THE last outpost of the Solar System is a weird little world. It takes 248
years to dawdle round the Sun and is locked in a perpetual waltz with its
nearest planetary neighbour. It has a lot in common with the Earth, and its
origins might even explain how our planet came to develop conditions favourable
for life. Yet we know next to nothing about the outermost planet. Indeed, it
might not be a planet at all. It could simply be the lonely survivor of a band
of comets that was flung out of the Solar System long ago.
While spacecraft have visited all the other planets, there has never been a
mission to Pluto. That may change with the dawn of the new millennium. Next
week, NASA will invite scientists to start work on the detailed design of a pair
of spacecraft due to depart early in the next decade and scheduled to arrive at
Pluto around 2010. Dubbed the Pluto Express, the two spacecraft will send back
clear views of Pluto and its moon Charon. One or both may then journey on to the
mysterious Kuiper belt, a band of icy objects that orbits just beyond.
The little we know of the outermost planet comes from Earth-based
observations. Pluto first appeared as a tiny dot on a photograph in 1929. In the
1970s, telescope observations revealed that it has a very close moon, christened
Charon. Signs of methane ice, nitrogen and carbon monoxide have since appeared
on Pluto鈥檚 surface, along with water ice on Charon. Crude mapping of Pluto and
Charon became possible in the late 1980s. This showed that Pluto has a patchy
surface, with bright polar caps, and is also somewhat redder and brighter than
Charon.
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The 1990s have seen our knowledge of Pluto take off, thanks to the Hubble
Space Telescope. Hubble has provided astronomers with accurate measurements of
the radii of Pluto and Charon鈥�1160 and 610 kilometres respectively. And in
March, a team of scientists led by Alan Stern of the Southwest Research
Institute in Colorado released a map of Pluto鈥檚 surface generated from Hubble
images. This shows that Pluto has a more varied surface, in terms of brightness,
than any planet except Earth. As well as bright polar caps, there are bright
patches on a darker equatorial background. 鈥淚t鈥檚 fantastic,鈥� says Stern. 鈥淗ubble
has brought Pluto from a fuzzy distant dot of light, to a world which we can
begin to map, and watch for surface changes.鈥�
But from now on, mapping Pluto will not be so easy. Pluto has a highly
eccentric orbit鈥攊ts distance from the Sun on its 248-year orbit varies
from 29 to 49 times the distance from the Earth to the Sun. Our views have
become better over recent years because Pluto was moving closer to the Sun
reaching its innermost point in 1989. But now it is receding, and as it returns
to the darkness of the outer Solar System, Earth-based telescopes will be
hard-pressed to keep up.
Primeval drama
So the time is ripe to send spacecraft to Pluto, and there are a host of
questions this mission could resolve. One relates to the drama in the very early
life of the Solar System, and the origin of Pluto itself. As a small outer
planet, Pluto may be a link between planets and the Kuiper belt objects (see
鈥淔rozen in time鈥�, New 杏吧原创, 13 April, p 36). These comets, some of
which are giants more than a hundred kilometres across, are mostly found just
beyond the orbit of Pluto. Since the first was discovered in 1992, 28 such
objects have come to light.
Signs of violent impacts in the outer Solar System suggest that the Kuiper
belt must originally have contained more than a thousand icy bodies measuring
about 1000 kilometres across, as well as many more small ones. The clues come
first from Neptune鈥檚 moon Triton. The satellites that orbit the outer planets
probably formed from a leftover disc of gas and dust surrounding each planet,
and most orbit in an anticlockwise direction as viewed from north. But Triton
orbits Neptune clockwise, suggesting that Neptune captured the moon from a solar
orbit. Second, Uranus has a curiously tilted axis. Compared to the Earth and the
other planets, it lies on its side. This is probably the result of a giant
impact with a Kuiper belt object.
Some astronomers suggest that Pluto was once one of the many balls of ice in
the Kuiper belt. 鈥淧luto was probably the largest member of the Kuiper belt,鈥�
says Jonathan Lunine of the University of Arizona, Tucson, who chaired the
science team for the Pluto Express. 鈥淏ut we won鈥檛 know for sure until we鈥檝e
studied both Pluto and a smaller Kuiper belt object closely.鈥� In a lottery of
celestial mechanics, most of the large objects on the Kuiper belt would have
been flung outwards by close encounters with Neptune and Uranus. But a lucky
few, like Pluto and Charon, became trapped in a stable resonance with Neptune,
preventing further encounters.
Strangely enough, if Pluto does turn out to have been a member of the Kuiper
belt, it could tell us something about the early history of the Earth. No one
knows how the Earth managed to retain its water as it formed. Conditions then
were too hot for water to condense. It is possible that the Earth may owe its
atmosphere and oceans to a shower of comets rich in volatile compounds, which
brought water and trapped gases from the cold Kuiper belt to the inner Solar
System. A detailed analysis of the volatile compounds on Pluto and a Kuiper belt
object could help to answer this question.
