ÐÓ°ÉÔ­´´

THE VOYAGER mission began in 1977 when NASA launched two small spacecraft
towards Jupiter and Saturn. But NASA left its options open: if Voyager 1
achieved the mission’s main objectives at Saturn, the space agency might
send its sister craft, Voyager 2, to Uranus and possibly on to Neptune.

In the event, Voyager 1 performed well. It passed close to Saturn’s
moon Titan, a manoeuvre that sent the spacecraft climbing above the plane
of the Solar System and beyond hope of any further planetary encounters.
But its success allowed Voyager 2 to follow a simpler path past Saturn and
continue towards Uranus and Neptune. Despite some technical problems, Voyager
2 survived to encounter Uranus in 1986. ÐÓ°ÉÔ­´´s at the Jet Propulsion
Laboratory in Pasadena, California, then directed it on to Neptune.

Voyager’s encounter with Neptune began two months ago, on 5 June. Voyager
was then starting to send back images of Neptune that are far better than
we can obtain from Earth, even though the spacecraft was months away from
its closest approach. At the beginning of the year, Voyager had already
revealed a dark band around the planet’s southern hemisphere. In April,
it had detected a large dark spot, proportionately similar in size to Jupiter’s
Great Red Spot, lying 25Degree south of the equator of Neptune.

In July, one of the Voyager scientists, Steven Synnott, announced his
discovery of a new moon, first seen in pictures taken in mid-June. The new
satellite, known for the moment as 1989 N1, is about 400 kilometres in diameter
and has a circular, equatorial orbit 117 000 kilometres from the centre
of Neptune.

The ‘far encounter phase’ begins on 6 August, 19 days before the closest
approach. At this distance, the planet with its rings has swollen in size
until it takes two or more images of the narrow-angle camera to cover the
entire system. The searches for the rings and further new satellites, together
with visible and infrared observations of Neptune, take place during this
two-week period.

The ‘near encounter phase’, six days around the closest approach on
25 August, will allow astronomers to make detailed observations of the planet,
its moons and its rings. Apart from 1989 N1, there are two known moons:
Triton and the smaller satellite Nereid. The closest approach to Nereid
takes place before the spacecraft passes Neptune but Voyager will still
be more than 4.5 million kilometres away, so the images of Nereid will show
little detail. Astronomers do not know the size of Nereid very accurately.
The best estimate is a diameter of 700 kilometres. If this is correct, its
width will occupy only 18 picture elements, or pixels, in an image 800 pixels
across – even in Voyager’s best pictures.

At about 4 am on 25 August, Voyager will sweep 4850 kilometres above
Neptune at a latitude of 76 degrees north, its closest approach to a planet
or satellite since it left Earth in 1977. Five hours later, it will pass
Triton at a distance of 40 000 kilometres, taking a series of images that,
if the atmosphere is clear enough, should show features on the surface of
Triton as small as 1 kilometre across.

Neptune orbits the Sun at an average distance of 4500 million kilometres.
The planet takes 165 years to complete one journey around the Sun, and so
it has not yet completed an orbit since it was discovered in 1846. Astronomers
tried for many years to detect structures in the atmosphere similar to the
bands and spots of Jupiter. Without these details, astronomers had considerable
problems in working out the period of rotation of the planet; early estimates
ranged between 7 and 18 hours. Neptune takes up only 2 seconds of arc when
viewed from Earth, one-thousandth of the apparent width of the Moon, so
it was also difficult to determine the planet’s diameter accurately. For
many years, no one was sure whether Neptune was larger or smaller than Uranus.

Today, astronomers think that the period of rotation at the level of
the clouds is 18 hours. The planet’s diameter across the equator is estimated
as 50 538 kilometres, slightly less than that of Uranus. Because the rotation
of both planets flattens them at the poles, however, Neptune is actually
wider at its equator than Uranus is from pole to pole. Paradoxically, Neptune
is considerably more massive than Uranus, weighing in at about 17.2 times
the mass of the Earth compared with 14.6 Earth masses for Uranus.

Astronomers usually group all four giant planets together, but Uranus
and Neptune both differ significantly from Jupiter and Saturn, the larger
pair. In particular, Jupiter and Saturn are made mostly of hydrogen and
helium with only small quantities of more complex molecules such as methane,
ammonia and water. For Uranus and Neptune, these proportions are reversed.
Because these planets formed further from the Sun than Jupiter and Saturn,
the differences presumably reflect variations in the composition of the
primaeval disc of gas and dust from which the planets were formed.

