The Great Christ Comet (17 page)

Tails

Basically there are two types of tails that comets may (or may not) have: (1) gas (sometimes called “plasma”) tails, which are straight, bluish, structurally fine, and point almost exactly away from the Sun; and (2) dust tails, which are famously yellowish white and thick and may be curved.
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One might imagine that comet tails are like long loose hair flowing out from a motorcyclist's head as she accelerates along at breakneck speed.
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However, comet tails always point in the direction opposite the Sun, regardless of whether they are moving toward it or away from it (see figs. 5.7–8).

Gas tails are formed when the electrically charged gas particles that exploded toward the Sun from the nucleus are pushed by the solar wind directly back behind the nucleus. Because of this, gas tails are generally straight, narrow, and point directly away from the Sun. The sunlight causes them to fluoresce.

Dust tails are formed when small dust particles expelled from the comet's nucleus toward the Sun are pushed behind the coma by solar radiation pressure. Because dust particles travel more slowly than gas particles, they lag behind the comet, with the result that dust tails tend to be more curved and broad than gas tails and not directly anti-solar, but rather angled back toward the direction from which the comet has just come (
fig. 5.7
). The section of the dust tail farthest from the coma is less dense, less bright, and more curved. A tail's curvature and length are typically greatest just after the comet's closest encounter with the Sun, when it is traveling fastest and is at its most productive. In the case of a comet steeply inclined to the plane of Earth's orbit around the Sun (the “ecliptic”
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; see
fig. 5.24
and
fig. 7.12
), this curvature may be very apparent to Earth-dwellers, and the dust tail may seem wider and be easily distinguishable from the gas tail. However, where a comet's orbit is angled narrowly to the ecliptic plane, the dust tail will generally appear to Earth-dwellers to be narrower, straighter, brighter, and longer, and will combine with the gas tail to form a single tail.
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Some comet tails are mindbogglingly large. Especially long and wide tails are associated with very productive comets that begin reacting to the Sun when far from it. Some hyperactive small nuclei (like that of Hyakutake) produce extraordinarily long tails. But the dream scenario for a long comet tail is a large, very volatile nucleus.

The longest known comet tails in space are Hyakutake's at 3.8 AU (570 million km),
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that of Messier's Comet of 1769 at 3.5 AU (520 million km),
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and that of the Great March Comet of 1843 at 2.15 AU (330 million km).
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See
fig. 5.9
.

As regards the apparent lengths of comets in history, the longest is a 300-degree one recorded in the year AD 905 by the Chinese (C/905 K1).
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200-degree comets were seen in AD 893
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and 1618.
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Tebbutt's Comet of 1861 was 120 degrees long, sufficient to extend two-thirds of the way across the dome of the sky.
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Great comets in 1618 and 1769, as well as Halley's Comet in AD 837, were longer than 90 degrees, and many other comets in history peaked at 90 degrees (e.g., 1106, 1680, 1843, 1910 [Halley], 1996).
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The apparent length of a comet is determined not just by the actual tail length but also by the comet's brightness, its distance from Earth, and the angle of the comet relative to the Sun and Earth.

Comet dust tails may also be very wide. The tail of Donati's Comet (1858;
fig. 5.12
) was as wide as the Big Dipper's handle (16 degrees),
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while that of Tebbutt's Comet (1861; see
fig. 5.21
) was as broad as the distance between the top two stars in the Big Dipper's bowl (10 degrees).
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Halley's Comet in AD 837 was as wide as four fingers at arm's length (7–8 degrees), and in 1066 it was as wide as the Big Dipper's bowl is high (5 degrees), as
were comets in 1106 and 1471.
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In 1965 the end of Comet Ikeya-Seki's tail was as wide as the distance between the 3 stars on Orion's belt (3 degrees).
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In addition, the Great Comet of 1577 (
fig. 5.10
) was as wide as the Andromeda Galaxy (2.5 degrees).
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The Great Comet of 1680 (figs. 5.15; 6.7; 10.19) was described as being “like a very wide belt” that stretched from one side of the horizon to the other with little difference in its breadth
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(2 degrees).
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It is surely one of the most remarkable wonders of our solar system that relatively small bodies like comet nuclei may give rise to tails so astonishingly long and wide.

Antitail

Occasionally, comets may have a thin “mini-tail” that points toward the Sun—the so-called antitail.

This happens, from the Earth-dweller's vantage point, when a comet's orbit takes it close to the plane of Earth's orbit. The antitail consists of larger particles of dust recently expelled from the comet that are not so easily pushed back behind the comet and remain in the same orbital plane as the coma. When Earth moves near to the plane of the comet's orbit, observers can see not just the main part of the tail on the side of the comet away from the Sun, but also some of this larger material still remaining close to its orbital plane, which appears to poke out in front of the coma on the sunward side.
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An antitail may also be seen when Earth's position relative to a productive comet is such that the rear part of the comet's tail is seen, in projection, on the sunward side of the coma.
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Antitails may take the form of a fan or a spike, depending on the position of the comet relative to the ecliptic plane.
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Comet Arend-Roland on April 22, 1957, had a fan antitail and on April 25 a spike (
fig. 5.13
).