A second mystery is how Pluto got its moon. Both the Charon-Pluto and the
Earth-Moon systems are unusual 鈥渄ouble planets鈥�. Both moons are large compared
to their parent planets, so it鈥檚 unlikely that they are the leftovers of the
planet-building process. Instead, giant impacts in the early Solar System
probably chipped chunks out of the Earth and Pluto, and the material then
accreted to form the satellites. That seems perfectly feasible for the Moon,
which is about a quarter the diameter of the Earth, but Charon is about half the
diameter of Pluto. Astronomers believe that an impact big enough to carve Charon
out of Pluto would have smashed Pluto into tiny bits.
There are yet more mysteries relating to the world of Pluto itself. For
instance, no one knows exactly how active it is. There is some tentative
indication from brightness measurements that the patterns of light and dark on
Pluto鈥檚 surface have changed over the past 20 years or so. The frosts of
nitrogen, methane and carbon monoxide are probably evaporating from some regions
and being deposited on others as the planet鈥檚 prolonged seasons change. A
spacecraft could confirm this is happening.
Curious atmosphere
More questions relate to Pluto鈥檚 atmosphere, which only came to light in 1988
when Pluto passed in front of a star. Instead of cutting off abruptly, the light
level fell smoothly as refraction by gases in the atmosphere bent the light. The
observations suggested that the atmosphere contains nitrogen, carbon monoxide
and methane, just like the frozen surface. A thin layer of organic smog,
produced by sunlight on methane, or a warm layer of gas like the Earth鈥檚
troposphere, seems to hug the surface. The atmosphere is diffuse and extends to
about ten times the height of Earth鈥檚, as it is only weakly bound by Pluto鈥檚
small gravity.
Astronomers believe this atmosphere may behave in a curious way. As the
planet moves from its nearest point to the Sun to its farthest, the amount of
sunlight it captures drops by about 40 per cent. This has led scientists to
speculate that there will soon be enormous changes in Pluto鈥檚
atmosphere鈥攊t may even be about to collapse. Temperatures should fall from
about 42 K to 34 K or less, causing the atmosphere to condense as frost on the
surface. The atmospheric pressure at the surface would then drop from 40
microbars to only 1 microbar (one millionth of the Earth鈥檚 atmospheric pressure
at sea level).
Thermostat
However, there are alternative views. John Stansberry of NASA鈥檚 Ames Research
Center in Moffett Field, California, suggests that Pluto might have a built-in
thermostat that prevents the temperature falling too far and the atmosphere from
collapsing. The temperature on Pluto depends on the balance of incident sunlight
and thermal radiation lost by the surface. As the planet moves away from the
Sun, its surface cools until the thermal radiation leaving the surface matches
the decreased incident radiation from the Sun. But some materials emit more
thermal radiation than others at a given temperature. So the final equilibrium
temperature depends on what the surface of Pluto is made of.
Much of Pluto鈥檚 surface is covered by solid nitrogen frost, which has two
states鈥攁lpha and beta鈥攖hat exist at different temperatures.
Stansberry suggests that the phase change acts as a thermostat. At the moment,
he says, all the frost is in the beta state, but as Pluto moves further from the
Sun and the temperature drops below 35 K, it will change to alpha. The alpha
state gives out less radiation for a given temperature than the beta state, so
if the outgoing radiation from the planet鈥檚 surface is to match incoming
radiation from the Sun, the temperature has to rise. The beta-alpha phase change
may buffer the temperature and maintain the pressure.
On the other hand, carbon monoxide and methane also exist on Pluto鈥檚 surface,
in small but uncertain concentrations, and these may affect the thermostat in
ways that no one yet understands. Only a visit to the planet by a spacecraft can
resolve these issues.
If the history of astronomy tells us anything, it is that ideas about the
outer planets inspired by Earth observations alone are likely to be very wide of
the mark. When the Voyager 2 spacecraft visited the outer planets, it found that
Uranus is much more exciting than anyone had predicted, with a complex ring
system. Neptune鈥檚 moon Triton鈥攕imilar to Pluto in size and apparent
composition鈥攚as expected to be a dead ball of ice, but turned out to have
a young, changing surface and active geysers spouting nitrogen. 鈥淲e were stunned
by the pictures we saw,鈥� says Richard Terrile of NASA鈥檚 Jet Propulsion
Laboratory in Pasadena, California, who was one of the Voyager science team and
is now the Pluto Express project scientist.
Terrile hopes that the results from Pluto Express will be just as spectacular
as Voyager鈥檚, despite the fact that the costs will have to fall well short of
those of earlier missions. For example Galileo, which reached Jupiter last year,
ate up over $1 billion. But in the past few years, NASA鈥檚 budgets have
been squeezed and squeezed again. 鈥淭he glory days of the dinosaurs are over,鈥�
says Terrile. 鈥淭here aren鈥檛 going to be any large missions.鈥� A lean Pluto
Express mission, with a tight cost cap of $250 million, probably
represents the only chance to get to Pluto.
