Missions to Jupiter
S
everal spacecraft have now passed by Jupiter. First
there were Pioneer 10 (December 1973) and Pioneer 11
(December 1974), which carried out preliminary surveys;
Pioneer 11 was subsequently sent on to a brief encounter
with Saturn. Next came the much more sophisticated
Voyagers, No. 1 (March 1979) and No. 2 (July of the same
year). Both these then went on to carry out detailed studies
of Saturn and its satellite system; Voyager 2 went on to
encounter Uranus and Neptune as well. All these four early
probes are now on their way out of the Solar System per-
manently. In February 1992 the Ulysses solar polar probe
passed close to Jupiter, mainly to use the strong gravita-
tional pull of the giant planet to send Ulysses soaring far
out of the plane of the ecliptic, but observations of Jupiter
were also made – the opportunity was too good to be missed.
The latest mission, Galileo, was dispatched in October 1990,
though it followed a somewhat circuitous route and did
not reach its target until 1996.
The most important results have come from the
Voyagers. Particular attention was paid to the magneto-
sphere, which is very extensive. It is not spherical, but has a
long ‘magnetotail’ stretching away from the direction of
the Sun and extends out to well over 700 million kilometres
(over 400 million miles), so that at times it may even engulf
the planet Saturn. There are zones of radiation ten thousand
times stronger than the Van Allen zones of the Earth, so that
any astronaut foolish enough to venture into them would
quickly die from radiation poisoning. Indeed, the unexpect-
edly high level of radiation almost crippled the equipmentATLAS OF THE UNIVERSE
in Pioneer 10, the first probe to pass by the planet, and sub-
sequent spacecraft were aimed so as to pass quickly over
the equatorial region, where the danger is at its worst. The
Voyagers were carefully constructed so as to tolerate twice
the anticipated dose of radiation; only minor effects were
noted when Voyager 1 approached Jupiter to within
350,000 kilometres (220,000 miles), but Voyager 2, passing
at the greater distance of 650,000 kilometres (400,000
miles), found the level to be three times stronger. Evidently
the zones are very variable. The magnetic field itself is
extremely complicated, and is reversed relative to that of the
Earth, so that a compass needle would point south; the mag-
netic axis is inclined to the rotational axis by 10 degrees.
An obscure ring was discovered, made up of three
components which are now known as Halo, Main and
Gossamer. The ring system reaches up to 50,000 kilometres
(31,000 miles) above the cloud tops, and is so faint that it
would have been well nigh impossible to detect from Earth.
It is quite unlike the glorious icy ring-system of Saturn.
Superb pictures of the planet’s surface were obtained,
showing the turbulent, vividly coloured clouds and spots.
Aurorae and lightning flashes were recorded on the night
side, and observations of all kinds were made. It was seen
that marked changes had occurred between the Pioneer
and Voyager passes, and even in the interval between the
Voyager 1 and 2 encounters, the shape of the Red Spot
was different, for example. Jupiter is a turbulent place, and
the ancients were indeed justified in naming it in honour
of the King of the Gods. Voyager views of
Jupiter. The photograph at
top left shows the Great Red
Spot from Voyager 1 on
1 March 1979. Long subject
to observation, the Spot
forms a hollow in the
adjoining belt, and though
it periodically disappears for
a while, it always returns.
Top right shows the North
Equatorial Belt from
Voyager 2, taken on 28 June
1979; the colours have been
deliberately enhanced to
bring out more detail. The
wisp-like plumes of the
equatorial zone can be seen
across the middle of the
image. Bottom left shows
the region just east of the
Great Red Spot in a view
from Voyager 1. The colours
have again been
exaggerated to bring out
subtle variations in shading.
The view at bottom right was
taken by Voyager 2 on
29 June 1979. It extends
from latitude 40ºS to 40ºN,
and shows an equatorial
zone similar to that at top
right. A region of turbulence
is visible in the bottom
righthand corner, just west
of the Great Red Spot,
where western and eastern
winds combine.C Atl of Univ Phil'03stp 2/4/03 3:08 pm Page 98