杏吧原创

THE engineer in charge of life-support scans his computer monitor and smiles.
Two months into a three-year mission to explore Mars and the tomatoes are doing
nicely. Potato and soya bean production is also on schedule. He tinkers with the
lighting to edge up oxygen output, then heads down to the galley. Dinner tonight
is a pizza鈥攎ade from soya flour, soya cheese, tomato sauce, and 鈥渟ausage鈥�
from textured vegetable protein. It鈥檚 not the best food he鈥檚 ever eaten, but it
beats three years of freeze-dried meals.

For decades, science fiction writers have speculated that long trips into
space would depend on self-sustaining biological systems to produce food and
oxygen for the voyagers. Now this concept is closer to being realised. Last
July, a team led by Don Henniger at the Johnson Space Center, in Houston, Texas,
enclosed a British chemist in a chamber 10 metres square containing a microwave
oven, a telephone, a computer, a VCR, some reading material and 10.8 square
metres of growing wheat. The chemist emerged a week later in good health. He had
survived on food and water passed to him through an airlock and oxygen generated
by the plants鈥攈e even managed to get some work done.

Henniger is one of a growing band of scientists from the US, Europe, Japan
and Russia who are working on a self-sustaining ecosystem for space travel.
鈥淲hat we鈥檙e doing is copying the way the Earth creates a life-support system,鈥�
says Mel Averner, programme manager for advanced life-support at NASA. 鈥淏ut
we鈥檙e trying to do it within the enormous constraints of a space habitat. We鈥檙e
trying to do what nature does鈥�in something the size of a bus.鈥�

Averner鈥檚 annual budget at $10 million is tiny, even by the standards
of today鈥檚 scaled-down NASA. But he is convinced that the future of space travel
depends on biological systems. It is a matter of simple economics. Boosting
material out of Earth鈥檚 gravity is enormously expensive, and the longer the trip
the heavier the payload. The 鈥渂reak-even point鈥� is about two years, says
Averner. For longer trips it would be cheaper to install self-sustaining
biological systems on board the spacecraft than to launch all the supplies
required. Missions that long are decades away, but Averner is confident they
will happen. 鈥淧eople need to explore. For people to continue to explore, sooner
or later they have to cut the umbilical cord,鈥� he says.

Balancing trick

But making it happen is much easier said than done. The complex ecological
systems on Earth have evolved over billions of years. They are successful in
part because vast sinks of water, atmospheric gases and carbon are on tap to
buffer minor disturbances in the system. If the weather is unusually cool and
cloudy over the Amazon rainforests this week, the shortfall in oxygen won鈥檛
leave everyone on the planet gasping for air until the sun comes out. But future
space voyagers will not be able to haul along tonnes of extra supplies of air
and water.

With reserves at a minimum and little room for error, the ecosystem will be
controlled by pumping in finely judged amounts of energy鈥攁 scarce
commodity in outer space. 鈥淵ou鈥檙e suddenly confronted with the idea that this
has to be very efficient,鈥� says Robert MacElroy, senior scientist at the space
technology division at NASA鈥檚 Ames Research Center, Moffett Field, California.
鈥淏ecause the currency you鈥檙e working with is energy passing through the system.鈥�
But how much energy to supply, and where? The answer will depend on precise
information about the physiology of plants and humans in conditions of
microgravity. It will also require sophisticated computer controls to ensure
that the energy is input at the right place and time.

The researchers begin, however, with a natural advantage. The amount of
biomass you need to feed someone, it turns out, is just about the same as the
amount you need to supply them with oxygen and recycle the carbon dioxide they
produce. A self-supporting space mission will need to carry around 20 square
metres of plants in continuous production for each crew member on board.

These plants will not be grown in anything as inefficient as soil, though.
Last month, Doug Ming and his team at the Johnson Space Center, announced that
they had developed a soil substitute. It contains granules of common minerals
called zeolites impregnated with nitrogen and potassium, together with a
specially synthesised version of another mineral, apatite, containing other
essential plant nutrients. The soil substitute looks and works like a mass of
tiny time-release pills, containing enough nutrients to last several years.

One of the major technical problems researchers face is providing the plants
with enough light. A long-term mission would probably be powered by solar
panels, and the energy from these would power banks of electric lights. Averner
points out, however, that converting solar to electric power is extremely
inefficient. One possibility might be to develop a fibreoptic system that could
siphon the sunlight directly to the plants.

But the conversion of solar to electric power isn鈥檛 the only wasteful step.
Photosynthesis itself is extremely inefficient. Under greenhouse conditions,
only about 7 per cent of the light that strikes the plant is put to good use.
That is because individual chlorophyll molecules can only process one photon at
a time, so that any others hitting the leaves will simply bounce off unused.

