
C/1999 F1 can have
apoapsis distances of nearly 70,000 AU with orbital periods estimated around 6 million years.
Single-apparition or non-periodic comets are similar to long-period comets because they also have
parabolic or slightly
hyperbolic trajectories
[79]
when near perihelion in the inner Solar System. However, gravitational
perturbations from giant planets cause their orbits to change.
Single-apparition or comets are those with a hyperbolic or parabolic
osculating, which makes them permanently exit the Solar System after a single pass of the Sun.
[81] The Sun's
Hill sphere has an
unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).
[82] Only a few hundred comets have been seen to achieve a hyperbolic orbit (e > 1) when near perihelion
[83] that using a heliocentric unperturbed
two-body best-fit suggests they may escape the Solar System.
No comets with an
eccentricity significantly greater than one have been observed,
[83] so there are no confirmed observations of comets that are likely to have originated outside the Solar System. Comet
C/1980 E1
had an orbital period of roughly 7.1 million years before the 1982
perihelion passage, but a 1980 encounter with Jupiter accelerated the
comet giving it the largest eccentricity (1.057) of any known hyperbolic
comet.
[84] Comets not expected to return to the inner Solar System include
C/1980 E1,
C/2000 U5,
C/2001 Q4 (NEAT),
C/2009 R1,
C/1956 R1, and
C/2007 F1 (LONEOS).
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),
[85] whereas others use it to mean exclusively short-period comets.
[79]
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).
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 km per second). If such
objects entered the Solar System, they would have positive
specific orbital energy
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.
[86]
Oort Cloud and Hills cloud
The Oort cloud is a vast cloud of comets that is thought to surround the Solar System.
The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)
[87] to as far as 50,000 AU (0.79 ly)
[68] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).
[87]
The region can be subdivided into a spherical outer Oort cloud of
20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud
of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound
to the Sun and supplies the long-period (and possibly Halley-type)
comets to inside the orbit of
Neptune.
[68] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.
[88] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;
[88][89][90]
it is seen as a possible source of new comets to resupply the
relatively tenuous outer cloud as the latter's numbers are gradually
depleted. The Hills cloud explains the continued existence of the Oort
cloud after billions of years.
[91]
Exocomets
Exocomets beyond our Solar System have also been detected and may be common in the
Milky Way Galaxy.
[92] The first exocomet system detected was around
Beta Pictoris, a very young type
A V star, in 1987.
[93][94] A total of 10 such exocomet systems have been identified as of 2013, using the
absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.
[92][93]
Effects of comets
Connection to meteor showers
Diagram of Perseids meteors
As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by
radiation pressure and the solar wind.
[95] If the comet's path crosses the path the Earth follows in orbit around the Sun, then at that point there are likely to be
meteor showers as Earth passes through the trail of debris. The
Perseid meteor shower, for example, occurs every year between August 9 and August 13, when Earth passes through the orbit of
Comet Swift–Tuttle.
[96] Halley's comet is the source of the
Orionid shower in October.
[96]
Comets and impact on life
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 portion of it. Other
researchers have cast doubt on this theory.
[97] The detection of organic molecules in significant quantities in comets has led some to speculate that comets or
meteorites may have brought the precursors of life—or even life itself—to Earth.
[98] In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the
amino acids that make up
proteins through shock synthesis.
[99]
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.
[100] Comet and
meteoroid impacts are also believed responsible for the existence of
tektites and
australites.
[101]
Fate of comets
Departure (ejection) from Solar System
If a comet is traveling fast enough, it may leave the Solar System;
such is the case for hyperbolic comets. To date, comets are only known
to be ejected by
interacting with another object in the Solar System, such as Jupiter.
[102]
Volatiles exhausted
Jupiter-family comets and long-period comets appear to follow very
different fading laws. The JFCs are active over a lifetime of about
10,000 years or ~1,000 orbits whereas long-period comets fade much
faster. Only 10% of the long-period comets survive more than 50 passages
to small perihelion and only 1% of them survive more than 2,000
passages.
[35]
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.
[103] Some asteroids in elliptical orbits are now identified as extinct comets.
[104] Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer emit gas.
[35]
Breakup
The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.
[106] A significant cometary disruption was that of
Comet Shoemaker–Levy 9,
which was discovered in 1993. A close encounter in July 1992 had broken
it into pieces, and over a period of six days in July 1994, these
pieces fell into Jupiter's atmosphere—the first time astronomers had
observed a collision between two objects in the Solar System.
[107][108] Other splitting comets include
3D/Biela in 1846 and
73P/Schwassmann–Wachmann from 1995 to 2006.
[109] Greek historian
Ephorus reported that a comet split apart as far back as the winter of 372–373 BC.
[110] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.
[111]
Comets
42P/Neujmin and
53P/Van Biesbroeck
appear to be fragments of a parent comet. Numerical integrations have
shown that both comets had a rather close approach to Jupiter in January
1850, and that, before 1850, the two orbits were nearly identical.
[112]
Some comets have been observed to break up during their perihelion passage, including great comets
West and
Ikeya–Seki.
Biela's
Comet was one significant example, when it broke into two pieces during
its passage through the perihelion in 1846. These 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 it is caused when the Earth crosses the orbit of Biela's Comet.
[113]
Collisions
Some comets meet a more spectacular end – either falling into the Sun
[114]
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
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.
