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Next week an American military spacecraft is due to begin the first
journey to a near-Earth asteroid. During the mission, Clementine will test
technology that could one day be used to track nuclear missiles in the Earth’s
atmosphere. Although primarily a military project, the mission is a cooperative
venture between the Department of Defense and NASA. Before heading off in
late May to the tiny asteroid Geographos for a close encounter around the
end of August, Clementine will spend much of the intervening period mapping
the surface of the Moon for civilian scientists (see Diagram).

The mission, which is costing less than $80 million, was originally
conceived by the Strategic Defense Initiative Organisation as part of the
Star Wars programme. It is now controlled by the Ballistic Missiles Defense
Organisation, which replaced the SDIO last year. The BMDO admits it has
no real interest in either the Moon or Geographos, but over the past decade
Star Wars scientists have developed military technologies, such as lightweight
components and sensors for tracking missiles, that the BMDO now wants to
test in space.

Light array

For instance, parts of the spacecraft will use carbon fibres instead
of aluminium. Clementine will generate electricity using an array of solar
cells made from gallium-arsenide-germanium semiconductors – the array will
be one of the lightest ever sent into space. And it will save this energy
in a nickel hydrogen battery that has twice the storage capacity of previous
power cells. A solid state data recorder, holding three times as much information
as conventional devices, will store information from sensors designed to
detect and track incoming missiles. However, Clementine will track a harmless
asteroid, thus saving the estimated $10 million needed to launch a missile
for the spacecraft to track.

Unlike most asteroids, which are confined to orbits in the region between
Mars and Jupiter, Geographos orbits the Sun in an elliptical path that
intersects the Earth’s orbit of the Sun. According to Pedro Rustan, joint
leader of the Clementine project at BMDO, the asteroid is a challenging
target because it is cold, unlike a missile that gives off exhaust as it
takes off. And although Geographos is about 4 kilometres long and 1.5 kilometres
wide, which is many hundreds of times as large as an intercontinental ballistic
missile (ICBM), it will present a much smaller target than an ICBM because
the spacecraft will begin to track the asteroid at a distance of between
1 million and 2 million kilometres. Clementine will start tracking the asteroid
a day or two before the flyby and will approach it at a relative velocity
of about 11 kilometres per second – similar to the closing velocity of an
anti-ballistic missile needed to intercept an ICBM. Clementine’s high resolution
camera will take thousands of images showing features as small as 1 metre
across, giving astronomers clues to the asteroid’s origin.

Only two asteroids have previously been photographed from space. En
route to Jupiter, the Galileo spacecraft captured photographs of Gaspra,
from a distance of 1600 kilometres in October 1991, and Ida, from about
2400 kilometres, nearly two years later. Clementine will approach to within
100 kilometres of Geographos.

But this will only be possible if the tracking technology works. BMDO
scientists are anxious to see how the spacecraft’s electronic systems withstand
the barrage of solar radiation and cosmic rays, which can damage microchips.
In a top-secret phase of the mission, part of the spacecraft known as the
Interstage Adaptor will separate from the main vehicle and remain in an
elliptical orbit around Earth. To maximise its radiation dose, the Adaptor
will pass repeatedly through the Van Allen radiation belts – dense pockets
of electrons and protons trapped by the Earth’s magnetic field – measuring
radiation levels. How the Adaptor performs is important because any space-based
missile defence system would have to operate successfully under similar
levels of radiation. In fact, conditions in space mimic some of those present
after a nuclear explosion on Earth, and military scientists hope to find
out how similar equipment might work after a nuclear detonation on Earth.

If the military technology works well, scientists and astronomers will
learn a great deal about Geographos and the Moon. During the two months
it spends in lunar orbit, Clementine will obtain hundreds of thousands
of images. This data – enough to fill a small library of compact discs –
is comparable to the total amount of data collected by the Magellan spacecraft
during its first 243-day radar mapping of Venus. There are two reasons
why Clementine can capture as much data as Magellan in a quarter of the
time. First, Clementine’s cameras will obtain sharper images that convey
more information. Its high resolution optical camera, for example, will
resolve features on the lunar surface down to a limit of 10 metres. Magellan’s
images, by comparison, have a resolution of about 100 metres. Secondly,
Clementine’s cameras take pictures of the Moon using 11 separate wavelengths
of visible and infrared light, while Magellan made only radar observations.

Quantity surveying

The lunar surface, unlike that of Venus, has never been systematically
mapped. Photographs of variable quality exist but ‘our knowledge of the
Moon is essentially limited to those regions in the lower latitudes that
lie under the paths followed by Apollo spacecraft’, says Eugene Shoemaker
of the US Geological Survey and leader of the Clementine science team. Clementine
will map the entire lunar surface at wavelengths chosen because they are
absorbed by common minerals. The amount of light reflected at each wavelength
will indicate the kind and quantity of substance present, helping scientists
create a mineral inventory of the lunar surface. Shoemaker hopes this information
will help scientists to reconstruct the geological history of the Moon.

