Comets and impact on life
Many comets and asteroids collided with
Earth in its early stages. Many scientists think that comets
bombarding the young Earth about 4 billion years ago brought the vast
quantities of water that now fill Earth's oceans, or at least a
significant portion of it. Others have cast doubt on this idea. The
detection of organic molecules, including poly-cyclic aromatic
hydrocarbons, in significant quantities in comets has led to
speculation that comets or meteorites may have brought the precursors
of life—or even life itself—to Earth. 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. The speed at which the comets entered the
atmosphere, combined with the magnitude of energy created after
initial contact, allowed smaller molecules to condense into the
larger macro-molecules that served as the foundation for life. In
2015, scientists found significant amounts of molecular oxygen in the
out-gassings of comet 67P, suggesting that the molecule may occur
more often than had been thought, and thus less an indicator of life
as has been supposed.
It is suspected that comet impacts
have, over long timescales, also delivered significant quantities of
water to Earth's Moon, some of which may have survived as lunar ice.
Comet and meteoroid impacts are also thought to be responsible for
the existence of tektites and australites.
Fear of comets
Fear of comets as acts of God and signs
of impending doom was highest in Europe from AD 1200 to 1650. The
year after the Great Comet of 1618, for example, Gotthard Arthusius
published a pamphlet stating that it was a sign that the Day of
Judgment was near. He listed ten pages of comet-related disasters,
including "earthquakes, floods, changes in river courses,
hail storms, hot and dry weather, poor harvests, epidemics, war and
treason and high prices".
By 1700 most scholars concluded that
such events occurred whether a comet was seen or not. Using Edmond
Halley's records of comet sightings, however, William Whiston in 1711
wrote that the Great Comet of 1680 had a periodicity of 574 years and
was responsible for the worldwide flood in the Book of Genesis, by
pouring water on Earth. His announcement revived for another century
fear of comets, now as direct threats to the world instead of signs
of disasters. Spectroscopic analysis in 1910 found the toxic gas
cyanogen in the tail of Halley's Comet, causing panicked buying of
gas masks and quack "anti-comet pills" and
"anti-comet umbrellas" by the public.
Fate of comets
If a comet is traveling fast enough, it
may leave the Solar System. Such comets follow the open path of a
hyperbola, and as such, they are called hyperbolic comets. Solar
comets are only known to be ejected by interacting with another
object in the Solar System, such as Jupiter. An example of this is
Comet C/1980 E1, which was shifted from an orbit of 7.1 million years
around the Sun, to a hyperbolic trajectory, after a 1980 close pass
by the planet Jupiter. Interstellar comets such as 1I/ʻOumuamua and
2I/Borisov never orbited the Sun and therefore do not require a
3rd-body interaction to be ejected from the Solar System.
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. Eventually most of the
volatile material contained in a comet nucleus evaporates, and the
comet becomes a small, dark, inert lump of rock or rubble that can
resemble an asteroid. Some asteroids in elliptical orbits are now
identified as extinct comets. Roughly six percent of the near-Earth
asteroids are thought to be extinct comet nuclei.
Breakup and collisions
The nucleus of some comets may be
fragile, a conclusion supported by the observation of comets
splitting apart. 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. Other splitting comets include 3D/Biela in 1846 and
73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus
reported that a comet split apart as far back as the winter of
372–373 BC. Comets are suspected of splitting due to thermal
stress, internal gas pressure, or impact.
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.
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 minor meteor shower,
the Andromedids, occurs annually in November, and it is caused when
Earth crosses the orbit of Biela's Comet.
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 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.
The names given to comets have followed
several different conventions over the past two centuries. Prior to
the early 20th century, most comets were 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 Edmond Halley demonstrated that
the comets of 1531, 1607, and 1682 were the same body and
successfully predicted its return in 1759 by calculating its orbit,
that comet became known as Halley's Comet. Similarly, the second and
third known periodic comets, Encke's Comet and Biela's Comet, 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 appearance.
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. For example, in
2019, astronomer Gennadiy Borisov observed a comet that appeared to
have originated outside of the solar system; the comet was named
2I/Borisov after him.
History of study
From ancient sources, such as Chinese
oracle bones, it is known that comets have been noticed by humans for
millennia. 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.
Aristotle (384–322 BC) was the first
known scientist to use various theories and observational facts to
employ a consistent, structured cosmological theory of comets. He
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. Aristotle's cometary theory arose from his
observations and cosmological theory that everything in the cosmos is
arranged in a distinct configuration. Part of this configuration was
a clear separation between the celestial and terrestrial, believing
comets to be strictly associated with the latter. According to
Aristotle, comets must be within the sphere of the moon and clearly
separated from the heavens. Also in the 4th century BC, Apollonius of
Myndus supported the idea that comets moved like the planets.
Aristotelian theory on comets continued to be widely accepted
throughout the Middle Ages, despite several discoveries from various
individuals challenging aspects of it.
