Why does crab grass grows through cracks in the sidewalk? Why do trees grow in tiny crevices on sheer granite cliffs? Why did life crawl out of the oceans and onto land? Answer these questions and you'll know why we will colonize space.
Our ancestors have lived on this planet for over three billion years. They survived asteroid hits that burned the planet to a crisp. They survived deep freezes that may have covered the entire globe with hundreds of meters of ice. Our ancestors, all the way back to the first bacteria, survived because they were very tough and took every possible opportunity to grow, covering every nook and cranny - no matter how miserable -- with life.
Everywhere life has gone, life made things better for number one. Namely, more life. That's life's best trick: never miss an opportunity to mold a dreadful mess into a nice, or at least bearable, place to live. Trees make a hot day shady, grass makes a barren hill lovely, plants make oxygen for the animals, and animals make CO2 for the plants. Earth was once nothing but rock and radiation, but now we've got Hawaii, Cancun, New York, and Paris. Most of this solar system is little more than rock and radiation today. Perhaps its time to change that.
As the solar system's only space-faring species it falls to humanity to turn the unimaginably hostile environment of outer space into nice place to raise the kids; with good schools, clean air and water, peaceful neighborhoods, and enough excitement that the teenagers don't die of boredom.
It's a little hard to argue against making great places to live for people, but there's a catch. It will be very expensive. True, the current space program is not particularly expensive, less than 1% of the U.S. federal budget. However, space colonization will cost a great deal. Why should we spend money on space when there are so many problems here on Earth? After all, isn't feeding the hungry and healing the sick more important than building cities in space?
Yes, it is, which is why we spend far more money on feeding the hungry and healing the sick than on space exploration. However, our elected representatives, in unusual fit of wisdom, seem to understand that if we spend all of our money on today's problems we will never create tomorrow's solutions.
This wisdom has been handed down by a thousand generations of farmers. Every farmer knows that he must save some of his crop for next year's seed. If you eat your seed corn, you'll starve next year. Similarly, every successful society spends some of its wealth developing new capabilities. New capabilities that will create even more wealth in the future. Space colonization can create such vast wealth that tomorrow's poor would seem rich to us. Furthermore, space colonization can directly solve some of today's most critical problems.
For example, space has already contributed to global understanding. Communication satellites have made world-wide communication, and occasionally even dialogue, an everyday reality. While there is still plenty of misunderstanding to go around, every TV carries footage of foreign lands every day. Any of the hundreds of millions (soon to be billions) of people on the internet can connect to foreign web sites in a few minutes. The full implications of this radical change in communication patterns is yet to be seen.
Space has helped keep the peace. In particular, spy satellites were critical in keeping the U.S. and the U.S.S.R. from destroying each other with nuclear weapons in the Cold War. Each side could see what the other was doing, rather than assuming the worst. It is quite likely we owe spy satellites our lives. Without them the two great super-powers may well have blundered into war, destroying much of civilization with their massive nuclear arsenals.
Space has also supplied on of the most powerful images of our times: the Earth as seen from space, a small, beautiful blue and white ball hanging in the utter desolation of our solar system. This image has driven home the indisputable fact that we are all passengers on spaceship Earth, and our fates are intertwined for better or worse.
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On a more mundane level, satellites allow us to predict the weather much more accurately than otherwise possible. While it may seem that the weatherman couldn't get the next rainy day right to save his life, weather prediction is far more accurate than only a few decades ago largely because weather satellites let the weatherman see what's going on. Avoiding rain on your picnic is nice but trivial, the ability to see hurricanes coming hours or days in advance can mean the difference between life and death.
Space has also been crucial to develop our admittedly limited understanding of our world. Perhaps the most dramatic example was that Brazilian government wouldn't believe the rain forest was being burned down until they saw satellite photos. The ozone hole is monitored by satellite. Much of the data supporting global warming theory is based on satellite data. The list is endless. There is no hope of understanding the environment without satellite data, and our obedient robot Earth-observation satellites are supplying us with more and better images of Earth year after year.
The $16 billion a year we give NASA has paid off handsomely. If we are smart enough to colonize the solar system it will pay off a million-fold. However, to my eye, NASA today seems a bit confused.
If you ask NASA what the space program is about they will say science and exploration, and for the robotic spacecraft program that's probably true. But most of the money doesn't go to robots, it goes to human space flight. Unfortunately for humans in space, robots have vastly outperformed humans in space exploration and science.
We like to think of ourselves as great explorers, and we are. Humans have explored every nook and cranny of Earth above the water line, and a great deal of the oceans as well. In space, humans have visited the Moon and Low-Earth Orbit. On the other hand, robots have visited Mercury, Venus, the Moon, Mars, Jupiter, Saturn, Uranus, Neptune, and several comets and asteroids - at far less cost.
Not only are robots much cheaper, they have produced far more scientific data. Deciding which data to gather and analyze is best done by humans, but this can be done by sending orders from Earth to robots in space. Some people will wave their hands and say that humans are smarter, more capable, and more flexible than robots. That's true, but it doesn't mean humans do a better job of gathering data.
We don't need to make theoretical arguments about whether humans or robots do the best space science. We've flown human and robot space missions for over 40 years and we've got the results. If we could measure scientific output, then we would know if humans or robots are best.
Science output can be measured by Nobel prizes and papers published in the best journals. Its not a perfect measure, but this contest isn't even close; so an imperfect measure is plenty good enough. Neither robots nor humans in space have won any Nobel prizes. However, in the scientific pecking order, brownie points are awarded for publishing in scientific journals. Some journals are considerably more equal than others. In particular, Science and Nature are at the top of the heap. After these two, there are better and worse (meaning more or less prestigious) journals in every scientific field. Data from robotic spacecraft appear regularly in the best journals. Data from the human spaceflight program rarely do. This is not a small effect. In years of reading Science I have never seen a paper derived from the human space program. Robotic programs do, however, get papers published there from time to time.
