Early observations and thought 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 BCE) 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. In the 1st century CE,
Seneca the Younger questioned Aristotle's logic concerning comets. Because of their regular movement and imperviousness to wind, they cannot be atmospheric, his arguments would spark much debate among Aristotle's critics in the 16th and 17th centuries. His system for classifying comets according to their color and shape was used for centuries. appeared in 1066, prior to the
Battle of Hastings, and is depicted in the
Bayeux Tapestry. There is a claim that an Arab scholar in 1258 noted several recurrent appearances of a comet (or a type of comet), and though it's not clear if he considered it to be a single periodic comet, it might have been a comet with a period of around 63 years. 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. s of the Great Comet of 1577 in his
notebook 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. Despite being a skilled astronomer, in his 1623 book
The Assayer,
Galileo Galilei rejected Brahe's theories on the parallax of comets and claimed that they may be a mere optical illusion, despite little personal observation. During the
early modern period comets were studied for 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. 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. 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 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. Correspondence within the observatory, particularly between Santini and another astronomer Giuseppe Toaldo, mentioned 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."
Spacecraft missions • The
Halley Armada describes the collection of spacecraft missions that visited and/or made observations of Halley's Comet 1980s perihelion. •
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 , 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 passed through the tail of the comet
C/2006 P1 (McNaught) and recorded data on the interaction of the tail and solar winds over a period of five days. •
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 . 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 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 ==