杏吧原创

WHEN three Japanese spacecraft plough into the dusty surface of the Moon in
just over a year鈥檚 time, a new age of space exploration will begin. Instead of
landing daintily on the surface, these torpedo-shaped craft will bury
themselves at huge speed in the bedrock. From here they will be able to measure
the properties of the rocks and peer into the core many thousands of kilometres
below.

The mission is called Lunar A and it is the first of a new generation of
spacecraft called penetrators that will pierce the surface of planets, moons and
other bodies throughout the Solar System. Penetrators will help to explain
one of the great mysteries of the Solar System鈥攈ow the planets and moons
formed and why the Earth is a warm, wet, hospitable planet while other planets
experience hostile extremes of heat and cold.

Digging deep is crucial. In the past, space scientists have built up their
knowledge of the Solar System by sending spacecraft to photograph planets and
moons, land on them and even take samples from the surface. The data gathered
have given them extraordinary insights into the processes at work on the
surfaces of the planets and moons, but it has also left gaping holes in their
knowledge.

Comets and asteroids are even less well understood. In 1986, the Giotto
spacecraft made a close fly-by of Halley鈥檚 Comet and and passed Comet
Grigg-Skjellerup in 1992. The European Space Agency鈥檚 mission was a huge
success, but basic facts about comets remain a mystery. Nobody knows how dense
these bodies are and even the best guesses vary by more than an order of
magnitude鈥攃omets could be as dense as marble or as light as meringue. To
understand the deeper structure and composition of a planet, moon or other body,
a spacecraft has to go underground.

Deep down

Getting beneath the surface is not as hard as it might seem. Over the past
thirty years, Russian and American spacecraft have carried drills, rods, shovels
and arms to poke about in the top few centimetres of the Moon and Mars. But
these tools cannot be scaled up to dig many metres down. A scooping device
digging twice as deep must remove eight times as much material. This would use
up at least eight times as much power and dramatically increase the complexity
of the spacecraft. Complicated mechanical systems are the bane of spacecraft
designers, because even the most ingenious and well-tested designs can seize up
under the severe conditions on other planets.

Fortunately, designers have another way to get under a planet鈥檚 skin. In
future, penetrators will slam into planets and moons, using their kinetic energy
to force their way beneath the surface. Depending on the density of the soil it
hits, a probe could end up several metres underground.

On most non-terrestrial bodies the bedrock is hidden by a layer of
fine-grained rock particles called the regolith. The key reason why penetrators
are so useful is that this homogenous layer of dust obscures the differences in
the depth, thickness and composition of the bedrock beneath. Because a
penetrator鈥檚 payload is in direct contact with the bedrock material, it can
supply the information which is crucial to understanding the origin and
evolution of a planetary body鈥檚 crust and interior.

Snugly embedded in the ground, a penetrator is isolated from the temperature
swings that sweep over the surface of a rotating, airless body such as the Moon.
This makes the subsurface an ideal place to take precise measurements of the
heat generated flowing in and out of the body. A further benefit is that seismic
activity can be monitored much more accurately here than on the surface, where
the regolith tends to muffle quakes.

The first penetrator was launched in November 1996 with the ill-fated Russian
Mars 96 mission. The penetrator was just over a metre long and shaped like a
giant golf tee. The idea was that it would divide on impact, the narrow spike
penetrating the Martian soil while the head of the tee remained on the surface.
The two sections contained a range of instruments that would have analysed the
Martian soil, photographed the surface and broadcast the results to a spacecraft
orbiting the planet above. Sadly, Mars 96 failed to boost itself into the
correct orbit and the mission was lost.

The Lunar A spacecraft, which must now carry the torch for the penetrator
revolution, was designed and built by the Institute of Space and Astronautical
Science (ISAS) near Tokyo (鈥淲atch this Space鈥�, New 杏吧原创, 25
November 1995, p 46). It comes in two halves, a mothership that carries a
mapping camera, and three penetrators that will be dropped onto the near and far
side of the Moon to form a network of sensors beneath the surface. Each
penetrator is just under a metre long, 13 centimetres in diameter, and weighs a
hefty 13 kilograms.

From an orbit 100 kilometres above the Moon, the penetrators will detach from
the mother ship and decelerate out of orbit using small rocket motors. Then they
will plummet towards the surface at a speed of up to two kilometers per
second鈥攁bout the speed of an anti-tank round. Nothing could survive an
impact at this speed, so just before hitting the surface a pair of rockets on
each penetrator will blaze into life, slowing it to 300 metres per second. Even
so, the impact will create a deceleration force equivalent to ten thousand times
the Earth鈥檚 gravity before the penetrator comes to rest up to two metres under
the surface.

Designing probes that can survive such an impact and then go on to make
precise measurements is hugely challenging. In some ways these shock troops of
astronomy are spacecraft turned inside-out. Instead of having the load-bearing
structures on the inside and sensors mounted on the outside, as is normally the
case for satellites and probes, a penetrator packs its precious sensors safely
inside an armoured exterior.

Each penetrator will contain a computer, heating devices and thermometers, a
seismometer and an antenna for broadcasting the data back to the mother ship.
These devices can be protected by ensuring that there are no gaps between parts
so that nothing moves or collides during the impact. To achieve this the parts
must be machined to tolerances measured in micrometres and assembled precisely.
And the materials from which they are made have to be carefully chosen to
minimise any thermal contraction and expansion which would otherwise create gaps
that could endanger the spacecraft.