[115]
Nomenclature
The names given to comets have followed several different conventions
over the past two centuries. Prior to the early 20th century, most
comets were simply referred to by the year when they appeared, sometimes
with additional adjectives for particularly bright comets; thus, the "
Great Comet of 1680", the "
Great Comet of 1882", and the "
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
Halley's Comet.
[116] Similarly, the second and third known periodic comets, Encke's Comet
[117] and
Biela's Comet,
[118]
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.
[119]
In the early 20th century, the convention of naming comets after
their discoverers became common, and this remains so today. A comet can
be named after its discoverers, or an instrument or program that helped
to find it.
[119]
History of study
Early observations and thought
From ancient sources, such as Chinese
oracle bones, it is known that their appearances have been noticed by humans for millennia.
[120] Until the sixteenth century, comets 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.
[121][122]
Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the
Zodiac and vary in brightness over the course of a few days.
[123] Pliny the Elder believed that comets were connected with political unrest and death.
[124]
In the 16th century
Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the
parallax of the
Great Comet of 1577
from observations collected by geographically separated observers.
Within the precision of the measurements, this implied the comet must be
at least four times more distant than from the Earth to the Moon.
[125][126]
Orbital studies
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.
[127]
In 1705,
Edmond Halley
(1656–1742) 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.
[128] 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.
[129]
When the comet returned as predicted, it became known as Halley's Comet
(with the latter-day designation of 1P/Halley). It will next appear in
2061.
[130]
Studies of physical characteristics
"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."
James Thomson The Seasons (1730; 1748)[131]
|
Isaac Newton described comets as compact and durable solid bodies
moving in oblique orbit 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.
[132]
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.
[133] In 1836, the German mathematician
Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the
jet forces
of evaporating material could be great enough to significantly alter a
comet's orbit, and he argued that the non-gravitational movements of
Encke's Comet resulted from this phenomenon.
[134]
In 1950,
Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.
[135] This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of
spacecraft (including the
European Space Agency's
Giotto probe and the Soviet Union's
Vega 1 and
Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.
[136]
Spacecraft missions
View from the impactor in its last moments before hitting the comet in the Deep Impact mission
NASA is developing a comet harpoon for returning samples to Earth.
Debate continues about how much ice is in a comet. In 2001, the
Deep Space 1 spacecraft obtained high-resolution images of the surface of
Comet Borrelly.
It was found that the surface of comet Borrelly is hot and dry, with a
temperature of between 26 to 71 °C (79 to 160 °F), and extremely dark,
suggesting that the ice has been removed by solar heating and
maturation, or is hidden by the soot-like material that covers
Borrelly's.
[137]
In July 2005, the
Deep Impact probe blasted a crater on
Comet Tempel 1
to study its interior. The mission 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.
[138] Renamed
EPOXI, it made a flyby of
Comet Hartley 2 on November 4, 2010.
Data from the
Stardust mission
show that materials retrieved from the tail of Wild 2 were crystalline
and could only have been "born in fire," at extremely high temperatures
of over 1,000 °C (1,830 °F).
[139][140]
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,
[141]
so comets also contain crystalline grains that formed in the hot inner
Solar System. This is seen in comet spectra as well as in sample return
missions. More recent still, the materials retrieved demonstrate that
the "comet dust resembles asteroid materials".
[142] These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.
[143]
The
Rosetta probe is presently in erratic orbit around
Comet Churyumov–Gerasimenko; later in 2014 it will stabilise its orbit and place a small lander on its surface.
[144]
Examples
-
Comet
C/2006 P1 (McNaught) taken from Victoria, Australia 2007
-
-
Great comets
Approximately once a decade, a comet becomes bright enough to be
noticed by a casual observer, leading such comets to be designated as
Great Comets.
[110]
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.
[145]
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 has 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.
[146] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.
[147]
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
C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.
[148]
Sungrazing comets
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.
[149] Although 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.
[150]
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.
[151]
The remainder 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.
[152]
Unusual comets
"Active asteroid" P/2013 P5 with several tails.
[153]
Of the thousands of known comets, some exhibit unusual properties.
Encke's Comet orbits from outside the asteroid belt to just inside the
orbit of the planet
Mercury
whereas the Comet 29P/Schwassmann–Wachmann currently travels in a
nearly circular orbit entirely between the orbits of Jupiter and Saturn.
[154] 2060 Chiron, whose unstable orbit is between Saturn and
Uranus, was originally classified as an asteroid until a faint coma was noticed.
[155] Similarly,
Comet Shoemaker–Levy 2 was originally designated asteroid 1990 UL
3.
[156]
Observation
X-ray emission from Hyakutake, as seen by the
ROSAT satellite.
A 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 sungrazing comet online by downloading images accumulated by some satellite observatories such as
SOHO.
[32] SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010
[157] and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).
Lost
A number of periodic comets discovered in earlier decades or previous centuries are now
lost comets.
Their orbits were never known well enough to predict future appearances
or the comets have disintegrated. However, occasionally a "new" comet
is discovered, and calculation of its orbit shows it 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.
[158]
Comets & 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.
[159]
Halley's Comet alone has caused a slew of sensationalist 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)
[159] and
Eudora Welty, to whose life
Mary Chapin Carpenter dedicated the song
Halley Came to Jackson.
[159]
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,
[160] whereas the appearance of
Comet Hale–Bopp in 1997 triggered the mass suicide of the
Heaven's Gate cult.
[161]
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).
[159] In
Jules Verne's
Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir
Arthur C. Clarke's novel
2061: Odyssey Three.
[162]