There is one major exception (sort of): the Hubble Space Telescope. The Hubble is a large telescope in orbit around the Earth. From its perch above the atmosphere, it has an unmatched view of the cosmos. Furthermore, although Earth-bound telescopes can only be used at night, the Hubble can be used 24/7 so long as it doesn't point at the Sun, the Earth, or the Moon. All of these are so bright that their light would destroy sensitive components of the Hubble Space Telescope.
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The Hubble has revolutionized our view of the universe, producing thousands of beautiful and important images of the heavens (see hubble.nasa.gov). Dozens of important scientific papers based on Hubble data have appeared in the best astronomy journals, as well as in the flagship journals Science and Nature. All this even though when the Hubble was first launched it had a terrible flaw.
The Hubble is a robot, but it was put in orbit by the shuttle with human pilots. The shuttle is a winged spaceplane developed by NASA in the 1970's and early 80's to transport people and equipment to and from low Earth orbit (LEO). The shuttle is the most capable launcher in the world. It can take 10 or more people and many tons of equipment from Earth to orbit. Among the great accomplishments of the shuttle was putting the Hubble Space Telescope in orbit. However, there was a problem.
The Hubble telescope has a large mirror, called the primary mirror, which concentrates the faint light given off by objects such as the Cat's Eye Nebula. Primary mirrors must be shaped very carefully to produce a sharp image. When the Hubble was first placed in orbit, it was discovered that the mirror had a terrible flaw. The Hubble had blurry vision. The primary mirror was misshapen. If the Hubble had been like almost all satellites before or since this would have meant that Hubble would have been useless for much, although not all, of its mission. However, the Hubble was different. It was designed and built to be repaired by shuttle astronauts.
NASA designed the equivalent of a pair of glasses for the Hubble, and some very capable and courageous astronauts installed them, fixing the Hubble's blurry vision. Since then, astronauts have visited the Hubble several times fixing problems and upgrading the great telescope. It sounds like a rousing success for humans in space enabling great science, and it is. But there's one minor catch.
Each shuttle mission costs about $800,000,000. To see this note that the shuttle program costs about $4 billion per year. Since 1981 when the first shuttle flew, there have been about 113 shuttle flights, roughly five flights per year. Thus, each flight costs about $800 million. For something close to that price, we could have built another great space telescope and launched it on a cheaper vehicle. Then, instead of having one Hubble in orbit, we'd have two or three space telescopes. The shuttle is the most capable launcher in all of history, but it is also very, very expensive. On a dollar for dollar basis, the record shows that it can't produce science competitive with robots.
Science is all about answering questions. For some questions humans in space are essential, or at least very useful. However, none of these questions needs to be answered right now. We can afford to wait a bit. Once space tourism makes humans in space cheap and ubiquitous we can get to them. In the meantime, for most space science robots are better and much, much cheaper.
Does this mean the human spaceflight program is a giant waste of taxpayer money? No. Human spaceflight is unquestionably the most important project mankind has ever, or will ever, undertake. Only human spaceflight can bring our mostly dead solar system to life. Only human spaceflight can insure the survival of our civilizations, our species, or for that matter life itself. Only spaceflight can give us weightless sex lasting more than thirty seconds :-). By comparison, space science, however fascinating, is of minor importance.
There are three noble reasons to colonize space.
People can be noble, and will sometimes act out of the best intentions. However, it's a much safer bet that people will grasp power and wealth when the opportunity arises. Of course, wealth and power significantly improve survival and growth, so perhaps the difference between noble and craven motivation is not as great as one might like to think. Let us now take some time to examine the major reasons for space colonization one at a time.
As far as we know, there is no robot transported from the future trying to wipe out the human race. But sooner or later, for one reason or another, the Earth will become uninhabitable. Perhaps slowly as the Sun dies, or in a flash when the next asteroid slams into us; perhaps in a billion years, perhaps next week, but it is inevitable. If we do nothing, our extinction is as sure as tomorrow's sunrise; although timing is significantly less predictable.
Space colonization can insure the survival of civilization, humanity, and life itself for the indefinite future by spreading all of them out over billions of kilometers (miles). Without space colonization, all of these are limited to one small rocky planet which will be destroyed sooner or later, although perhaps much later. Nothing lasts forever, and life on Earth as we know it is no exception. Space colonization can insure survival by a time-tested means - spreading out so any disaster only gets some of us. Simultaneously, space colonization will give us the power to avoid the most devastating disasters Earth will face in the next few million years - asteroid collisions.
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It's unlikely, but a large comet or asteroid could impact the Earth in 15 minutes and eventually we will be be hit, for sure. We are hit by thousands of smaller asteroids every year and we don't see any of them before the collision. Detection of larger, Earth-threatening rocks is very far from complete. At the present rate it will take years before we find just 90% of them. Beside these inevitable cosmic disasters the long list of potential human-induced potential calamities -- nuclear war, ecological collapse, global warming, epidemics, etc. -- are less certain and far less dangerous, although much more likely in the near term. After all, the worst of these would probably kill less than three quarters of the people on the planet. A good sized asteroid will get us all.
We have been warned. In October of 1990 a very small asteroid struck the Pacific Ocean with a blast about the size of the first atomic bomb; the one that leveled Hiroshima, Japan killing roughly 200,000 people in seconds. If this asteroid had arrived ten hours later it would have struck in the middle of more than a million U.S. and Iraqi soldiers preparing for war. It could have struck near U.S. forces. The U.S. would have thought Iraq attacked with a nuclear weapon. America would have used its immense nuclear arsenal to turn Iraq into a radioactive wasteland, and even one nuclear bomb can ruin your whole day. Don't worry though, these small asteroid strikes only happen about once a month [Lewis 1996b]. Besides, it gets worse.
In 1908 a small asteroid (perhaps 50 meters across) hit Tunguska, Siberia and flattened 60 million trees. That asteroid was so small it never even hit the ground, just exploded in mid-air. If it had arrived four hours and fifty-two minutes later it could have hit St. Petersburg [Lewis 1996b]. At the time St. Petersburg was the capital of Russia with a population of a few hundred thousand. The city would have cease to exist. As it was, dust from the blast lit up the skies of Europe for days. Asteroid strikes this size probably happen about once every hundred years. However, this is just an average. Just because we got hit once doesn't mean we're safe for another hundred years. Indeed, there was another Tunguska-class strike in the Brazilian rain forest on 13 August 1930 [Lewis 1996b]. But don't worry, it gets worse.
