OK, If the title sounds vaguely familiar to some of you, it's because you know about Ben Bova's book, "Welcome to Moonbase," published back in the 1980s, but my interest in lunar colonization goes back many years to my youth, while growing up on the works of Robert Heinlein and Arthur C. Clark. In his books, Robert Heinlein made it abundantly clear that allowing mankind to limit itself to inhabitting our one planet would eventually lead to disaster for our species.
Maybe not now, or in the next century, but eventually (inevitably -- as in 'it IS going to happen') a large enough global-scale disaster will strike and wipe out most of the species on earth and us humans too. Perhaps the disaster will be an asteroid impact, or runaway global warming, or a disease, or a large-scale volcanic eruption. In any event, human civilization could be completely wiped out. That is, it will if we don't spread ourselves out throughout the solar system and beyond. The only guaranteed strategy for survival of any species facing large scale disasters is to spread out as much as possible, to expand into new environments as soon as the capability to move into them becomes available. Therefore, since global scale disasters are a potential threat to our species, spreading out to the other planets and into space is the only guarantee of our survival. It's also much more entertaining and educational than just sitting around on our crowded planet, waiting for the asteroid to arrive, or the big volcano to explode, or the new plague to erupt, etc.
So if we are to spread out into space, what should be the first steps?
Here are some options:
LOW EARTH ORBIT
This is where the ISS is being built now. There are several advantages to being located in low earth orbit. The primary advantage is the relatively easy access to earth and earth resources. Initially -- and probably always -- low earth orbit colonization will be heavily dependent upon earth resources. The primary reason to establish space stations in general is to take advantage of the nearly constant supply of solar energy, the hard vacuum around the station, and the microgravity environment. Space tethers could also be used to harvest electrical energy directly from the low earth orbit environment as well.
The disadvantage with the low earth orbit environment is that the accumulated
effects of infinitessimal atmospheric drag will eventually cause the space
station colony to "de-orbit", which is NASA's nice way of saying "burn
up in the atmosphere and crash into the earth like a big meteor."
This is corrected by boosting the station back into its orbit with rockets.
This activity is called station keeping, and needs to be done every six
months or so to avoid "de-orbitting". Also, depending upon
orbit height, radiation may become a problem due to the Van Allen radiation
belts. For these reasons and others, it is probably unlikely that
low earth orbit would be used for space colonies.
GEOSYNCHRONOUS EARTH ORBIT
This is where a close-in earth orbitting space station should be built. A geosynchronous earth orbitting station is located about 35850 km (22400 mi.) above the surface of the earth. Since its position in the sky never changes (if it's put in an orbit that exactly lines up with the equator), then the station can be used as a communications platform (like a communications satellite), weather monitoring platform, etc.
Like the low earth orbit station, the geosynchronous station can also
take advantage of the nearly constant solar energy supply, the microgravity
environment, and the hard vacuum around the station. Additionally,
the geosynchronous station, for all intents and purposes, is no longer
affected by drag at that orbital height, and so will be highly "de-orbit
resistant". The disadvantages are that it would be more difficult
to get to from the earth than the low earth orbit station due to it's higher
altitude, and would most likely still be highly dependent on earth for
its physical resources.
L-5 COLONY
The term L-5 generically refers to locations in the lunar/earth orbittal system called Lagrangian Libration points that are really nice places to locate satellites and space colonies due to their natural stability. Satellites in low earth orbit and geosynchronous orbit tend to drift in their orbits. A satellite located in one of the lagrangian libration points may require no additional station keeping whatsoever (technically not true for some of the lagrangian points). The two most stable, and therefore most useful lagrangian libration points have been designated L-5 and L-4. L-5 is roughly located in the lunar orbital path, about 60 degrees behind the location of the moon. The L-4 shares the same characteristics as the L-5 point, but is 60 degrees ahead of the location of the moon. If you were looking up at the moon when it was directly overhead, then the L-5 would be in the general direction of a point in space about two thirds of the way down towards the western horizon, and would be about 380,000 km away, or roughly the same distance as the earth to the moon; the L-4 point would be about two thirds of the way down towards the eastern horizon and also about 380,000 km away.
The L-5 station is very much like the low earth orbit and geosynchronous
stations: Sunlight is constant, the vacuum is hard, and microgravity abounds.
However, from a resource point of view, L-5 is very far away from home.
The L-4 and L-5 points only make sense if resources can be moved to the
lagrangian points easily, which implies shipping material up from the moon
or moving an asteroid into the lagrangian point and mining it for resources.