Previous lunar missions ‘will make it easier to interpret the Clementine
data because we know what minerals exist at certain sites’, explains geologist
Paul Spudis of the Lunar and Planetary Institute in Houston, Texas. ‘When
Clementine looks at these areas, we will calibrate the instruments so that
we can decide what sort of minerals and rock types exist at other locations.’

Interpreting the Geographos data will be more difficult, Spudis says,
because it has never been studied at close range. The only views of the
asteroid have come from ground-based telescopes that do not show any detail.
Measurements of the brightness of the asteroid indicate nevertheless that
one side is at least six times as bright as the other. ¿ìè¶ÌÊÓƵs believe
the asteroid may have a weird, elongated shape, perhaps resembling a cigar.
They suspect that when the long side of the cigar faces the Earth, more
light is reflected in our direction than when the tip faces us. ‘It’s a
really bizarre object. But we really won’t know what it is until we get
a close-up view,’ comments Shoemaker, one of the world’s foremost authorities
on asteroids.

As with the Moon, scientists hope to determine the structure and composition
of Geographos. Knowledge of the asteroid’s mineral content could provide
clues about the matter which formed the Solar System. ¿ìè¶ÌÊÓƵs also hope
to discover whether the asteroid consists of only one type of rock or a
complicated blend, and whether it is one big lump or many small pieces moving
together.

Geographos appears to spin on its axis once every five hours. During
the days that Clementine’s cameras will be trained on it, the asteroid will
rotate several times, giving scientists a chance to observe its shape. ‘That
shape will tell us something about its history,’ Spudis says. ‘Is it a single
object that’s been battered in collisions with other asteroids or is it
two or more objects that have joined together?’

Collision course

The information collected by Clementine will be invaluable to scientists,
weapons specialists and government officials pondering the prospect of an
asteroid impact on Earth. Collisions with space rocks can have global implications.
Many scientists believe that a gigantic impact or series of impacts 65
million years ago wiped out the dinosaurs and half the species on Earth.
‘If we are serious about modifying the orbit of potentially threatening
asteroids, we need to take a good look at them first,’ Shoemaker notes.
‘Trying to push a big rock is different from trying to push a pile of rubble.’

Figuring out the shape is another important piece of the puzzle. ‘We
won’t know exactly where to push an object like Geographos until we get
a better idea of its shape,’ he says. ‘If you hit it in the wrong place,
you may spin it around without really moving it off course.’

The next generation of spacecraft to investigate asteroids is already
on the drawing board. Clementine 2 is expected to fire missiles at two asteroids
and, although funding for the venture is not secure, the BMDO has already
built parts of the craft. According to current plans, it would travel to
a comet and to two near-Earth asteroids, firing a total of four warheads
without their explosive payloads. ¿ìè¶ÌÊÓƵs expect the resulting impacts
to throw up material from the asteroids’ surfaces that could be captured
and analysed by the spacecraft’s sensors.

‘Instead of just a flyby, we’d like to scratch the surface,’ Rustan
says. ‘The goal is to prove that we can actually intercept an object, whether
it is an asteroid or an ICBM.’ Although hitting a rock the size of a city
does not seem demanding, Rustan explains that they would be aiming at a
small spot on the surface. ‘That’s a difficult challenge, especially when
you start from millions of kilometres away.’ He stresses that the ultimate
objective of Clementine 2, as with the current Clementine mission, is to
develop more reliable methods of protecting against a missile attack, not
an asteroid impact. ‘But if ever we want to deflect approaching asteroids
in the future, people may look back to these missions for some answers.’

The Clementine mission could set the standard for other NASA missions,
too. NASA’s chief, Daniel Goldin, wants the next generation of space missions
to be ‘faster, cheaper and better’. In particular, Goldin is promoting smaller
missions that could be completed in three years at a total cost of $150
million or less.

Clementine may provide a blueprint for this formula. It took only two
years to carry the idea from approval to launch. And, at a cost of $50 million
for the spacecraft and launch costs of between $20 million and $30 million,
Clementine is about ten times cheaper than a typical NASA mission.

But John Pike, a space policy expert at the American Federation of ¿ìè¶ÌÊÓƵs,
a private research and advocacy group based in Washington DC, suggests that
Goldin’s formula can be achieved only by taking greater risks. ‘Over the
last few years, about half the Star Wars missions failed or had serious
problems. If NASA had that kind of track record, we would have shut down
the agency long ago.’ BMDO is able to carry on because it operates outside
the limelight, Pike adds. ‘When NASA has a failure, it makes headlines.
When Star Wars has a failure, no one hears about it.’ Shoemaker, however,
contends that Clementine is worth the risk. ‘At this price,’ he says, ‘we
can afford to lose an uncrewed mission every now and then.’

Stave Nadis is a freelance journalist based in Cambridge, Massachusetts.