There are about 1,000 asteroids a kilometer or more in size that cross Earth's orbit (the path Earth takes around the Sun). About a third of these will eventually hit Earth [Lewis 1996a] if we don't do something about it. An asteroid strike this large can be reasonably expected to kill a billion people or so, depending on where it hits. A strike in China or India will kill more, in Antarctica less. Even a strike in the ocean would create a tsunami so enormous most people living near the coast would be drowned. A strike of this size is expected about once every 300,000 years or so. We might as well be playing Russian roulette. Admittedly, the revolver has 300,000 cylinders, but if we keep pulling the trigger long enough we'll blow our head off, and there's no guarantee it won't be the next pull.
It's not just Earth. In 1178 our Moon was hit with by an asteroid creating 120,000 megatons explosion (about six times the force of Earth's entire atomic arsenal). The collision dug a 20 km (12 mile) crater. This strike was recorded by a monk in Canterbury, England. We are extremely lucky it didn't hit us. The moon is a smaller target and has much less gravity to attract an impactor. If a 120,000 megaton blast had hit the Earth our history would have been dramatically different. We're just lucky this one hit the Moon instead.
The most recent large strike also missed Earth. In July 1994 the comet Shoemaker-Levy 9 plowed into Jupiter. The comet broke up into roughly 20 large pieces before contact, but when the pieces hit they left a string of enormous explosions clearly visible to our telescopes. The scale of the destruction was staggering. Each impact was the equivalent of about 10 million megatons of TNT. If Shoemaker-Levy had hit Earth instead of Jupiter, in the extremely unlikely event you were alive you certainly wouldn't be reading this book. You'd spend every waking moment trying to survive. But don't worry, it gets worse.
Sixty-five million years ago a huge asteroid several kilometers across slammed into the Yucatan Peninsula in Mexico. The explosion was the equivalent of about 200 million megatons of dynamite, about the equivalent of all 20 pieces of Shoemaker-Levy. The blast turned the air around it into plasma - a material so hot electrons are ripped from the atomic nucleus and molecules cannot exist. This is the stuff the Sun is made of. Enormous quantities of red-hot materials were thrown into space, most of which rained down worldwide burning literally the entire planet to a crisp. Anything not underground or underwater was killed. Evidence gathered by the University of Colorado at Boulder suggests that all the dinosaurs above ground were incinerated in a few hours reference. Surprisingly, only about 75% of the plant and animal species on Earth were exterminated. What's surprising is that everything wasn't wiped out. This scenario has been repeated over and over, perhaps once every 100 million years or so. Each collision killed up to 95% of all species on Earth. As many as two-thirds of all species that ever existed may have been terminated by asteroids hitting the Earth.
We know about the asteroid that killed the dinosaurs because we found the crater. But what happens when an asteroid hits the ocean? After all, oceans cover two-thirds of the Earth's surface. Most asteroid strikes must be in water. Unless the asteroid is very large there won't be a crater. However, if you drop a rock into a lake it makes a wave. The larger the rock the bigger the wave. Drop a 400 meter (three football fields) diameter asteroid into the Atlantic Ocean and you get a tsunami 60 meters (yards) high [Willoughby and McGuire 1995]. Do that today and beach side property values will plummet due to the sudden and complete absence of any people or buildings. Almost every human culture has a flood story (for example, Noah's Arc). These may be the living memory of asteroids hitting the oceans. This is not idle speculation, there are several hundred thousand asteroids in near Earth orbits large enough to cause world- wide casualties by creating tsunamis [Freedman 1995].
Of course, if the asteroid is big enough, even a hit in the ocean will rearrange the Earth's crust. Researchers from the University of Toronto and the Geological Survey of Canada determined that an asteroid the size of Mt. Everest probably hit the Earth about 1.8 billion years ago, and literally turned part of the Earth inside out [http://www.spaceref.com/news/viewpr.html?pid=14341]. The crater is about 250 kilometers (156 miles) wide. It's amazing that anything survived at all, but somehow a few of our single-celled ancestors lived through the ensuing hell. Life started on Earth over 3 billion years ago, but no large animals appeared until about 700 million years ago - perhaps because of the devastating bombardment Earth was still suffering through.
These are just a small sampling of the cosmic threats to Earth. We are living in an orbital shooting gallery. There are lot of objects out there and, someday, many of them are going to hit something - the Sun, another planet, or Earth (some will also be ejected from the solar system). Of the known near earth asteroids, between 25 and 875 large objects will hit the Earth causing global devastation and another 400 to 6250 smaller objects will strike the Earth's oceans causing tsunamis devastating coastal regions [Willoughby and McGuire 1995]. We just don't know when. We don't know if the next strike will be in five minutes or 50 million years, but we do know it will happen. It's just a matter of time. Only mankind can end this threat. If we don't do it, no one else will. No one else can. Fortunately, we are starting to pay attention.
The first step is to simply find the dangerous asteroids and comets. NASA has a small program to locate potential Earth killing asteroids - those more than a kilometer (a bit more than half a mile) across. At the current rate it will be decades before they are all found. If one of the unfound has our number on it, it's not only Houston that will have a problem. Even an asteroid only a couple hundred meters (yards) across may, if it hit the Atlantic Ocean, create a wave that would completely wash over Florida, making it irrelevant in the next Presidential election. We aren't even trying to find the near-Earth asteroids in this size range, although NASA is reportedly formulating a plan to present to Congress.
We have found hundreds of kilometer-sized Earth orbit-crossing asteroids and determined their orbits well enough to know that these, at least, pose no threat in the immediate future, although one does have a small chance of getting us in 2036. If we took this particular threat seriously, we would have time to divert it. In principle, such an asteroid can be given just a little shove and will then miss Earth. While we're not sure of the best way to do this, if we knew a collision was coming one can be confident that every scientist and engineer on the planet would be bent to the task. Funding would not be a problem. Who knows, faced with such a cosmic threat we might even stop killing each other temporarily.