Taking a pulse

Roger Binot, an agronomist at the European Space Agency in Belgium, thinks
using pulsed electric lights might be the solution. His team is experimenting
with LEDs that pulse high-frequency light hundreds of times a second, so that
each time a photon hits the plant the chlorophyll is ready to process it. 鈥淭he
idea is to send the plants the right energy at the right time,鈥� says Binot. For
his test plant, Chorella pyronoidosa, this turned out to be 5
millisecond flashes of light with a frequency of between 2 and 12 kilohertz.

So which plants are being prepared for long-haul space flight? To keep it
simple, researchers are working on a handful of basic food crops鈥攄ietary
staples that have already been studied in detail. Potatoes, tomatoes, soya
beans, wheat, sugar beets, lettuce, onion and strawberries are all being
considered. Another strong contender is an edible blue-green algae called
Spirulina. Binot has been experimenting with Spirulina because it
grows easily in tanks of water, giving a good source of calories without taking
up much space. He believes it could provide up to 20 per cent of each voyager鈥檚
energy needs, though it would probably have to be processed to make it
palatable.

Detailed analysis of the physiology of all these plants will be a huge task.
鈥淓verybody thinks it鈥檚 simple,鈥� says Harry Janes, director of a NASA-funded
programme in bioregenerative life-support systems at Rutgers University in New
Jersey. 鈥淭hey say, `Tomatoes? I grow tomatoes鈥�. But when you really get down to
it, it鈥檚 not that simple. NASA is one of the few places that understands that.鈥�
Janes points out that much of the information needed to create a successful
biological life-support system just isn鈥檛 available because there鈥檚 been no call
for it until now.

At Rutgers University, they will spend the next five years studying a few
crops in minute detail. Starting with tomatoes, and later potatoes, soya beans
and wheat, the team will try to work out exactly how these plants behave when
levels of light, carbon dioxide and water vapour fluctuate. They aim to collect
enough information to build reliable computer models simulating the
interconnections between all the variables that affect plant growth.

According to Janes there is already plenty of research showing optimum
growing conditions for all sorts of plants. 鈥淏ut what does it mean to the
development of a crop if you increase the temperature by five degrees for five
days, two months after germination?鈥� he asks. 鈥淚t鈥檚 obviously a different effect
than if you increase the temperature by five degrees, one day after germination.
We hope to model what鈥檚 going on.鈥� And once that is done in a terrestrial
laboratory, they will need to take the effects of microgravity into account.

Jane Bramm, a plant biologist at Rice University in Houston is working on
this problem. With a grant from NASA, she is looking at three genes associated
with the way plants react to touch stimuli, which also seem to play a role in
their response to gravity. Plants grown in protected conditions such as
greenhouses tend to be tall and spindly. Those buffeted by wind or other
disturbances, grow thicker and sturdier. Bramm has found that in normal
conditions the three 鈥渢ouch鈥� genes release proteins that cause the cell walls to
thicken. Gravity also seems to turn on these genes. 鈥淎s for microgravity in
space, we鈥檙e still at the observation stage,鈥� admits Bramm. Experiments with
plants in space are hard to interpret, she adds, because researchers must
account for the forces at take-off and landing.

There is no doubt that plants can be grown successfully in space. But can
they satisfy the dietary needs of voyagers for years on end? Specially bred and
genetically engineered plants will provide the correct balance of
micronutrients鈥攁lways assuming we can work out what that is. Researchers
suspect that years in space may alter human dietary needs, but no one knows
exactly how. Later this year researchers will start to address the problems of
nutrition and space travel at a conference at Rutgers University. They will
consider, for example, how to combat the loss of calcium from bones and whether
space voyagers could protect themselves from the carcinogenic effects of cosmic
radiation by eating large amounts of carotenoids鈥攖he yellow and red
pigments found in plants such as tomatoes and carrots.

If so, the diet of tomorrow鈥檚 space travellers will certainly look colourful.
But will it also be bland and monotonous, with endless meals of baked potatoes
and stewed tomatoes followed by strawberries? Not if T. C. Lee, a food scientist
at Rutgers University can help it. He is trying to develop ways for space
travellers to process their limited ingredients and create a varied menu with
very little effort. Ideally, Lee wants to invent a 鈥渕agic box鈥�, that would
accept raw ingredients at one end and spit out ready-to-eat food at the other.
He is experimenting with extruders鈥攎achines that grind, mash, cook and
texture food. These are widely used in industry, especially for shaped snack
foods.