The structure of Neptune is similar to that of Uranus. It has a rocky
core about 16 000 kilometres in diameter, overlain by a slushy layer dominated
by water, liquid methane and liquid ammonia. Above this lies a thick atmosphere
containing most of the planet’s hydrogen and helium. It also contains methane
and ethane, as shown by the spectrum of sunlight reflected from the atmosphere.
Gases in the atmosphere absorb light from the Sun at distinctive wavelengths,
so an analysis of the spectrum allows scientists to identify atoms and molecules
present in the Neptunian atmosphere. An interesting, although probably academic,
possibility is that at the temperatures and pressures prevailing at the
centre of the planet, methane will decompose into hydrogen and carbon. The
carbon may then crystallise, forming a layer of diamond around the core.

There are also significant differences between Uranus and Neptune. The
atmosphere of Uranus is clear and featureless to great depths, for example,
but Neptune has clouds. There is a high-altitude layer of haze, probably
made up of ice crystals or tiny particles. This layer, whose extent can
change over a matter of a few weeks, absorbs sunlight and warms the upper
atmosphere. Another important difference is that Neptune, unlike Uranus,
has an internal heat source. The evidence for this is straightforward. Despite
being much further from the Sun, Neptune has almost the same temperature
as Uranus, implying that Neptune radiates 2.4 times as much energy as it
receives from the Sun.

Astronomers do not know what provides this energy. Frictional heating
from tides raised in the planet by its large moon Triton would not supply
enough energy. The most likely explanation seems to be that gravitational
potential energy is released as denser material within Neptune settles out
and sinks towards the core of the planet.

At present, before the Voyager encounter, what astronomers know of the
moons, Triton and Nereid, indicates that they are both rather unusual. Gerard
Kuiper, an American astronomer who specialised in studying the Solar System,
discovered the smaller satellite, Nereid, in 1949. Nereid follows a very
elongated orbit, ranging between 1.4 and 9.7 million kilometres from Neptune.
Observers on Earth have found that Nereid can vary in brightness by a factor
of four from day to day. This implies that its reflectivity is remarkably
different from one hemisphere to another. Images from the Voyagers showed
this is the case on Saturn’s moon Iapetus. Unlike Iapetus, however, Nereid
does not keep the same face towards its parent planet. The orbit of Nereid
is so eccentric that its rotation does not match its orbital period.

A British amateur astronomer, William Lassell, discovered the large
moon, Triton, on 10 October 1846, only a few weeks after the discovery of
Neptune itself. The orbit of Triton is nearly circular, about 350 000 kilometres
from the surface of Neptune and tipped at 20Degree to the planet’s equator.
It completes one revolution every 5 days and 21 hours. The precise diameter
of Triton is uncertain, with estimates ranging between 3000 and 4000 kilometres,
but in any event it is one of the largest satellites in the Solar System.

Unusually for such a large body, Triton moves around Neptune in the
opposite way to most of the rest of the Solar System: its motion is ‘retrograde’.
Retrograde orbits are fairly common among small planetary satellites, which
are probably captured asteroids, but Triton is by far the largest moon to
behave in this way. Its large size poses problems for theories that explain
Triton as a body captured by Neptune. One consequence of its retrograde
motion is that Triton’s orbit is unstable; it is spiralling down towards
Neptune. ÐÓ°ÉÔ­´´s have calculated that within 10 to 100 million years
the moon will approach so close to Neptune that it will be torn apart by
the planet’s gravity.

Triton is one of the few moons to possess an atmosphere. Spectroscopic
studies using telescopes on Earth have revealed nitrogen and methane, but
no one is sure exactly how they might be distributed. Some astronomers think
that the surface is dominated by a sea of liquid nitrogen dotted with icebergs
of solid methane. Others favour a rocky surface with a few patches of methane
ice and most of the nitrogen in the atmosphere. There may also be heavier
hydrocarbon compounds present, because Triton has a reddish colour, which
may come from complex organic molecules produced by photochemical processes
in the atmosphere.

The highly inclined orbit of this moon could produce dramatically varied
seasons. Each of Triton’s poles has a winter lasting 28 years while the
opposite pole basks in prolonged summer sunlight.

At the warmer pole, frozen nitrogen, methane and other gases will vaporise,
thickening the atmosphere, while on the other side, the gases condense into
a polar cap. The result may be an atmosphere that changes size through the
year, waxing and waning with the seasons.

In 1666, Christiaan Huygens, the Dutch physicist, realised that Saturn
had a system of bright rings. In 1977, astronomers observing Uranus passing
across and blocking light from a background star – a process called occultation
– noticed the star flick on and off several times before and after disappearing
behind the planet. They realised that there was a set of thin, dark rings
around Uranus. Two years later, Voyager 1 photographed a diffuse ring round
Jupiter; and searching for Neptunian rings became a popular pastime in the
1980s.

Possible rings?