Increasing the speed of an asteroid by only 1 cm/sec (about a quarter of a mile an hour, some babies crawl faster) a few years before a collision is enough to turn a hit into a miss [Willoughby and McGuire 1995]. Such a small push can work because of way asteroids move. Asteroids orbit the Sun. When an asteroid speeds up or slows down the orbit to go up or down a bit. You can use this to turn a hit into a miss, but there is another, perhaps more effective way. A gentle shove will also change the time an asteroid will arrive at the Earth's location. Specifically, the asteroid will arrive at the collision point either before or after the Earth does. It's like driving a car towards a pedestrian crossing the street a few blocks ahead. If you stay at the same speed you'll hit the guy, but if you slow down just a little bit he'll have crossed the street by they time you get there. The bottom line is, if you are careful, know what you are doing, and see it coming, the threat of asteroid collision with the Earth can be eliminated.
Willoughby and McGuire describe twelve ways to deflect asteroids[Willoughby and McGuire 1995]. Here's a brief description of each approach.
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Credit : NEAR Project, NLR, JHUAPL, Goddard SVS, NASA |
One of the problems with orbital real estate is a distinct lack of materials. Actually, a complete and total lack of materials. Oh, there are a few atoms here and there, a stream of solar particles (the solar wind), and perhaps even a speck of dust, but no where near enough matter to make a grain of sand, much less a kilometer-scale space colony. If an asteroid is headed on a collision with Earth, the best thing to do with it is to use the materials to build space habitats! After all, the materials are already headed for Earth, it's just a matter of changing the aim point a bit and converting the materials into something we can use. In the section on growth we will see that these materials can make truly astronomical amounts of new land available. In the section on wealth we will see that these materials can make someone, maybe you, filthy rich. Thus, an asteroid that once threatened billions of lives can be turned into land and money. Not a bad deal.
We have at least a chance of finding the asteroids that will hit us in time to do something about it. Asteroids almost always stay in the inner solar system and can usually, although not always, be observed decades or even centuries before they hit Earth. Not so comets. Typical comets spend most of their life far from the Sun in highly elliptical (elongated) orbits. Occasionally, one comes screaming through the inner solar system at extremely high speed before heading back out. While asteroids tend to hit Earth at about 20 km/sec (45,000 miles per hour), long period comets strike at around 50 km/sec (112,500 miles per hour) [Willoughby and McGuire 1995]. For example, Halley's comet visits us only once every 86 years, spending only a short time near enough for Earthlings to observe it and decades lolly gagging out past Pluto. Even with a truly first-rate observation system (which we don't have at the moment), if a comet is on a collision course with Earth we will have a few months warning, or less. There simply won't be enough time to deflect it, no matter how hard we try. We will be doomed.
Fortunately, since comets spend much less time than near-Earth asteroids in the inner solar system, there is a much smaller chance of collision, so we probably have some time. Given the short warning times a civilization limited to Earth would have, only a solar system wide observation system can identify problem comets in time. Worse, the system has to keep functioning for millions of years. This will never work.
However, comets contain vast quantities of water. They will be of great value to space colonies. The truly large-scale orbital civilization this book advocates (ten trillion or more people) will almost certainly exploit the comets as resources and, in the process, end the threat the Earth. Although this may take tens of thousands of years, the chance of cometary collision is so small that we probably have the time.
Civilization, humanity, and perhaps life itself cannot survive forever on Earth. At a minimum, we need the ability to detect and deflect incoming asteroids and comets. We don't have that now, but we can develop it. Failing to do so would be a gross abdication or responsibility, roughly equivalent to leaving a six month old alone in the house full of loaded guns for the day. Fortunately, what is needed is not self-denial, but rather growth. This should not be surprising, growth is a common survival strategy among nearly all successful life forms. We just need to keep doing what our ancestors have done for the last three billion years.
Ceres, the largest asteroid, has enough material to build orbital colonies with a livable surface area about 200 times greater than the entire Earth. That's a lot of elbow room. In fact, that one asteroid there's enough space for at least one trillion people, assuming the same density as Earth. Of course most of the Earth is essentially unpopulated - the oceans cover 2/3 of the Earth and most deserts and mountains are pretty empty. But just in case my calculations are optimistic, lets say there's only room for ten trillion folks. Ten trillion is a nice round number and will do for now.
While growth is its one reward, there are others. One immediate Earthly benefit of this growth potential is less war. Specifically, wars motivated by the desire for territorial won't make sense any more. Of course, some people think territorial wars don't make sense now and they have a point. However, if a country wants more land there's only one way to get it: force.
No one really knows how many people died in the war, but 50-100 million is a good guess. If space colonization had been an option the Germans and Japanese could have met their need to grow by peaceful means - building space colonies. This is not to say that that space colonization would definitely have prevented World War II. Maybe, maybe not. But rather that one of the primary, and most legitimate, motivations could have been satisfied by peaceful means.
Although space colonization may or may not have prevented World War II, it probably would have prevented the Arab-Israeli conflict. Besides conquering much of Europe, the Nazis also tried to exterminate Europe's Jews and nearly succeeded. Not only did the German's nearly exterminate them, but Jews who fled Europe were often sent back to their death. The U.S. was one of the many countries that did this.
After the war, the Jews, not surprisingly, believed that they needed a country of their own for protection from future genocide. The Jews wanted a country that would always take them in. There was a great debate in the Jewish community over where the new Jewish state should be. Needless to say, outer space was not an option. Too bad. Palestine (much of which is now known as Israel) was chosen.
Israel/Palestine was attractive since Jews had viewed it their homeland for thousands of years. The Jewish religion holds that Palestine was promised the Jews by no less an authority than God himself. However, the Jews had been expelled from Palestine by the Romans and hadn't controlled that land since. Furthermore, many devout Jews believed that they should not return to Israel until the Messiah came.
