THE SOLAR SYSTEM
Short-period Comets
Encke’s Comet, as
photographed by Jim Scotti
on 5 January 1994, using the
0.91-m Spacewatch
Telescope on Kitt Peak.S
hort-period comets come from the Kuiper Belt, but
long-period comets come from the so-called Oort
Cloud, a swarm of these icy bodies orbiting the Sun at a
distance of more than a light-year. If one of the members
of the Cloud is perturbed for any reason, it may start to
fall inwards towards the Sun, and eventually it will invade
the inner part of the Solar System. One of several things
may happen. The comet may simply swing round the Sun
and return to the Oort Cloud, not to be back for many
centuries – or even thousands or millions of years. It may
fall into the Sun, and be destroyed. It may be perturbed
by a planet (usually Jupiter) and either thrown out of
the Solar System altogether, or else forced into a short-
period orbit which brings it back to perihelion after a
few years. Or it may collide with a planet, as Comet
Shoemaker–Levy 9 did in July 1994, when it impacted
Jupiter. But really brilliant comets have periods so long
that we cannot predict the date of their appearance, and
they are always apt to arrive without warning and take us
by surprise.
It is a pity that all the comets with periods of less than
half a century are faint. No doubt they were much more
imposing when they first plunged sunwards, but by now
they are mere ghosts of their former selves. Encke’s
Comet, the first to be identified, is a case in point. It was
originally found in 1786 by the French astronomer Pierre
Méchain, when it was of the fifth magnitude, and had a
short tail. It was seen again in 1795 by Caroline Herschel,
William Herschel’s sister, and yet again in 1805 by
Thulis, from Marseilles. In 1818 it turned up once more,
and was detected by Jean Louis Pons, whose grand total
of comet discoveries amounted to 37. (Pons’ story is
unusual, because he began his career as an observatory
doorkeeper and ended it as an observatory director.) The
orbit was calculated by J. F. Encke, of Berlin, who con-
cluded that the comets of 1786, 1795, 1805 and 1818
were one and the same; he gave the period as 3.3 years,
and predicted a return for 1822. The comet duly appeared
just where Encke had expected, and, very appropriately,
was named after him. Since then it has been seen at
every return except that of 1945, when it was badly
placed and when most astronomers had other things on
their minds.
At some returns during the last century Encke’s Comet
was quite prominent; in 1829 it reached magnitude 3.5,
with a tail 18 minutes of arc in length. Nowadays it does
not achieve such eminence, and although it is hard to be
sure – estimating comet magnitudes is far from easy – it
does seem to have faded. Whether it will survive into the
22nd century remains to be seen.
Encke’s Comet has a small orbit; at perihelion it
ventures just inside the orbit of Mercury, while at its
furthest from the Sun it moves out into the asteroid zone.
Modern instruments can follow it all around its path;
its period is the shortest known. In 1949 a new comet,
Wilson–Harrington, was believed to have a period of only
2.4 years, but it was not seen again until 1979, when it was
recovered – this time as an asteroid, designated No. 4015!
There is little doubt that it has changed its status, and
it may well be that many of the small close-approach
asteroids, such as Phaethon, are ex-comets which have lost
all their volatiles.
Biela’s Comet met with a sad fate. It was discovered
in 1772 by Montaigne from Limoges, recovered by Pons
in 1805 and again by an Austrian amateur, Wilhelm von
Biela, in 1826. The period was given as between six and
seven years, and it returned on schedule in 1832, when it
was first sighted by John Herschel. (It was unwittinglyresponsible for a major panic in Europe. The French
astronomer Charles Damoiseau had predicted that the
orbit of the comet would cut that of the Earth; he was
quite right, but at that time the comet was nowhere near
the point of intersection.) Biela’s Comet was missed in
1839 because of its unfavourable position in the sky, but
it came back once more in 1846, when it astonished
astronomers by splitting in two. The pair returned in
1852, were missed in 1859 again because they were
badly placed, and failed to appear at the expected return
of 1866 – in fact they have never been seen again. When
they ought to have returned, in 1872, a brilliant meteor
shower was seen coming from that part of the sky
where the comet had been expected, and there is no doubt
that the meteors represented the funeral pyre of the
comet. The shower was repeated in 1885, 1892 and 1899,
but no more brilliant displays have been seen since then;
to all intents and purposes the shower has ceased, so
that we really have, regretfully, seen the last of Biela’s
Comet.
Other periodical comets have been ‘mislaid’, only to
be found again after a lapse of many years; thus Holmes’
Comet, which reached naked-eye visibility in 1892 and
had a period of nearly seven years, was lost between 1908
and 1965; it has since been seen at several returns, but is
excessively faint.
Comet Brooks 2 made a close approach to Jupiter in
1886, when it actually moved inside the orbit of Io and
was partially disrupted, spawning four minor companion
comets which soon faded away. During the Jupiter
encounter, the orbit was changed from 29 years to its pre-
sent value of seven years.
Comet Schwassmann–Wachmann 1 is of unusual
interest. Its orbit lies wholly between those of Jupiter and
Saturn, and normally it is very faint, but sometimes it
shows sudden outbursts which bring it within the range of
small telescopes. Large instruments can follow Comet
Schwassmann–Wachmann 1 all round its orbit, as is also
the case with a few other comets with near-circular paths,
such as Smirnova–Chernykh and Gunn. Oterma’s Comet
used to have a period of 7.9 years, but an encounter with
Jupiter in 1973 altered this to 19.3 years, and the comet
now comes nowhere near the Earth, so that its future
recovery is very doubtful.Name Year of Period, Eccentricity Incl. Dist. from Sun,
discovery years astr. units
min. max.
Encke* 1786 3.3 0.85 12.0 0.34 4.10
Grigg–Skjellerup 1902 5.1 0.66 21.1 0.99 4.93
Tempel 2 1873 5.3 0.55 12.5 1.38 4.70
Pons–Winnecke 1819 6.4 0.64 22.3 1.25 5.61
D’Arrest 1851 6.4 0.66 16.7 1.29 5.59
Kopff 1906 6.4 0.55 4.7 1.58 5.34
Schwassmann–
Wachmann 2 1929 6.5 0.39 3.7 2.14 4.83
Giacobini–Zinner 1900 6.6 0.71 13.7 1.01 6.00
Borrelly 1905 6.8 0.63 30.2 1.32 5.83
Brooks 2 1889 6.9 0.49 5.6 1.85 5.41
Finlay 1886 7.0 0.70 3.6 1.10 6.19
Faye 1843 7.4 0.58 9.1 1.59 5.96
Wolf 1 1884 8.4 0.40 27.3 2.42 5.73
Tuttle 1790 13.7 0.82 54.4 1.01 10.45
Schwassmann–
Wachmann 1* 1908 15.0 0.11 9.7 5.45 6.73
Halley 240 BC 76.0 0.97 162.2 0.59 34.99(* = comets which can be followed throughout their orbits)PERIODICAL COMETS WHICH HAVE BEEN OBSERVED
AT TEN OR MORE RETURNSD108- 151 UNIVERSE UK 2003CB 7/4/03 5:18 pm Page 139