Lee鈥檚 lab is full of bags containing what looks like puffed rice cereal but
is in fact processed potatoes, corn, rice and other crops. So far the results
are quite ordinary, but his plans are ambitious. Lee wants to make machines that
can take soya beans and produce oil, soya wheat, cheese-like tofu and even
textured protein that could be used as a meat substitute. His dream machines
will automatically turn tomatoes into tomato paste or mill wheat into flour and
then bake it into bread鈥攚hile the voyagers get on with the more important
aspects of their mission such as monitoring their oxygen supply.

Put crudely, plants process carbon dioxide and produce oxygen, while humans
do it the other way around. But preserving a fixed atmosphere in a small closed
system is not so simple. The balance of gases is bound to vary on a day-to-day
basis. And if these minor fluctuations are not carefully controlled, the whole
system could go haywire, with disastrous results. 鈥淲ith an open system, the
disturbance goes through the system, then out, and you don鈥檛 see it again,鈥� says
David Auslander, professor of mechanical engineering at the University of
California at Berkeley. 鈥淚n a closed system it goes around and around and
around. As the disturbance goes around and around, it gets bigger or smaller. If
it gets bigger it鈥檚 an unstable system.鈥�

To maintain the equilibrium, carbon dioxide and oxygen will be removed or
added to the atmosphere from reserves on board the spacecraft. Physical and
chemical processes would also be used to iron out any fluctuations. Ideally,
researchers would like to develop direct ways of converting carbon dioxide to
oxygen and vice versa. But they have yet to begin work on the problem. Air will
also have to be scrubbed to prevent toxins building up. Plants, for example,
produce ethylene, people exhale carbon monoxide and the plastics and glues used
in spacecraft construction give off all sorts of chemical fumes. When the
International Space Station Alpha is built (work begins in 1998) such toxins
will be removed using activated charcoal, a conventional technology.

Customised cleaners

Eventually, though, the atmosphere could be detoxified by membranes
containing many different microorganisms, each customised to break down a
particular poison. Binot is working on a prototype biological filter
containingXanthobacter autotrophicus, a microorganism found in
sediments from the River Rhine. It converts dichloroethane鈥攁 toxic organic
solvent given off by paints, plastics and resins鈥攊nto carbon dioxide,
water and chloride ions. The filter works successfully on the ground and is now
being tested on the Mir space station.

A different mix of microorganisms鈥攁naerobic bacteria鈥攚ill
probably be used to purify waste water, using the same sort of technology as
municipal sewage plants. It should prove possible to recycle urine by separating
out the urea and converting it into nitrates which could then nourish the
plants. Solid waste, mostly plant material and faeces could be incinerated at
about 500 掳C, generating carbon dioxide and water for recycling. Any smoke would
have to be left in the chamber to settle, and researchers hope to develop a
biological system to digest left-over ash.

Many of the technologies required to keep a biological life-support system
flowing smoothly are quite simple. The tricky part will be getting everything
working together and geared to cope with the changing needs of the crew. 鈥淲hat
do you do [on a space station] if you have three new crew members to stay for a
week?鈥� asks MacElroy. You will need enough carbon dioxide in the system to meet
the demands of accelerated crop growth. And because extra solid waste will be
produced you鈥檒l need to be sure that you have spare oxygen to incinerate it and
still leave enough for your crew to breath. But the main difficulty will be
maintaining a balanced ecosystem while preparing for the visit and with extra
people on board.

The engineers designing self-supporting biological systems can plan for such
eventualities. They can also build in other safety features, for example,
different varieties of the same plant to increase resistance to disease, and
isolating garden chambers to prevent diseases spreading. But what happens if
something goes badly wrong鈥攊f the plants start dying in droves? 鈥淲e fully
expect we will have back-up systems on board,鈥� says Henniger. 鈥淚f there is a
catastrophic failure you can go to the physical chemical systems and you鈥檒l have
life-support while you rectify the problem and replant the plants.鈥�

Given all the problems, will biological life-support ever take off? Only
experiments with real systems will tell. At the Johnson Space Center, Henniger鈥檚
team is planning a series of increasingly ambitious prototypes. Following their
success last summer, the next chamber will be designed to support four people
for up to a month and, will recycle both air and water with the help of some
high tech apparatus. By 2005, Henniger wants to create a completely
self-contained chamber to support a rotating crew for months on end.

Building the chamber, says Henniger, will highlight the engineering and
science problems that must be solved before similar systems can be sent into
space. He believes automation is the key to ensuring that the crew are not
simply full-time gardeners and chefs. 鈥淥bviously, you鈥檙e not going to mount a
mission in space and send up a crew to do nothing but keep themselves
补濒颈惫别.鈥