On a more practical note, most knowledgeable people believed that the Arabs, who had lived in Palestine for a very long time and considered it their own, would destroy Israel. The Arabs had professional armies and navies whereas the Jews did not. There were perhaps 20 million Arabs in the Mid-East and North Africa and only a few hundred thousand Jews in the holy land. Arabs controlled all of North Africa and most of the Mid-East. Israel is only about 300 miles long and 70 miles wide. In 1948, when Israel was established in part of Palestine, it seemed that the Jewish state would be extremely short lived.
On the other hand, no matter where the Jews sought territory they would have had to fight. Every inch of the Earth is already claimed by someone. The Jews chose war. Absent space colonies, this isn't surprising.
The state of Israel was created, survived the first and later Arab assaults and has been fighting the Arabs for over 50 years now. Had space colonization been an option in the late 1940s is it quite possible that the Jews would have chosen to go into space rather than the Mid-East. In effect, choosing to build rather than fight. In space there is no one who considers the land their own. You don't fight for land, you build it. This would have completely avoided the entire Arab-Israeli conflict, the creation of hundreds of thousands of Palestinian and Jewish refugees, and saved an enormous amount of money not to mention countless lives.
World War II and the Arab-Israeli conflict are only two of many territorial wars throughout history. Many wars, although not all, are driven by a desire for land. The Mongol conquest, the European conquest of the Americas, the Roman conquests, the early expansionist wars of Islam, and countless others fall into this category.
In some cases two peoples are very set on a particular piece of land and space colonization won't help there. However, often a people simply want more land and don't really care much where it is. In this case orbital space colonization can fill the need without war. Large-scale space colonization will eliminate the motive for this class of wars and, with just a little bit of luck, make our world a significantly safer place. For the first time in thousands of years it will be easier to build land than conquer it.
Today we are trying to end war by fixing the boundaries of all countries, effectively preventing them from growing. This has worked to a certain extent, but stifling growth is a Sisyphean task. Countries are a lot like living things, and all living things grow. The desire to grow is one of the fundamental properties of life. It's not going to go away. Colonizing the solar system can give us somewhere to go.
In case you haven't noticed, Earth is getting a bit crowded. Most of the nice places have plenty of people, and more is not really a good thing. Some societies, such as China, have taken draconian measures. China limits couples to a single child in the name of population control. But children are wonderful, as any good parent knows. They are, to a great extent, what makes life truly worth living. An unhappily childless couple, or a couple with fewer children than they want, is a tragedy.
We need more room and space can supply it. While most individual colonies may only hold 10-50,000 people, when one colony fills up we can build another. If a child's home colony is full up when they get to adulthood, it is perfectly reasonable to make another exactly like it (or with improvements!). The new colony can be put in a compatible orbit so that taking the babies to see grandma is a very quick trip. There will always be more land, plenty of land for as many children as humanity wants to produce.
Earth is rapidly becoming more homogeneous, you can eat a MacDonald's hamburger in nearly any city on the planet. As transportation and communication systems improve, the diversity of social systems is declining. Earth's communities are closely tied together by a common environment and easy movement of people and materials. Thus, the ills of one society are readily visited on the next. For example, the polluted air and water of one country often comes to rest in another. The environmental sins of one are a problem for us all.
Orbital space colonies can provide thousands, if not millions, of completely separate ecosystems with easy-to-control borders. Poorly run space colonies cannot easily export their environmental problems. Each colony, holding from a few hundred to perhaps as many as a million people, is a completely enclosed ecosystem separated from every other colony by a very complete vacuum. The environmental sins of one will not be visited on another.
If a colony chooses to experiment, to try new things that don't work and creates some horrendous consequences, no other colony will be directly affected. There will be no need, as there is on Earth, for global treaties preventing a society from doing pretty much as it pleases. Not only is the environment of each colony separate and unique, immigration will be very easy to control.
Getting in or out of a space colony requires going through an airlock. An air lock is a small room with two airtight doors, one opening into a colony and the other into space. When someone wants to get into a colony their ship attaches to the empty air lock. After the attachment has been sealed properly, air is pumped into the air lock then the colony door can then be opened and you can go in. This procedure doesn't lend itself to illegal immigration. Colonies that so choose can isolate themselves very easily. Although I personally wouldn't like the uniformity within my colony, this isolation can be expected to lead to great variety between settlements, everything from strict Muslim settlements where everyone is covered head-to-toe in public to colonies of nudists.
Not only can individual colonies easily isolate themselves, the nature of transportation between orbits lends itself to creating groups of colonies between which transportation is easy. On the other hand, colonies in incompatible orbits are very hard to reach, requiring a great deal of energy, time, or both.
Colonies in compatible orbits are on essentially the same path, but follow each other by a few thousand miles or so. If we spaced settlements 10,000 kilometers (about 6,250 miles) apart along Earth's orbit around the sun, there is enough room for about 94,000 space colonies . Traveling between these should be fairly easy and, for those nearby, quick.
If you put another 94,000 colonies in an orbit a million kilometers further from the Sun and tilted at an angle to Earth's orbit, it will take a great deal of energy to get from the first group of colonies to the second. Communication will be easy, but moving people and materials hard. Since 94,200 space colonies with 50,000 people each is about 475 million people, we now have two large societies physically isolated from each other, but with excellent communications (if they so choose). There are thousands of similarly isolated orbits, each capable of hosting a half billion or more people. Isolation begets diversity. Each of these groups is likely to take a different social tack.
Although some colonies may choose to isolate themselves, most of humanity can be expected to stay connected. Even for groups of colonies that cannot easily transport people and materials back and forth, communication will be easy. In many cases one can simply point a laser at the colony you want to communicate with and send huge amounts of data. On Earth we do this by sending laser light through fiber optic cables. Most of the world's long distance communication takes this approach today.
The laser light makes very small changes very quickly. The data is in the changes, just as when we talk we vibrate the air. Small changes in air pressure are recognized by our ear and brain as words. The data in the laser light can easily encode voice, sound, text, pictures, video, or whatever. In space the fiber optic cable is unnecessary. Just aim the laser where you want the communication to go. Thus, the majority of the ten trillion plus people that may someday inhabit the solar system can be in constant communication. The number of web pages will be staggering. No one, and I mean no one, will be able to keep up with the blogs.
