Advanced Life-Support
Suppose for a moment that you wanted to go to Mars - just for a visit. It
will take you about nine months to get there and about nine months to get
back. Since you have already invested a year and a half travel time, and
since you can only come and go to Mars based on planetary rotations around
the sun, you probably will have to stay on Mars another 18 months or so
before you can come home. All in all - you are going to be away for about
1000 days. That's a long time - especially when you remember there are no
convenience stores along the way and no grocery stores when you get there.
Not only that - you are going to have to bring along all your water, food
and even air to breathe. And if all that wasn't difficult enough - you're
going to have to collect and store all your wastes, including the
carbon dioxide that you exhale! Wow!
Given all that, there also is the reality that no human being has ever
been severed from the life-support system of their home planet for that
length of time - ever. Even though the Mir astronauts have stayed in space
close to 500 days, they were regularly re-supplied from Earth. A Mars-bound
astronaut cannot be re-supplied in the same way. Hence - not only is the
travel time very long, but it never has been done before!
In looking at life-support systems we now use on space vehicles, such
as the Space Shuttle, we will need to design and build very different
ones for such long voyages away from Earth. Today's systems are called
"simple life support systems" because they do not recycle anything. A
Mars-bound life-support system is an "advanced life-support system" because
it will have to recycle many of the life-support commodities.
For example, in some simple life-support systems, exhaled carbon dioxide
is absorbed by chemicals and then discarded on return to Earth. In others,
the carbon dioxide is temporarily absorbed on resins and released in vacuum.
In an advanced life-support system (ALS), the carbon dioxide will be captured,
concentrated, then converted to oxygen. Water vapor also will be captured,
condensed, purified, and returned to the crew. The water in urine will
be cleaned, purified and returned to the system. These are just a few
examples of how an ALS returns vital life-support materials back into
the system.
There are two basic kinds of ALS systems. They are known as "physio-chemical"
(PC) and "bioregenerative". The aim of both is to recycle the life-support-system
components (such as oxygen and water) but they do it in very different
ways.
A PC system works with principles of physics and chemistry to recycle.
In one example - "dirty" water may be purified by distillation (converting
water to steam and then re-condensing it as pure water). Another method
of PC water recovery would be simple filtration of the water. In yet another
example of a PC system - carbon dioxide can be "cracked" under the right
conditions to strip away the oxygen. Likewise, under intense pressure
and heat, carbon-based materials such as human wastes can be converted
to water vapor, carbon dioxide, and a small amount of ash (as oxidized
metals). This is a PC process called "supercritical wet oxidation".
A bioregenerative system works much differently. In a bioregenerative
system, natural biological processes are used alongside engineering control
systems to recycle life-support-system materials and commodities. For
example, photosynthetic plants take up the carbon dioxide vapor. Other
biological systems treat wastes - process known as "resource recovery".
This process enzymatically degrades organic substances to renewable resources
such as CO2 and H2O but consumes
O2 in respiratory and metabolic processes. The bioregenerative
system has the advantage of being able to produce food - not yet an potion
for a PC system. The advantage of the bioregenerative system is that resources
are recycled at normal temperatures and pressures and with much less operational
cost of energy than PC systems.
Combined systems using the best and most efficient PC and bioregenerative
processes are called "hybrid" systems - and are probably the best kind
of ALS.
On any Mars journey, some kind of advanced life-support system will
be essential. When the rockets fire, boosting the Mars-bound vehicle out
of Earth orbit - there can be no turning around.
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