The searchers have met with mixed fortunes. Astronomers make use of
the fact that methane in the Neptunian atmosphere absorbs light in the infrared
part of the spectrum, so that the planet is only a faint image if they observe
at a wavelength of 2.2 micrometres. This is a good technique to cut down
‘glare’, which might drown out faint rings, but so far these images have
not revealed anything. Studying the occultations of background stars remains
the favoured approach. In 1981, astronomers went back to data from occultations
observed more than a decade earlier and searched for evidence of rings.
The observations showed a distinct fading of the star several minutes after
a main occultation, suggesting that there is a ring about 30 000 kilometres
from the centre of the planet – about 5000 kilometres above the cloud tops.
In the same year, observers in Arizona reported a brief occultation which
they suggested might be due to a satellite 180 kilometres across.

A concerted search for rings began in 1983, involving six different
observing sites around the rim of the Pacific Ocean. It proved negative;
but in the next year, astronomers at two observatories in Chile, La Silla
and Cerro Tololo, both detected a brief fading during an occultation. One
group suggested that they had detected a small satellite. The other group
thought that because the fading did not occur on both sides of the planet,
the structure that they had detected was not a full ring but an arc.

Although many observers found it difficult at first to accept the idea
of partial rings, they can exist in theory. For example, clouds of dust
could accumulate in the same orbit as a moon, but 60Degree ahead or behind
it. There would also need to be undiscovered satellites to confine material
into partial rings. Part of the key to the problem is the moon Triton, whose
retrograde and inclined orbit makes the existence of rings possible only
at certain angles to Neptune’s equator. Furthermore, the gravitational influence
of Triton may significantly warp any rings that exist.

Searches since 1985 involving more than 100 occultations or near occultations
have supported the idea of incomplete rings, or at least rings whose thickness
varies dramatically at different points around their circumference. The
detailed structure of the system remains obscure. The responsibility for
resolving this problem now falls on Voyager 2. During its journey through
the Neptunian system, Voyager will make several searches for rings, ring
arcs and moons that could ‘shepherd’ them. Because of the danger of a collision
damaging the spacecraft, NASA must plan Voyager’s trajectory so that it
is close enough to look for rings but not so close that it hits material
in the rings. Mission scientists believe there are three or four narrow
rings, between 8 and 20 kilometres wide, close to the equatorial plane of
the planet. They could extend about 42 000 kilometres from the cloud tops.
The controllers have aimed Voyager outside this danger zone.

Neptune is the last encounter planned for Voyager 2, so it does not
have to leave the planet in any particular direction. Of several possible
trajectories, NASA chose a path that passes over the north pole before swinging
south past Triton and below the plane of the Solar System. This takes Voyager
2 deep into Neptune’s magnetosphere, where it can measure trapped charged
particles. After this, it will travel behind the planet, where controllers
plan to monitor the refraction of radio waves by the atmosphere of Neptune
to find out about pressure and temperature at different depths. This route
will also allow NASA to use Voyager’s ultraviolet spectrometer to observe
the Sun shining through the planet’s atmosphere, so probing the chemical
composition of the atmosphere.

The chosen path is also close enough to Triton for Voyager to provide
good-quality images and to examine the moon’s atmosphere. Unfortunately,
Voyager cannot pass close to Triton without using Neptune’s gravity to turn
sharply southwards after crossing the north pole. This means flying very
close to the planet. Mission planners have had to consider the problems
of flying through the upper atmosphere of Neptune and risking damage from
frictional heating, loss of control over the attitude of the spacecraft
and unwanted changes in trajectory. Voyager could also collide with the
material that makes up Neptune’s rings.

As well as planning the encounter, NASA has continued to develop new
techniques to obtain the best possible data during the brief encounter so
far away from the Earth. Voyager’s radio signals, taking more than four
hours to reach the Earth, will be faint, so NASA has improved several of
its receiving stations. The radio dishes of the Deep Space Network have
been increased in size from 64 to 70 metres, and various radio telescopes,
including the 64-metre dish at Parkes in Australia and the Very Large Array
at Socorro, New Mexico, will work together with NASA’s receiving stations.

NASA has also improved Voyager’s control system, giving it a more stable
camera platform during the long exposures required to obtain photographs
where the level of sunlight is about one-thousandth of that at the Earth.
The cameras will pan during long exposures (up to 15 minutes during searches
for faint rings) to prevent blurring caused by the relative motion of the
spacecraft and the object being photographed. The control team can make
Voyager turn at a rate one-tenth that of the hour hand on a clock – incredible
precision for a remotely controlled object so far away from earth.

Once past Neptune, Voyager 2 will observe the planet for another five
weeks, studying the dark side and carrying out a further series of instrument
calibrations. Then, its final flyby finished, Voyager 2 will return to ‘cruise
mode’, probing the interplanetary medium towards the edge of the heliosphere,
the point that marks the boundary between the Sun’s sphere of influence
and interstellar space. Although increasing distance and decreasing power
from the tiny nuclear power plants will make them increasingly difficult
to track, both Voyagers could remain in operation well into the next century.
If they do, these robot explorers will have been very well named.

John Davies is an astronomer at the Royal Observatory, Edinburgh, specialising
in infrared observations of stars and objects in the Solar System.