Missions to Uranus
ATLAS OF THE UNIVERSE
O
nly one spacecraft has so far encountered Uranus. On
24 January 1986, Voyager 2 flew past the planet at a
distance of 80,000 kilometres (50,000 miles), and told us
more in a few hours than we had been able to find out all
through the whole of scientific history.
Several new inner satellites were discovered during
Voyager’s approach, but little could be seen on the disk
itself; of course the planet was being seen pole-on, so that
the equator lay round the rim of the disk (it was rather like
aiming at the bull’s eye of a dart board). Ten new satellites
were discovered in all, all within the orbit of Miranda.
Finally some cloud formations were made out, mainly in
the range of latitudes from 20 to 45 degrees where sunlight
can penetrate to slightly warmer levels, but all the clouds
are very obscure, and in general Uranus appears almost
featureless even from close range. Windspeeds could be
measured, and, to general surprise, seem to be strongest
at high levels in the atmosphere; there is a westwards air
current at low latitudes, and a meandering eastwards jet-
stream further from the equator.
Uranus has a decidedly bluish-green hue. This is
because of the abundance of methane in the upper clouds;
methane absorbs red light and allows the shorter wave-
lengths to be reflected. In the Uranian atmosphere it seems
that water, ammonia and methane condense in that order
to form thick, icy cloud-layers. Methane freezes at the
lowest temperature, and so forms the top layer, above
which comes the hydrogen-rich atmosphere. Aurorae were
seen on the planet’s night side, and on the day side ultra-
violet observations showed strong emissions, producing
what is termed the electroglow – the origin of which is
still unclear.
As expected, Uranus is a source of radio waves, and
there is a fairly strong magnetic field. The surprising
fact about this is that the magnetic axis is displaced by
58.6 degrees from the axis of rotation. Moreover, the
magnetic axis does not even pass through the centre of
the globe; it is displaced by more than 7,500 kilometres
(4,700 miles), and the polarity is opposite to that of the
Earth. The reason for the tilt of the magnetic axis is not
known. Initially it was believed to be connected in some
way with the 98-degree tilt of the axis of rotation, but
since Neptune has since been found to share the same
peculiarity we must think again. The windsock-shaped
magnetosphere is so extensive that all the members of the
satellite family are engulfed by it.
Voyager was able to make a detailed survey of the
ring-system. Ten individual rings have been identified,
plus a broad sheet of material closer-in than the main sys-
tem; the nomenclature is frankly chaotic, and one can only
hope that in the future it will be revised. All the rings are
very thin, with remarkably sharp borders; their thickness
cannot be more than a few tens of metres (40 to 100 feet),
and they are probably made up of boulders a metre or
two (3 to 7 feet) in diameter. There are not many smaller
centimetre-sized objects.
All the rings of Uranus are not alike. The outer or
Epsilon ring is not symmetrical. It is variable in width.
The part of it closest to Uranus is around 20 kilometres
(12.5 miles) wide, while the part furthest from the planet
has a maximum width of around 100 kilometres (about
60 miles). All the other rings are much narrower, and some
of them show definite structure. The satellites Cordelia
and Ophelia act as ‘shepherds’ to the Epsilon ring; a close
search was made for shepherd satellites of the other rings,
but without success. The rings of Uranus are as black as
coal-dust, and are totally unlike the magnificently colourful
icy rings that surround Saturn.▲ Complete ring system
of Uranus (Voyager 2).
In addition to the ten rings,
there is a broad sheet of
material closer-in than Ring
6, extending from 39,500 to
23,000 km from Uranus.▼ Uranus on 25 January
1986.As Voyager 2 left
and set forth on its cruise
to Neptune, the spacecraft
used its wide-angle camera
to photograph this crescent
view of Uranus. Voyager 2was then 1 million km
(about 600,000 miles)
beyond Uranus. The picture,
a composite of images taken
in blue, green and orange,
resolves features 140 km
(90 miles) across.D108- 151 UNIVERSE UK 2003CB 7/4/03 5:15 pm Page 124