Observational history of comets

The Book of Miracles (Augsburg, 16th century).

Comets have been observed by humanity for thousands of years, but it is only in the past few centuries that they have been studied as astronomical phenomena.

Early observations and thought

Until the first 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. From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. One very famous recording of a comet is the appearance of Halley's Comet as a terrifying omen on the Bayeux Tapestry, which records the Norman conquest of England in AD 1066.

In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky. Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.

A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky. However, the Aristotelian viewpoint proved more influential, and it was not until the 16th century that 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.

Orbital studies

The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia

Though comets had now been demonstrated to be in space, the question of how they moved would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the Sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies - he believed that comets travel among the planets along straight lines, and it required Edmund Halley to prove that their orbits are in fact curved. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelian notion of comets moving on straight lines through the upper atmosphere.

The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In 1681, the Saxon pastor Georg Samuel Doerfel set forth his proofs that comets are heavenly bodies moving in parabolas of which the Sun is the focus. Then Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.

In 1705, Edmond Halley (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. (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618, while Giovanni Domenico Cassini had suspected the identity of the comets of 1577, 1665, and 1680. Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. When the comet returned as predicted, it became known as Halley's Comet (with the latter-day designation of 1P/Halley). It next appears in 2061.

Among the comets with short enough periods to have been observed several times in the historical record, Halley's Comet is unique in that it is consistently bright enough to be visible to the naked eye while passing through the inner Solar System. Since the confirmation of the periodicity of Halley's Comet, quite a few other periodic comets have been discovered through the use of the telescope. The second comet found to have a periodic orbit was Encke's Comet (with the official designation of 2P/Encke). During the period 1819–21 the German mathematician and physicist Johann Franz Encke computed the orbits for a series of comets that had been observed in 1786, 1795, 1805, and 1818, and he concluded that they were same comet, and successfully predicted its return in 1822. By 1900, seventeen comets had been observed through more than one passage through their perihelions, and then recognized as being periodic comets. As of July 2014, 305 comets have achieved this distinction, although several of these have disintegrated or been lost.

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)

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. Newton also believed that the vapors given off by comets might replenish the planets' supplies of water (which was gradually being converted into soil by the growth and decay of plants) and the Sun's supply of fuel.

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor 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.

However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift–Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and was never seen again after 1852. A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.

By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted 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.

According to research, large comets with a radius of over 10 kilometers could contain liquid water at their cores by the decay of radioactive isotopes of aluminum or iron.

Spacecraft targets

In recent years, spacecraft have visited comets, generating a host of new findings. The following table lists comets that have been visited by spacecraft.

Comet	Year of discovery	Spacecraft	Year of visit	Closest approach (km)	Notes
Giacobini–Zinner	1900	ICE	1985	7800	First flyby of comet
Halley	Known since antiquity	Vega 1	1986	8889	Flyby
Halley		Vega 2	1986	8030	Flyby
Halley		Suisei	1986	151000	Distant flyby
Halley		Giotto	1986	596	Flyby
Grigg–Skjellerup	1902	Giotto	1992	200	Flyby
Borrelly	1904	Deep Space 1	2001	?	Flyby
Wild 2	1978	Stardust	2004	240	Flyby; first returned samples from comet to Earth
Tempel 1	1867	Deep Impact	2005	Impacted	Flyby; first impact to comet; blasted a crater using an impactor
Hartley 2	1986	EPOXI (was Deep Impact)	2010	700	Flyby; smallest comet visited
Tempel 1	1867	Stardust	2011	181	Flyby; imaged the crater created by Deep Impact
Churyumov–Gerasimenko	1969	Rosetta	2014	200	Currently orbiting; first orbiter of comet and first landing on comet

See also: Comet § Spacecraft missions, and List of comets visited by spacecraft