On the other hand, the L-5 site is sufficiently far away from the earth,
that the colony can safely take advantage of asteroid mining techniques
without endangering the earth. Many people are very interested
in the possibility of working and living in the L-5 region of space, and
you can find out more about them by checking out some of the National Space
Society's local chapters, for instance the Huntsville
Alabama Chapter.
LUNAR COLONY
The biggest advantage a lunar colony has over the previously mentioned locations is that it is resource rich. Admittedly, not all of those resources are easily extracted, but they do exist in mass quantities. This is important, because from an energy expending point of view, the moon is a bit farther away than an L-5 colony because a spaceship going to the moon has to land in a gravity field.
The primary disadvantage to a lunar colony is that in most places
on the moon sunlight, and thus cheap power, is only available for two weeks
out of the month. This means that a lunar colony most likely cannot
run on solar power alone. Nuclear power is one option, but it isn't
the only option. Another option might be to build a plant to break
down water into hydrogen and oxygen during the two weeks of sunlight, then
recombine the hydrogen and oxygen in fuel cells during the two weeks of
darkness to generate power. Another option is to locate the lunar
colony on one of the poles of the moon. While it is true that some
of the craters at the lunar poles are in perpetual darkness, some of the
lunar peaks are almost always lit1
, therefore a lunar polar base has the advantage of almost constant sunlight
and the close proximity of one of the moon's most important resources for
colonization: water ice.
MARS COLONY
A mars colony would be very nice. Mars has much more water than the moon, and it has an atmosphere, albeit a mighty thin atmosphere. Still, as long as there are compressors, the martian atmosphere can be collected and used. The atmosphere on mars is primarily carbon dioxide, so plant life would be needed to convert the carbon dioxide into oxygen. Of course, this is also true for the long term survival of any of the colony concepts mentioned so far. Atmospheres also provide weather and wind. Therefore wind power might be a viable energy source on mars, in addition to solar energy. The atmosphere also provides for easier planetary landings, because spacecraft and use heat shields and wings to reduce their speed instead of brute rocket power. The martian day is roughly equivalent to an earth day, and temperatures exist within a much narrower and less hostile range than most places in the solar system (besides earth).
The martian atmosphere has two disadvantages too: monsterously
fast wind storms occasionally scrape the surface of mars and may play havoc
with a mars colony; and atmospheres limit the rocket launch options available
to a colony. Mars has twice the gravity of the moon, so rocket launches
require much more energy to achieve orbit. The most significant disadvantage
with mars, however, is that it is profoundly far away in comparison to
everything previously mentioned. Although the energy needed to get
to mars may not be excessively more than the energy needed to get to the
moon, a nine month journey is required to get to mars (in comparison, going
to L-5 or the moon takes about 3 days). Also, there is a limited
window of opportunity to travel between mars and earth. That window
of opportunity only exists every two and a half years. Consequently,
a mars colony is very isolated indeed.
WHY THE MOON
In a nutshell, the moon has ready access to resources, and it's relatively close. All other sites, with the exception of mars, are at the current moment resource limited. Low earth orbit and geosynchronous earth orbit colonies have the advantage of being close to the earth, but require station keeping to stay in orbit (which limits their size -- especially true for low earth orbit colonies).
In fact, for one of the other proposals to work (L-5), a lunar colony would be a prerequisite. Materials for an L-5 station could be sent to the L-5 region of space from the moon through the use of a device called a mass driver. A mass driver is essentially a very fast, high acceleration maglev train. Building materials, and even spacecraft, could be sent into orbit this way: A maglev train car on the moon would be loaded with a shipment of material bound for the L-5 station and accelerated to near lunar escape velocity, or about 8500 km/h (5300 mph). When the train had reached the appropriate speed, the shipment or spacecraft, would simply be released from the train. At such high speed the natural curvature of the lunar surface would be more pronounced than the shipment's orbital path, so for an observer on the train, the shipment would appear to rise off of the train on its own and slowly accelerate away into space. From a stationary point on the moon, the shipment's path would look more like a cannon shot.
This would work well on the moon, because the moon lacks an atmosphere that would otherwise prevent the train from achieving such high speed, or that would slow down the shipment once it was released from the train. After payload release, the train decelerates along a section of track about as long as the original acceleration section of track. If the train could be accelerated and decelerated at 10Gs the launcher would need about sixty kilometers of track2 to meet the acceleration and deceleration requirements for escape velocity launches.
So here is a construction project that could be built on the moon that
would be a delivery system for materials to the rest of the solar neighborhood.
Those materials and potentially spacecraft in turn would be the basis for
a lunar economy. L-4 and L-5 will eventually be prime real estate
sites in the solar system, especially when asteroid mining comes to fruition.
Until that time, the moon is the most convenient place in the solar system
for humanity to establish a completely self-sustaining colony off of the
earth.