杏吧原创

Washington DC

WELCOME back to Mars. Next week, after a twenty-year break, a machine from
Earth will touch down on the Red Planet. If all goes well, the Mars Pathfinder,
replete with a six-wheeled rover, will open a new era of planetary exploration.
And this really is just the beginning. Earth鈥檚 neighbour is about to become a
very busy place.

In September, the Mars Global Surveyor will go into orbit around the planet.
Its goals are to detect mineral deposits, create global weather charts and map
Martian topography. After that, every two years, when Mars reaches the correct
alignment with Earth, NASA plans to send a spacecraft to meet it. Over the next
few years the planet is going to be orbited, flown over, gouged, drilled into
and driven across by all manner of robotic spacecraft. These missions will test
a collection of ambitious technologies that will open the way to human
exploration of Mars early next century. If successful, these technologies could
change forever the nature of space travel.

Up to now, absolutely everything that might be needed for missions in space
has been taken from Earth and crammed into spacecraft before liftoff. For human
missions, that crucially includes fuel for the return journey. But a goal of the
proposed Mars missions is to do away with this. Instead, they will take along a
small factory that will sit on the planet鈥檚 rust-coloured soil and make the fuel
needed to bring a rocket safely home.

Officially, the rationale for this approach is economic. NASA is under
immense pressure to slash the cost of its space programme, and one of the big
drains on dollars is the expense of blasting heavy payloads out of the Earth鈥檚
gravitational grasp. So rather than dispatch tonnes of fuel into space, why not
take along a lightweight machine instead and let it produce the fuel? Some
people, however, see a more profound reason why NASA should follow this path. If
humans are ever to live beyond the confines of Earth, they must be able to 鈥渓ive
off the land鈥濃�攚herever that land might be. NASA鈥檚 plan takes the first
steps in this direction.

The starting point for making fuel on Mars is carbon dioxide, which makes up
95 per cent of the thin Martian atmosphere. From this gas, and possibly an
additional ingredient brought from Earth, the intention is to make not only
rocket fuel, but also the oxygen needed to burn it. Several research groups in
the US are in the early stages of developing ways to do this. They are funded by
NASA, which will choose the best option for missions in 2001, 2003 and 2005.
According to the present plan, the last of these missions will return samples of
rock and soil to Earth on board a rocket powered by Martian fuel.

The man in charge of deciding which fuel factory will fly on each flight is
David Kaplan, an aerospace engineer at the Johnson Space Center in Houston,
Texas. For the 2001 mission he needs to make up his mind by September. At
present, for this flight, Kaplan has pencilled in a zirconia cell developed by
Steven Crow and his colleagues at the University of Arizona in Tucson. 鈥淏ut if
someone can show they have something ready that will work better than this, then
I鈥檓 ready to evaluate it,鈥� he says.

Breaking loose

Zirconia, or zirconium oxide, is a ceramic that can be used as a solid
electrolyte. Its lattice is riddled with vacancies that are perfect refuges for
oxygen ions. Within the cell, a thin, flat crystal of zirconia鈥攚hich has
been doped with yttrium to increase the number of vacancies鈥攊s sandwiched
between porous electrodes. CO₂ diffuses through the cathode, where a
combination of high temperature and a free supply of electrons coaxes oxygen
atoms to break loose and become ions. From here, they migrate through the
crystal鈥檚 network of vacancies to the anode, where their surplus electrons are
stripped off and they combine to form oxygen molecules.

Crow鈥檚 zirconia cell has been rigorously tested in a simulated Martian
atmosphere. A more advanced model designed by K. R. Sridhar, also of the
University of Arizona, promises better performance, but has still to be
thoroughly tested. It could be a contender for the mission in 2003, says
Kaplan.

The reason for flying the cell in 2001, says Kaplan, would be to prove that
it is possible to produce pure oxygen on Mars and to make sure that we
understand exactly how the cell works. The device would also churn out carbon
monoxide, which could be used as a fuel, though it is far from ideal. Its
specific impulse鈥攖he rocket scientists鈥� measure of how much oomph you get
from a mixture of propellant and oxygen鈥攊s only about half that of the
liquid hydrogen that powers the space shuttle. 鈥淚t鈥檚 very poor,鈥� Kaplan says.
鈥淚t would be totally unacceptable for a human mission. But it might get a robot
mission back to Earth.鈥�

So something extra will be needed. And the most likely tactic will be to cart
something of low mass up to Mars to be used as a feedstock for fuel production.
One option, says Kaplan, is to use propane, which has a specific impulse of 80
per cent that of liquid hydrogen. A tank of propane not all that different from
the canisters people take on camping trips could be left on Mars with no special
treatment. And, because a rocket would need about 4 kilograms of oxygen for
every kilogram of propane, four-fifths of the fuel-oxygen mixture would still be
made on Mars, says Kaplan.

