Philips Atlas of the Universe

Comets

ATLAS OF THE UNIVERSE

▼ West’s Comet, below left,

and Neat’s Comet, below

right. West’s Comet was a

bright naked-eye object for

several mornings in March

1976, when this photograph

was taken by Akira Fujii.

As the comet receded from

the Sun it showed signs of

disruption in its nucleus,

so that it may not be so

bright when it next returns

to perihelion in

approximately 553,000

years! Neat’s Comet (C2002

V1) is seen here in a

photograph taken on

1 February 2003 by Gordon

Rogers, with his 16-inch

reflector. It has a very

eccentric path, and an orbital

period of about 37,000 years.

C

omets are the least predictable members of the Solar

System. All in all, a comet is a wraith-like object. The

only substantial part is the nucleus, which has been aptly

described as a dirty ice-ball and is never more than

a few kilometres across. When the comet is heated, as it

nears the Sun, the ices in the nucleus start to evaporate, so

that the comet develops a head or coma, which may be

huge; the coma of the Great Comet of 1811 was larger than

the Sun. There may be one or more tails, though many

small comets never produce tails of any sort.

A cometary nucleus is composed of rocky fragments

held together with ices such as frozen ammonia, methane

and water. Tails are of two kinds. A gas or ion tail is

produced by the pressure of sunlight, which drives very

small particles out of the head, while a dust tail is due to

the pressure of the solar wind; in general an ion tail is

straight, while a dust tail is curved. Tails always point

more or less away from the Sun, so that when a comet is

travelling outwards it moves tail-first.

Each time a comet passes through perihelion it loses

material to produce a coma and (in some cases) tails; for

example Halley’s Comet, with a period of 76 years, loses

about 300 million tons of material at each return to the

Sun. This means that by cosmical standards, comets must

be short-lived. Some short-period comets which used to be

seen regularly have now disappeared; such are the comets

of Biela, Brorsen and Westphal. (Comets are usually

named after their discoverers, though occasionally after the

mathematician who first computed the orbit – as with

Halley’s Comet.) A prefix P/ indicates that the comet is

periodical. A comet leaves a ‘dusty’ trail behind it as it

moves along, and when the Earth plunges through one of

these trails the result is a shower of shooting-stars. Most

meteor showers have known parent comets; for instance

the main annual shower, that of early August, is associated

with Comet P/Swift–Tuttle, which has a period of 130

years and last returned to perihelion in 1992.

Because a comet is so flimsy and of such low mass, it

is at the mercy of planetary perturbations, and orbits may

be drastically altered from one cycle to another. The clas-

sic case is that of Lexell’s Comet of 1770, which became a

bright naked-eye object. A few years later it made a close

approach to Jupiter, and its orbit was completely changed,

so that we have no idea where the comet is now.

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