In the 1st century AD, Seneca the
Younger questioned Aristotle's logic concerning comets. Because of
their regular movement and imperviousness to wind, they cannot be
atmospheric, and are more permanent than suggested by their brief
flashes across the sky. He pointed out that only the tails are
transparent and thus cloud-like, and argued that there is no reason
to confine their orbits to the zodiac. In criticizing Apollonius of
Myndus, Seneca argues, "A comet cuts through the upper
regions of the universe and then finally becomes visible when it
reaches the lowest point of its orbit." While Seneca did not
author a substantial theory of his own, his arguments would spark
much debate among Aristotle's critics in the 16th and 17th centuries.
Also in the 1st century, Pliny the
Elder believed that comets were connected with political unrest and
death. Pliny observed comets as "human like", often
describing their tails with "long hair" or "long
beard". His system for classifying comets according to their
color and shape was used for centuries.
In India, by the 6th century
astronomers believed that comets were celestial bodies that
re-appeared periodically. This was the view expressed in the 6th
century by the astronomers Varāhamihira and Bhadrabahu, and the
10th-century astronomer Bhaṭṭotpala listed the names and
estimated periods of certain comets, but it is not known how these
figures were calculated or how accurate they were. In 1301, the
Italian painter Giotto was the first person to accurately and
anatomically portray a comet. In his work Adoration of the Magi,
Giotto's depiction of Halley's Comet in the place of the Star of
Bethlehem would go unmatched in accuracy until the 19th century and
be bested only with the invention of photography.
Astrological interpretations of comets
proceeded to take precedence clear into the 15th century, despite the
presence of modern scientific astronomy beginning to take root.
Comets continued to forewarn of disaster, as seen in the Luzerner
Schilling chronicles and in the warnings of Pope Callixtus III. In
1578, German Lutheran bishop Andreas Celichius defined comets as "the
thick smoke of human sins... kindled by the hot and fiery anger of
the Supreme Heavenly Judge". The next year, Andreas Dudith
stated that "If comets were caused by the sins of mortals,
they would never be absent from the sky."
Scientific approach
Crude attempts at a parallax
measurement of Halley's Comet were made in 1456, but were erroneous.
Regiomontanus was the first to attempt to calculate diurnal parallax
by observing the great comet of 1472. His predictions were not very
accurate, but they were conducted in the hopes of estimating the
distance of a comet from the Earth.
In the 16th century, Tycho Brahe and
Michael Maestlin demonstrated that comets must exist outside of
Earth's atmosphere by measuring the parallax of the Great Comet of
1577. Within the precision of the measurements, this implied the
comet must be at least four times more distant than from Earth to the
Moon. Based on observations in 1664, Giovanni Borelli recorded the
longitudes and latitudes of comets that he observed, and suggested
that cometary orbits may be parabolic. Galileo Galilei, one of the
most renowned astronomers to date, even attempted writings on comets
in The Assayer. He rejected Brahe's theories on the parallax of
comets and claimed that they may be a mere optical illusion.
Intrigued as early scientists were about the nature of comets,
Galileo could not help but throw about his own theories despite
little personal observation. Maestlin's student Johannes Kepler
responded to these unjust criticisms in his work Hyperaspistes. Jakob
Bernoulli published another attempt to explain comets (Conamen Novi
Systematis Cometarum) in 1682.
Also occurring in the early modern
period was the study of comets and their astrological significance in
medical disciplines. Many healers of this time considered medicine
and astronomy to be inter-disciplinary and employed their knowledge
of comets and other astrological signs for diagnosing and treating
patients.
Isaac Newton, in his Principia
Mathematica of 1687, proved that an object moving under the influence
of gravity by an inverse square law 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. He describes 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. He
suspected that comets were the origin of the life-supporting
component of air. He also pointed out that comets usually appear near
the Sun, and therefore most likely orbit it. On their luminosity, he
stated, "The comets shine by the Sun's light, which they
reflect," with their tails illuminated by "the Sun's
light reflected by a smoke arising from [the coma]".
In 1705, Edmond Halley (1656–1742)
applied Newton's method to 23 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 caused 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.
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 Halley's Comet.
From his huge vaporing 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)
As early as the 18th century, some
scientists had made correct hypotheses as to comets' physical
composition. In 1755, Immanuel Kant hypothesized in his Universal
Natural History that comets were condensed from "primitive
matter" beyond the known planets, which is "feebly
moved" by gravity, then orbit at arbitrary inclinations, and
are partially vaporized by the Sun's heat as they near perihelion. 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.
In the 19th century, the Astronomical
Observatory of Padova was an epicenter in the observational study of
comets. Led by Giovanni Santini (1787–1877) and followed by
Giuseppe Lorenzoni (1843–1914), this observatory was devoted to
classical astronomy, mainly to the new comets and planets orbit
calculation, with the goal of compiling a catalog of almost ten
thousand stars. Situated in the Northern portion of Italy,
observations from this observatory were key in establishing important
geodetic, geographic, and astronomical calculations, such as the
difference of longitude between Milan and Padua as well as Padua to
Fiume. In addition to these geographic observations, correspondence
within the observatory, particularly between Santini and another
astronomer Giuseppe Toaldo, about the importance of comet and
planetary orbital observations.