Another chemical candidate for the trip to Mars is hydrogen鈥攂ut this is
not as easy to handle as propane. It would probably have to be kept very cold,
in liquid form, which would consume a lot of energy. The hydrogen would be
combined with carbon monoxide from a zirconia cell to produce methanol, a
high-performing fuel with a specific impulse 85 per cent of liquid hydrogen鈥檚.
This strategy would produce 18 kilograms of methanol and oxygen for every
kilogram of imported hydrogen.

Secret pebbles

With hydrogen on tap, the options for types of fuel and ways to make them
start to grow. At Lockheed Martin in Denver, Colorado, Larry Clark and his group
combine CO₂ and hydrogen in a Sabatier reactor to produce water and
methane, another excellent rocket fuel. They then electrolyse the water to
generate oxygen鈥攑lus hydrogen, which is recycled.

The Sabatier reactor is hardly new: it was invented in 1899, and has been
used in spacecraft to 鈥渟crub鈥� CO₂ out of the air. Clark鈥檚 version is a
high-tech descendant of the original鈥攁 small tube full of black pebbles
made of a secret ruthenium-based catalyst. The reactor has been tested in a
simulated Martian atmosphere, though not yet for the long periods that it would
have to function on Mars.

This is the reactor that would fly to Mars in 2001 if Robert Zubrin had his
way. Zubrin is president of Pioneer Astronautics in Denver, and an influential
advocate of the colonisation of Mars, but in an earlier incarnation he began the
work on which Clark has built. The Sabatier-electrolysis process is the most
energy-efficient option for making fuel, he reckons. And for every kilogram of
hydrogen landed on Mars, it will generate 12 kilograms of fuel and oxygen.

But this is not the end of the story. The process makes twice the mass of
oxygen as it does methane, while the mass ratio needed for a rocket engine is
more than 3 to 1. So some methane would have to go to waste, or another process
found to make more oxygen. It鈥檚 this latter path that he is pursuing.

Zubrin has turned his attention to the reverse water gas shift (RWGS)
reaction. Like the Sabatier reaction, it takes place in what is essentially a
steel tube packed with a catalyst. CO₂ and hydrogen are pumped in and
carbon monoxide and water come out. The big problem, though, has been to find a
catalyst that does the job. It was not until a few weeks ago that Zubrin finally
found what he was looking for: a copper and aluminium catalyst that stimulates
just the right reaction. The water produced can then be electrolysed to produce
more oxygen.

So now Zubrin has a dual approach in mind. Run the Sabatier-electrolysis
process, which operates at 400 掳C and gives off heat, and use the waste energy
to maintain the RWGS reaction. For a minimum amount of extra energy, this
arrangement would produce more than enough oxygen to burn methane in a rocket
engine, he says. Every kilogram of hydrogen would yield 18 kilograms of methane
and oxygen. 鈥淭his is the best idea I know will work today,鈥� he says.

Whichever process is chosen, the recipe is only one part of the tough task
facing the factory designers. These plants will have to work reliably for
between 300 and 500 Earth days at a stretch. And make no mistake: even for a
machine, working on Mars is going to be no picnic. The temperature can drop to
鈥�120 掳C at night and hit 22 掳C by day.

During the day, zirconia cells will work at about 1000 掳C, but at night they
will be shut down. Whether they will stand the continual cycling between 1000 掳C
and 鈥�120 掳C is not yet known. To find out, NASA will test them in an
explosion-proof vacuum chamber at the Johnson Space Center. 鈥淲e鈥檒l be able to
cool the chamber down with liquid nitrogen to simulate Martian diurnal
variations,鈥� says Kaplan.