A huge population in constant communication can create a society where artists, musicians, filmmakers, and others can easily find an audience to support their work. The potential audience is so vast that even a tiny fraction can give you an enormous fan base. For example, to sell a million CDs on Earth today you must sell a record to one out of every 6,000 humans on the planet. With a solar-system-wide population of ten trillion you can sell the same number of albums if only one person in ten million likes your music. In the U.S. today, if you only appeal to one in ten million, you will have a grand total of 30 fans. That's not enough to make a living.
A large population doesn't just supply an audience, is also supplies the artists. No one knows what makes one person a great artist and another not, but the more people there are the more great artists there will be. The same holds for scientists, engineers, humanitarians, philosophers, or whatever. Living in a solar system teaming with ten trillion living, breathing human beings could be very, very interesting.
Life started in the oceans nearly four billion years ago. For a very long time that's where it stayed. But by about 430 million years ago land animals had appeared. Why did life suddenly 'decide' to colonize land? What was wrong with the ocean? Well, nothing really. The ocean is still full of life, but land was colonized anyway. Those species that colonized land had a great deal of room to grow and have come to dominate the planet. Some even travel into space.
Colonization of land must have been incredibly difficult. Billions of plants and animals must have died when they left the hospitable ocean and tried to breath air. But a few survived, and we are descended from them. While ocean life is vibrant and beautiful, it is not space-faring. Thus, it cannot protect us from the asteroid bombardment, because ocean based life has not developed the technology to get into orbit. Its not for lack of intelligence, there are species of whales and dolphins that may be smarter than we are. That won't do much good when the next asteroid hits.
Homo sapiens started, as best we can tell, in a small corner of East Africa. For one reason or another, over the last few tens of thousands of years we colonized every continent. We spread from the warmth of Africa to the frigid cold of the European winter. This required the high technology of the day -- fire and clothing. We became big game hunters with stone spear tips to kill and teamwork supercharged by language. We spread throughout the world, probably a few miles at a time, until all of Earth was filled with small bands of humans. Today the population of Africa is a small fraction of all humanity. Almost everyone is descended from those who left home.
Now we are putting our first tentacles into space. With few exceptions, there have been 2-3 people in orbit almost continuously for about 25 years. While we mourn our dead, the astronauts and cosmonauts who have been consumed in fiery explosions of failing launchers and reentry vehicles, their numbers are a tiny fraction of those land colonization claimed, and the prize is much bigger. As we have seen our solar system has more than adequate materials to build orbital colonies totaling hundreds of times the surface area of the Earth with room for ten trillion people, perhaps more.
Actually, ten trillion humans is just my estimate. John Lewis, the eminent planetary scientist from the Arizona Space Engineering Research Center, makes a good argument that our solar system can support 10 quadrillion people [Lewis 1996a]. Maybe I'm just too conservative.
Gaia is all of life on Earth plus life's environment. Gaia is much like a single organism, actively keeping itself alive. Like most plants and animals Gaia doesn't think or plan in the same sense we do, but according to the Gaia hypothesis, life took control of Earth around 3.7 billion years ago. Since then life has manipulated the planet, especially the atmosphere, to maintain conditions beneficial to life. In my opinion, it is our destiny to become Gaia's midwives. We're going to help Gaia reproduce. This sounds pretty corny, and it is. Its also true.
There is a great deal of evidence for the Gaia hypothesis. For example, as long as they're alive, a person will keep themselves at a nearly constant temperature regardless of how cold or hot it is outside. People in Africa and Alaska have nearly the same body temperature. Over the 3.7 billion years of Gaia's existence on Earth the Sun has become perhaps 25% hotter, yet the Earth is roughly the same temperature. Gaia has maintained the Earth's temperature in the range suitable for life, probably by manipulating greenhouse gasses. To understand how this might work without forethought or planning, let us imagine what James Lovelock calls Daisy World.
Daisy World is an imaginary place the same size and shape as Earth, the same distance from the Sun, and with the same rotation rate. Unlike Earth, Daisy World has only one species, daisies. These can be either dark or light. Seeds from dark daisies usually, but not always, grow dark daisies and seeds from light daisies usually grow light daisies.
Our imaginary daisies grow best at 25oC, and less well at hotter or colder temperatures. For simplicity, we will ignore rainfall, soil conditions, grazing animals and so forth. If you've ever walked barefoot on a hot day, you know that asphalt, which is dark, gets a lot hotter than concrete, which is light colored. Similarly, dark daisies absorb more sunlight than light daisies, so the bits of Daisy World with lots of dark daisies tend to be hotter. Parts of Daisy World with mostly light daisies tend to be cooler.
In the beginning of time Daisy World has a weak sun, so Daisy World is colder than 25oC. Those areas that, by chance, have more dark daisies are a little warmer and grow better. As a result, dark daisies spread across Daisy World. As they do, Daisy World gets warmer. This continues until the temperature reaches 25oC or the world is covered by mostly dark daisies.
As the sun heats up (as we believe our sun has over the last few billion years) Daisy World, covered with mostly dark daisies gets hotter than 25oC. Places that, by chance, have a few more light daisies will be a bit cooler and closer to the ideal temperature. These areas will grow better and tend to have more and more light daisies. As the sun gets hotter light daisies will spread over the world, displacing the dark.
Notice that all this happens without forethought, planning, or even intelligence. It is inherent in the fact that dark daisies heat the environment and light daisies cool it, combined with the fact that dark daisies tend to create more dark daisies and light daisies behave similarly.
Computer simulations show that Daisy World can keep its temperature very close to 25oC even as the sun's temperature changes substantially. The temperature can be maintained even if large numbers of daisies are removed occasionally (simulating epidemics), when simulated rabbits eat the daisies, and under a wide variety of other conditions. These simple light and dark daisies control the temperature of Daisy World for their own benefit. The Gaia hypothesis claims that life does the same on the real Earth, and controls much besides the temperature. In effect, Gaiai is a selfish little so-in-so who runs the world for her own benefit. Works for me, I'm part of Gaia.