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 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.
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.
The detection was made by using the far-infrared abilities of the
Herschel Space Observatory. 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." 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).
Spacecraft missions
The Halley Armada describes the
collection of spacecraft missions that visited and/or made
observations of Halley's Comet 1980s perihelion. The space shuttle
Challenger was intended to do a study of Halley's Comet in 1986, but
exploded shortly after being launched.
Deep Impact. 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. 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 vaporized water
that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of
Comet Hartley 2 on 4 November 2010.
Ulysses. In 2007, the Ulysses probe
unexpectedly passed through the tail of the comet C/2006 P1
(McNaught) which was discovered in 2006. Ulysses was launched in 1990
and the intended mission was for Ulysses to orbit around the sun for
further study at all latitudes.
Stardust. 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).
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. As a
result, comets also contain crystalline grains that formed in the
early, 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". These new results have forced scientists to
rethink the nature of comets and their distinction from asteroids.
Rosetta. The Rosetta probe orbited
Comet Churyumov–Gerasimenko. On 12 November 2014, its lander Philae
successfully landed on the comet's surface, the first time a
spacecraft has ever landed on such an object in history.
Classification
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. 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 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.
Comet West, which appeared three years later, had much lower
expectations but became an extremely impressive comet.
The Great Comet of 1577 is a well-known
example of a great comet. It passed near Earth as a non-periodic
comet and was seen by many, including well-known astronomers Tycho
Brahe and Taqi ad-Din. Observations of this comet led to several
significant findings regarding cometary science, especially for
Brahe.
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.
Sun-grazing comets
A sun-grazing comet is a comet that
passes extremely close to the Sun at perihelion, generally within a
few million kilometers. Although small sun-grazers can be completely
evaporated during such a close approach to the Sun, larger
sun-grazers can survive many perihelion passages. However, the strong
tidal forces they experience often lead to their fragmentation.
About 90% of the sun-grazers 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 remainder contains
some sporadic sun-grazers, 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
exhibit unusual properties. Comet Encke (2P/Encke) 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. 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 1990 UL3.
Largest
The largest known periodic comet is
95P/Chiron at 200 km in diameter that comes to perihelion every 50
years just inside of Saturn's orbit at 8 AU. The largest known Oort
cloud comet is suspected of being Comet Bernardinelli-Bernstein at
≈150 km that will not come to perihelion until January 2031 just
outside of Saturn's orbit at 11 AU. The Comet of 1729 is estimated to
have been ≈100 km in diameter and came to perihelion inside of
Jupiter's orbit at 4 AU.
Centaurs
Centaurs typically behave with
characteristics of both asteroids and comets. Centaurs can be
classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT
was discovered while it exhibited a coma, and so is classified as a
comet despite its orbit, and 60558 Echeclus was discovered without a
coma but later became active, and was then classified as both a comet
and an asteroid (174P/Echeclus). One plan for Cassini involved
sending it to a centaur, but NASA decided to destroy it instead.
Observation
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 sun-grazing
comet online by downloading images accumulated by some satellite
observatories such as SOHO. SOHO's 2000th comet was discovered by
Polish amateur astronomer Michał Kusiak on 26 December 2010 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. There are at
least 18 comets that fit this category.
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 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) and Eudora Welty, to whose life Mary Chapin Carpenter
dedicated the song "Halley Came to Jackson".
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, Earth passed through the comet's tail, and erroneous
newspaper reports inspired a fear that cyanogen in the tail might
poison millions, whereas the appearance of Comet Hale–Bopp in 1997
triggered the mass suicide of the Heaven's Gate cult.
In science fiction, the impact of
comets has been depicted as a threat overcome by technology and
heroism (as in the 1998 films Deep Impact and Armageddon), or as a
trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies
(Night of the Comet, 1984). In Jules Verne's Off on a Comet a group
of people are stranded on a comet orbiting the Sun, while a large
crewed space expedition visits Halley's Comet in Sir Arthur C.
Clarke's novel 2061: Odyssey Three.
In Literature
The long-period comet first recorded by
Pons in Florence on 15 July 1825 inspired Lydia Sigourney's humorous
poem Wikisource-logo.svg The Comet of 1825. in which all the
celestial bodies argue over the comet's appearance and purpose.
Footnotes
"I do not think that a comet is
just a sudden fire, but that it is among the eternal works of
nature." (Sagan & Druyan 1997, p. 26)
Seneca is quoted as stating, "Why
... are we surprised that comets, such a rare spectacle in the
universe, are not yet grasped by fixed laws and that their beginning
and end are not known, when their return is at vast intervals? ...
The time will come when diligent research over very long periods of
time will bring to light things which now lie hidden."
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