Another potential weak spot is the electronics, which cannot stand extreme
cold. These will be placed in a 鈥渨arm box鈥� to protect them. But even this may
not be enough. During the 2001 mission, Mars will swing out to its farthest
point from the Sun. 鈥淓ventually, there won鈥檛 be enough light or heat to keep the
warm box warm,鈥� says Kaplan. 鈥淭he electronics will freeze one night and not wake
耻辫.鈥�

But the temperature swings are not all bad news: at least one component of
the fuel plant will put them to good use. CO₂ is forced into the
reactor vessel by a sorption pump containing pellets of a special type of clay,
called a zeolite, which absorbs CO₂ when cold and releases it when hot.
This will open to the atmosphere at night, and be sealed in the morning so that
the pressure builds up as the zeolite expels the gas. 鈥淚t鈥檚 an elegant
solution,鈥� boasts Kaplan. 鈥淲hat was an obstacle we鈥檝e used to our benefit.鈥�

The pump has not been all plain sailing, however. The daytime temperature
rise will not generate enough pressure inside the pump, so some extra heating
will be needed to expel more CO₂, and insulation will have to be added
to keep the heat in. But this creates another problem: to cool the pump quickly
enough at night, it will probably need to have a radiator attached to it.
鈥淭hat鈥檚 a non-trivial engineering job,鈥� says Donald Rapp, senior research
scientist at the Jet Propulsion Laboratory (JPL) in Pasadena, California.

He also points out that small quantities of other gases in the
atmosphere鈥攎ainly argon and nitrogen鈥攎ay form a barrier round the
zeolite pellets. So a small, robust fan to keep the Martian gases flowing
through the pump is now on the shopping list, says Rapp.

Yet another environmental hazard on Mars is wind-blown sand. Evidence from
the Viking landers twenty years ago suggests that Mars is covered with a coat of
caustic dust. But we still have no idea exactly what this super-oxidising dust
will do, says Mark Adler, the architect of the Mars exploration programme at
JPL. It might eat away at seals, clog filters or otherwise gum up the works. If
dust coated the photovoltaic panels, the fuel plant would simply shut down.
鈥淭hese are issues which only have answers right at Mars,鈥� Adler says.

With all these uncertainties, plans for the Mars-fuelled 2005 mission to
bring Martian rock back to Earth are still only provisional. Another
alternative, now being discussed, is to fill a small rocket with oxygen and fuel
produced by the 2003 mission. The rocket would arc through the Martian sky,
perhaps taking high-resolution pictures of the surface, before landing back on
the planet. For less money than a journey to Earth, this would demonstrate
clearly that the principle of 鈥渓iving off the land鈥� can work.

Such a demonstration would be crucial to space exploration, says Doug Cooke,
manager of the exploration office at the Johnson Space Center. Some time early
next century, NASA will have to go cap in hand to Congress to get its plan for a
human mission to Mars approved. Estimates for that mission start at $20
billion and rise steeply. If NASA cannot show that living off the land will help
it to cut the bill, he believes, Mars could stay forever distant.

And Cook sees another less tangible dimension to the Mars venture. 鈥淲e tend
to talk about machines [and] hard figures too much,鈥� he says. 鈥淎 large benefit
of going to a place like Mars is the experience of people going and doing the
exploration.鈥� Zubrin is even more vociferous on this point. A Martian colony, he
says, is essential if the human race is to thrive. 鈥淎dvanced communication and
transportation technologies have eroded the healthy diversity of human cultures
on Earth,鈥� he argues. 鈥淥n the other hand, if the Martian frontier is opened,
then this same process of technological advance will also enable us to establish
a new branch of human culture.鈥�

Mars is a better place to live than the Moon, Zubrin says. While both have
metals and other important elements, only Mars has the carbon and nitrogen that
are essential to life. It also has an abundance of easily recoverable hydrogen
and oxygen in water at its poles, and possibly in perma-frost beneath the
ground.

Going back to the chemistry that NASA is exploring, Zubrin points out that
it鈥檚 just a short step from the RWGS reaction to making ethylene (C₂H₄),
which is not only a great rocket fuel but also an excellent raw
material for plastics. So the first astronauts to be propelled back to Earth
with Martian fuel will have been only a whisker away from the foundation of a
new industry. 鈥淲e鈥檙e taking the first step,鈥� Zubrin says. 鈥淲e鈥檙e like
hunter-gatherers learning to use Martian resources.鈥