Although apparent regulation of the atmosphere's temperature is strong evidence for Gaia, the most definitive evidence is the composition of the atmosphere. Our atmosphere is mostly oxygen and nitrogen. If it weren't for life, our air would be mostly carbon dioxide like Venus and Mars. The Earth is between Venus and Mars and one would expect a similar atmosphere, but life has changed our air almost beyond recognition. Consider this table:
| Gas | Venus | Mars | Earth without life | Earth today |
| Carbon dioxide (%) | 96.5 | 95 | 98 | 0.03 |
| Nitrogen (%) | 3.5 | 2.7 | 1.9 | 79 |
| Oxygen (%) | trace | 0.13 | 0 | 21 |
| Argon (%) | 70 ppm | 1.6 | 0.1 | 1 |
| Surface temp (oC) | 459 | -53 | 240-340 | 13 |
| Pressure (bars) | 90 | 0.0064 | 60 | 1 |
We see that, without life, our atmosphere would be fairly similar to Venus and Mars, but with life Earth's atmosphere has far less carbon dioxide, much more nitrogen and oxygen, and is much less dense (the pressure also indicates the density).
Not only is the atmosphere much different with and without life, Earth's atmosphere is unstable. For example, our atmosphere contains both oxygen (O2) and methane (CH4). If you mix oxygen and methane together and shine sunlight on it, the methane will react with oxygen to produce carbon dioxide (CO2) and water (H20). If life were not constantly replenishing the methane it would quickly disappear.
Recent discovery of small amounts of methane in the martian atmosphere is, by far, the best evidence found that life may exist there. Unfortunately, the methane is probably just venting from reservoirs inside the planet. Pity.
Mars and Venus have chemically stable atmospheres, Earth does not. This is a direct result of Gaia's control of Earth's atmosphere, and definitive evidence that little if any life exists on Mars or Venus. Gaia keeps enough oxygen for us to breath, enough carbon dioxide in the air for plants to grow, keeps the temperature within reasonable bounds and much else as well. Mother Earth really is taking care of you.
Humans don't need forethought, planning, or intelligence to reproduce. Our sex drive takes care of that just fine. Gaia has no forethought, cannot plan, and is basically mindlessly, but without these capabilities Gaia is stuck on the planet. Gaia cannot reproduce, and it's not just because there are no hot guy planets in the neighborhood (Mars may be the macho god of war, but Mars is absolutely freezing). Getting into space requires intelligence, technology, planning, forethought, and much else. That's where we come in. We can think and we can build.
Like every other Gaian species, everywhere mankind went he changed the landscape. For example, the California Indians burned the fields each fall. This recycled nutrients quicker and kept the forest from taking over the fields. This resulted in an extraordinarily rich bio system in Central and Northern California, and the tribes benefited immensely.
But the changes wrought by the hunter-gatherers were trivial compared to what farming brought. Agriculture has completely changed the plant and animal life of vast tracts of land and created species of plants and animals that can only survive by the hand of man. A cow wouldn't last long in North Dakota without a farmer, it takes a buffalo to make it through a Great Plains winter. Buffaloes can push aside the snow to get at the grass beneath. Cows just starve if they don't freeze to death first.
In any case, forests have been cut down, deserts turned into gardens by irrigation, rivers and swamps drained, and anything that eats cows exterminated. Even industrial pollution has not made such a massive change to the planet. As a direct result of the agricultural revolutions of the last 10,000 years, Earth supports over six billion people where before perhaps a few million could live. We are very good at molding the environment to our own benefit.
As these examples suggest, life doesn't simply grow to exploit particular conditions, life creates the conditions necessary for its own growth. This is essentially what space colonization is all about, taking uninhabitable vacuum and making it fit for life. This cannot be accomplished by Daisy World or anything like it. Gaia has molded our Earth without forethought or planning, but space cannot be colonized this way. To take the next step requires intellect, intention, purpose, planning and forethought. The human brain and hand are key to space colonization and space colonization is how Gaia will reproduce.
When an oak tree is ready to reproduce, it creates thousands of acorns, each with a hard shell protecting a tiny bit of life inside. This is one of life's favorite tricks. In keeping with that strategy, humankind makes small, metal-skinned spacecraft to protect tiny bits of life - a few humans, some air, food, water and perhaps a few plants or animals -- and sends them into space. These are Gaia's first, short lived, children. Unlike the oak, we have only sent a handful of life-containing spacecraft into the void. It's our destiny to change that. All we need is the will to reproduce, bequeathed to us by billions of years worth of ancestors, and the wisdom to do our job.
When the USSR blasted Yuri Gagarin into orbit on April 12 1961, Gaia changed in a fundamental way. The first step towards reproduction, the final characteristic of life that Gaia has lacked, was taken. Gaia's fate is in our hands. We can chose to help her reproduce, supply the technology, the launch vehicles, the spacecraft, the endless gadgets and materials necessary for life to move into space; or we can wait for the next asteroid to wipe us out. It seems to me that failure is not an option. We must move into space, but how?
The obvious way to reproduce Gaia is to make other planets enough like Earth to support life. This might just barely be possible for Mars or perhaps Venus, but it will be extraordinarily difficult; a little like having a six foot, two hundred pound baby. A much easier approach is to build spacecraft big enough to live in; in other words, orbital space colonies.
Since mankind finished colonizing the globe, perhaps a few thousands years ago, all human colonization has involved conquering someone and stealing their land. Often this includes exterminating the losers, or perhaps just killing the men and enslaving the women and children. Fortunately, orbital space colonization won't displace a single local, won't destroy a single native species, will not wipe out a single culture, for the simple reason that there isn't anything alive in orbit. Actually, there's nothing there at all. It's a vacuum.
