THE SOLAR SYSTEM
Saxons, and it is shown on the Bayeux Tapestry, with
King Harold toppling on his throne and the courtiers look-
ing on aghast. In 1301 it was seen by the Florentine artist
Giotto di Bondone, who used it as a model for the Star of
Bethlehem in his picture The Adoration of the Magi– even
though Halley’s Comet was certainly not the Star of
Bethlehem; it returned in 12 BC, years before the birth
of Christ. At the return of 1456 the comet was condemned
by Pope Calixtus III as an agent of the Devil. It was
prominent in 1835 and in 1910, but unfortunately not in
1986, when it was badly placed and never came within
39 million kilometres (24 million miles) of the Earth, and
though it became an easy naked-eye object it was by no
means spectacular. The next return, that of 2061, will be
no better. We must wait until 2137, when it will again be a
magnificent sight.
The fact that Halley’s Comet can still become brilliant
shows that it came in fairly recently from the Oort Cloud.
It loses about 250 million tonnes of material at each
perihelion passage, but it should survive in more or less
its present form for at least 150,000 years to come.
The comet was first photographed at the 1910 return,
after which it remained out of range until 16 October 1982,
when it was recovered by D. Jewitt and E. Danielson, at
the Palomar Observatory, only six minutes of arc away
from its predicted position; it was moving between the
orbits of Saturn and Uranus. As it drew in towards
perihelion, four spacecraft were sent to it: two Japanese,
two Russian and one European. The European probe
Giotto, named after the painter, invaded the comet’s head,
and on the night of 13–14 March 1986 passed within
605 kilometres (376 miles) of the nucleus. Giotto’s camera
functioned until about 14 seconds before closest approach,
when the spacecraft was struck by a particle about the
size of a rice-grain and contact was temporarily broken; Comet Halley through the
1.5-m (60-inch) telescope at
Mount Wilson Observatory,
California. The head region,
known as the coma, is well
seen in this photograph from
8 May 1910 taken by G. W.
Ritchey. The comet’s tail
flows to the right, away from
the Sun. Short, straight
streaks are background stars.OBSERVED RETURNS
OF HALLEY’S COMET
Year Date of
perihelion
1059 BC 3 Dec
240 BC 25 May
164 BC 12 Nov
87 BC 6 Aug
12 BC 10 Oct
AD 66 25 Jan
141 22 Mar
218 17 May
295 20 Apr
374 16 Feb
451 28 June
530 27 Sept
607 15 Mar
684 2 Oct
760 20 May
837 28 Feb
912 18 July
989 5 Sept
1066 20 Mar
1145 18 Apr
1222 28 Sept
1301 25 Oct
1378 10 Nov
1456 9 June
1531 26 Aug
1607 27 Oct
1682 15 Sept
1759 13 Mar
1835 16 Nov
1910 10 Apr
1986 9 Febin fact the camera never worked again, and the closest
image of the nucleus was obtained from a range of 1675
kilometres (about 1000 miles).
The nucleus was found to be shaped rather like a
peanut, measuring 15 8 8 kilometres (9 5 5 miles),
with a total volume of over 500 cubic kilometres (120
cubic miles) and a mass of from 50,000 million to 100,000
million tonnes (it would need 60,000 million comets of
this mass to equal the mass of the Earth). The main
constituent is water ice, insulated by an upper layer of
black material which cracks in places when heated by the
Sun, exposing the ice below and resulting in dust jets. Jet
activity was very marked during the Giotto pass, though
the jets themselves were confined to a small area on the
sunward side of the nucleus. The central region of the
nucleus was smoother than the ends; a bright 1.5-kilometre
(one-mile) patch was presumably a hill, and there were
features which appeared to be craters with diameters of
around a kilometre (3280 feet). The comet was rotating
in a period of 55 hours with respect to the long axis of
the nucleus.
Tails of both types were formed, and showed marked
changes even over short periods. By the end of April the
comet had faded below naked-eye visibility, but it provided
a major surprise in February 1991, when observers using
the Danish 154-cm (60-inch) reflector at La Silla in Chile
found that it had flared up by several magnitudes. There
had been some sort of outburst, though the reason for it
is unclear.
Giotto survived the Halley encounter, and was then
sent on to rendezvous with a much smaller and less active
comet, P/Grigg–Skjellerup, in July 1992. Despite the
loss of the camera, a great deal of valuable information
was obtained. Unfortunately Giotto did not have enough
propellant remaining for a third cometary encounter.Line of nodes(^192619211915)
1948
(^197219771983)
Neptune Uranus
Saturn Jupiter
1985
1986, Feb. 09▲ Orbit of Halley’s Comet.
Aphelion was reached in
1948; the comet passed
perihelion in 1986, and has
now receded once more as
far as the orbit of Uranus.
The next perihelion will be
in 2061. Nucleus of Halley’s Comet,
from the Halley Multi-colour
Camera carried in Giotto.
The range was 20,000 km
(12,500 miles).D108- 151 UNIVERSE UK 2003CB 7/4/03 5:18 pm Page 141