The penetrators鈥� onboard computers are unique. About as powerful as a desktop
computer, each has been handbuilt by Satoshi Tanaka, the chief engineer for the
Lunar A payload. Such a computer cannot be assembled on a flat, two-dimensional
circuit board, which would break during an impact. Instead, Tanaka has crafted
each computer in three dimensions, connecting the components by hand with a web
of wires and encasing the entire assembly in epoxy resin. The result is a green,
semitransparent cylinder that is strong enough to allow its microelectric
components to survive the impact.

Understanding the effects of the explosive deceleration during impact has
been a central concern for Tanaka and the team which is lead by Hitoshi
Mizutani, a planetary scientist at ISAS. When they began designing the
penetrators in the early 1990s, nobody knew what shape they would have to be to
withstand this force. 鈥淚t was really hard,鈥� says Tanaka.

At the leafy ISAS campus in the outskirts of Tokyo, Tanaka has the battered
remains of dozens of aluminium prototypes that have undergone impact testing to
answer this question. Some have hemispherical heads, others are conical, some
are even bent in half. All are pitted and scarred by the impact, almost as if
they had been sandblasted.

Smooth landing

The aim is to bury the penetrator vertically, but inevitably there will be
small deviations as it hits the surface. Tanaka鈥檚 team soon found that a
cone-shaped head amplified these deviations, making the penetrator tumble as it
entered the ground. Removing the top of the cone makes entry into the surface
much smoother, and this truncated cone turns out to be the best design, says
Tanaka.

The impact also affects the nature of the soil nearby. The sudden increase in
temperature and pressure tends to fuse grains together and this changes the
thermal characteristics of the soil. ISAS has run hundreds of experiments to
determine the characteristics of the fused soil, so that researchers will be
able to work backwards and determine what the soil was like before the
impact.

Each Lunar A penetrator contains a seismometer that will listen for
moonquakes and meteorites crashing into the surface. Because there are three
penetrators, it will be possible to pinpoint the position of these events by
triangulation. The way these sound waves pass through the Moon will also tell
researchers what the core is made of and its size. In turn, these measurements
will provide vital clues about the origins of the Moon.

The number of impacts that the Moon experiences will also provide some unique
insights. Nobody knows how heavily the Earth is bombarded from space since most
meteors burn up in the atmosphere unnoticed. But recent satellite observations
suggest that every day hundreds of house-sized lumps of ice are vaporised as
they enter the Earth鈥檚 atmosphere
(鈥淣ot a snowball鈥檚 chance鈥︹��, New 杏吧原创, 12 July, p 24).
If this is true, a proportionately lower number of objects
should strike the smaller Moon. Working out how heavy this rain of cosmic debris
really is will help scientists to predict the likelihood of a major asteroid
impact on Earth.

Following closely on the heels of the Lunar A mission will be the Mars
Microprobe, now being built by NASA at the Jet Propulsion Laboratory (JPL) in
Pasadena, California. This mission consists of two thumb-sized penetrators that
will travel to Mars with the Mars Surveyor 98 lander. Just before the lander
arrives in December 1999, the two penetrators will drop away from the main
vehicle and fall through the rarefied Martian atmosphere. Each will be protected
by an aeroshell the size of a large cereal bowl and will hit the surface at
around 200 metres per second. Because of its large surface area, the aeroshell
should remain on the surface while the penetrator punches through it and into
the ground, ending up some two metres beneath the surface
(see Diagram).FIG-21095201.jpg

Ice on Mars

The microprobes are designed to search for ice beneath the surface of Mars.
Each penetrator contains a tiny drill that will take samples from the
surrounding soil and transport them to a cavity inside the penetrator. Here, the
sample will be heated while its temperature is measured. Any ice present will
sublime, generating a characteristic temperature curve.

In some ways, the JPL researchers were faced with even more daunting
challenges than the Japanese team. The thumb-sized microprobes will decelerate
even more rapidly than the Lunar A or Mars 96 devices, even though they will
strike Mars at a lower speed than Lunar A will hit the Moon. The probe itself
will experience up to 30 000 g while the aeroshell which will
decelerate even faster will experience up to 80 000 g.

Another problem is that a penetrator must leave a radio antenna on the
surface so that it can broadcast its data. The Lunar A penetrators have short,
rigid antennae that will poke out of the craters created by the impact. But each
Mars microprobe will penetrate many times its length into the soil, so it must
be connected to a small radio transmitter on the surface by a cable that can
carry data.

Designing a cable that can survive the impact has turned out to be one of the
most daunting tasks for the JPL team. The cable must contain 30 separate wires
and be able to withstand the tension created when it deploys, reaching a speed
of 400 kilometres per hour in just a few milliseconds. In tests, JPL researchers
found that ordinary copper wires were too bulky and too weak to take the strain.
Instead they used a plastic strap coated with a thin film of metal. This plastic
harness is only one-tenth the weight of its copper counterpart, but is strong
enough to withstand the punishing impact with the Martian surface.

Other penetrators are also planned. Early next century, the European Space
Agency hopes to land a spacecraft called Roland on a comet Wirtanen and plant a
penetrator beneath its surface. The Huygens probe, which will land on Saturn鈥檚
moon Titan in 2004, has a type of penetrator that will measure the granularity
of the surface. Researchers at NASA are also thinking about using a probe to cut
through the icy surface of Jupiter鈥檚 moon Europa and search for the oceans of
liquid water that may lie below. Such a probe might even look for signs of
life.

It will be some years before these projects produce real data. In the
meantime, everyone involved in designing penetrators will hold their breath as
the Lunar A penetrators slam into the Moon and the researchers listen all over
the world for the radio signals that will tell them that the shock troops have
survived.