To keep the mission within budget, a NASA committee has narrowed down the
list of priorities to the essentials: to look at the global geology of Pluto and
Charon, measure their shapes, map the composition of compounds across Pluto鈥檚
surface, analyse the atmosphere, and find out if it is escaping into space.
Short list
According to Lunine, even this short list of measurements could dramatically
change our knowledge of the outer planet. 鈥淭hese would teach us more about Pluto
than we learnt about Triton from Voyager, and the Voyager encounter completely
revolutionised our understanding of Triton,鈥� he says. NASA also hopes to take
some 鈥渘ice-to-have鈥� measurements, if costs allow, such as surface temperatures
and Pluto鈥檚 effect on the solar wind.
Later this year, scientists will start designing the mission in detail. At
this early stage, the plan is to launch the Pluto Express in 2001, to arrive at
Pluto about 10 years later. The closest approach will be about 15 000
kilometres, three times as close as Voyager got to Triton. A single spacecraft
cannot map all of both Pluto and Charon. So to complete the mapping and monitor
any changes over time, the Pluto Express mission will comprise a pair of similar
spacecraft, the second arriving about 6 months after the first. One or both
could then journey on to a Kuiper belt object.
To achieve the project鈥檚 scientific objectives, the spacecraft will have to
map features in black and white with a resolution of about 1 kilometre, and in
colour and infrared with a resolution of 3 to 10 kilometres. The instruments are
likely to include a visible-light camera, an infrared mapping spectrometer and
an ultraviolet spectrometer. The infrared mapping spectrometer will be vital for
identifying the ices and minerals on the surface, while the ultraviolet
spectrometer will spot UV light emitted by gases in the atmosphere. 杏吧原创s
hope to economise on cost and weight by combining these into one package,
sharing the same electronics and structure.
The Pluto Express will probably also carry a precise radio transmitter for
gravity and atmospheric measurements. By comparing its signal with that of an
identical transmitter here on Earth, scientists will be able to measure the
Doppler shift in the wavelength due to the spacecraft鈥檚 motion. The tiny changes
in the spacecraft鈥檚 trajectory that this reveals will indicate subtle variations
in Pluto鈥檚 gravity. This in turn will provide telling clues about its interior,
for instance whether all the dense rocky material has sunk to the centre to form
a core. An irregular gravity field, along with signs of an impact scar, might
give an insight into the collision that chipped Charon out of Pluto. Also, the
bending of the radio beam by refraction in Pluto鈥檚 atmosphere will help
scientists to pin down its structure.
鈥淓xtras鈥� may be added to the mission through international cooperation. The
Russian Space Research Institute has proposed that the Express carry 鈥淒rop
Zonds鈥濃�攕mall disposable probes that would sample Pluto鈥檚 thin atmosphere
for a few seconds and send back data before smashing onto its surface. These
could spot gases too scarce to be detected by the spectrometers. The German
Space Agency may contribute instruments to study dust and plasma in the outer
Solar System, or a Drop Zond to release onto Jupiter鈥檚 moon, Io, en route. For
now, the options are still open.
The cost restraints on the mission call for a new philosophy in building the
spacecraft, which will weigh less than a tenth of a Voyager craft. Usually
spacecraft are built by a team of engineers working in isolation, while
different teams of scientists design instruments to 鈥渂olt鈥� on. But with the
Pluto Express, engineers and scientists will collaborate closely on every part
of the project from the beginning. By coordinating these design aspects the hope
is to achieve more with less. For instance, scientists will choose the best
focal length for the cameras, depending on the relative timing of the
observations, which in turn will affect which way the spacecraft points and
when. By integrating the designs of the camera and the spacecraft pointing
system, the performance of the overall mission can be maximised.
Staffing will be kept to a minimum. During design and construction, the much
more ambitious Galileo mission involved roughly four thousand people. The Pluto
Express will use about 700. And instead of having about 200 people operating the
control room, mission control for the Pluto Express will manage with fewer than
ten.
To make this possible, the designers will give the spacecraft a new, more
autonomous mode of operation in which it will constantly broadcast one of just
three tones. One tone would mean 鈥渆verything okay鈥�, while another might be 鈥淚
have data ready to download 鈥攋ust say when you鈥檙e listening.鈥� About five
people, perhaps based at a university, would take care of the routine running of
the Pluto mission during its quiet cruise. The third tone would indicate some
kind of anomaly, and then an emergency team would swing into action.
The race is on to get the mission up and running in time for a launch early
in the next decade, and scientists are working hard to prepare their instrument
proposals. Results from the Pluto Express will be a long time coming, as the
earliest the spacecraft could reach its destination is around 2010. But ask any
planetary scientist and they will tell you it is worth the wait. 鈥淲e know very
little about Pluto,鈥� says Terrile. 鈥淏ut what we do know makes it seem like a
very exciting place.鈥