Of course, that's not quite true. There is a lot of energy in the inner solar system, faithfully supplied by the giant nuclear fusion reactor we call the Sun. Furthermore, as we have seen, the near Earth asteroids have quite a bit of reasonably accessible materials. It is our job to develop the technology, and build the life-enclosing hard-shelled homes, to give life access to these bountiful energy and materials resources. If we don't, no one else will, no one else can. It's taken almost four billion years of evolution to produce us, Gaia's first space faring species. Now is the not the time to choke, to blow it, to hide in a little defensive shell saying we can't. Now is the time to seize the day, to become truly all that we can be, to be life's taxi to the stars.
While all this fine-sounding rhetoric is absolutely true, and this vision of our destiny can be a great motivator, not much gets done solely for high-sounding motives. To get people to really work hard, dangle power and wealth in front of them. Fortunately, space colonization will provide power and wealth in truly astronomical quantities.
Countries that have failed to seize the power that comes from growth have paid a heavy price indeed. The classic example is China. Between 1400 and 1450 China's Ming dynasty sent a mighty fleet to explore Asia, the Mid-East, and Africa. This fleet could have easily crushed the small ships the Europeans sent out a few decades later. Had China continued this expansive policy this book would have been written in Mandarin. However, the Chinese who favored continuing exploration and expansion lost a series of internal power struggles, and by about 1500 the fleet had been destroyed by court factions opposed to growth. Laws were even passed prohibiting the construction of large ocean- going ships. The opponents of growth claimed that the fleet was too expensive and the resources were needed to solve China's problems at home. Sound familiar?
The coming disaster was as predictable as a Hollywood movie. A few years after the Chinese fleet was destroyed, the Portuguese seized the Chinese island of Macao. Expansionistic European powers eventually divided China into spheres of influence and the Chinese spent the next few centuries under the thumb of foreigners. The people suffered horribly and China did not become truly independent until the 20th century. While space colonization need not involve subjugation or repression of anyone, the Chinese experience is an important lesson, and a warning.
The space age was born of a power struggle. In the 1960's the U.S. and the Soviet Union were locked in a struggle for world domination called the Cold War. It never became a shooting war because each combatant had nuclear tipped missiles capable of devastating the other; and there was no defense. Neither the U.S. nor the U.S.S.R. made China's mistake, both vigorously tried to expand. The traditional way to do this is warfare, which was suicide. Since they couldn't fight each other directly, the U.S. and U.S.S.R. fought a series of proxy wars around the globe, and engaged in a wonderful competition to put a man on the Moon. This ended when the American Neil Armstrong set foot on the lunar surface in 1969.
The Moon race was expensive but it was a big improvement over the proxy wars in Korea, Vietnam, Afghanistan, Africa, Nicaragua, El Salvador and elsewhere that killed millions and wrecked entire countries. The Moon race only killed three Americans and not a whole lot more Russians. No towns or cities were destroyed. A great deal of wealth producing knowledge was gained, we learned how to work in space, and got some moon rocks to show off. The U.S. won the Moon race in 1969 and by about 1990 the Cold War as well. I don't think that was a coincidence. In any case, losing the Cold War cost the Soviet Union dearly. The USSR collapsed and disappeared, replaced by roughly fifteen independent countries. Things are so bad today the population of the old USSR is actually declining.
Space dominance has served the U.S. well. Our spy satellites allowed us to keep track of what's going on in the world, particularly the status of the Soviet Union's nuclear weaponry. Spy satellites played an important role in defeating the Soviet Union's invasion in Afghanistan, giving the rebels a birds-eye view of Soviet positions.
The Global Positioning System (GPS), a satellite system developed by the U.S. military allows one to know exactly where in the world one is. It has been crucial in the U.S. military victories in the Persian Gulf. GPS not only lets troops know exactly where the are, but allows the U.S. to accurately hit military targets with long range weapons with far, far fewer civilian casualties that and previous long range weapon. Space is a crucial component of the current U.S. military global dominance.
It is perfectly reasonable to expect space to play an increasing role in Earth's wars, as long as those wars persist. The nations that control space will tend to win, those that don't will tend to lose - and the only thing worse than fighting a war is losing one. However, military dominance is only one form of power that space provides. Another important one is economic.
And not just in mining. Consider the 300 square miles of real estate we can make from a 1km diameter asteroid. That's a lot of land. We can easily house 160,000 people per asteroid. Assuming a low $100,000 per home and families of four, we are looking at $4 billion dollars worth of residential real estate alone. That doesn't count commercial, industrial, and agricultural land values. If all 1000 or so near-Earth asteroids in this size range were developed, the residential real estate would be worth perhaps $4 trillion. However, these are just theoretical arguments. The numbers could be off (although probably low). What does history tell us?
Ferdinand and Isabella are quite likely the only Spanish king and queen most people have ever heard of. Is this because of their domestic policies? Was it because they pushed the Muslims out of the Iberian Peninsula? No. It's because they invested in Christopher Columbus and his obviously insane scheme to go west to India; at a time when everyone knew that India lay to the East. At the time, Spain was not a particularly dominant power. But Spain was able to steal vast amounts of gold from the 'new' world 'discovered' by Columbus and other Spanish expeditions (the 'new' world was actually quite well known to the local inhabitants). This wealth made Spain the premier power in Europe for centuries. The wealth that comes from growth flows to societies not just to individuals.
I don't mean to pick on Spain. Many European powers of the time were involved in a bloody worldwide expansion. Empires throughout history have murdered and pillaged their way to great wealth. Spain is, however, an unusually clear example of discovery and colonization leading to great wealth, even if that wealth was derived from American gold obtained by pretty ugly means. However, there are two ways to gain wealth: by stealing or producing. Spain's was a combination, after all the Spanish did produce a very capable fleet even if they stole the gold. Fortunately, space colonization does not require theft in any form. Orbital space colonization can generate wealth and power almost purely by production. Production of, at least, metals, real estate, energy and knowledge.
In 1968 Peter Glaser had a great idea. Why not build large satellites with enormous solar arrays to generate electricity? This could be converted to microwaves and sent to Earth. Subsequent analysis suggests that these can work quite nicely. A string of solar power satellites could provide all the centralized electrical power the Earth needs or even wants with zero greenhouse gas emissions, or emissions of any kind for that matter. No mines, no drilling, and no radioactive waste, nothing. While decentralized power generators like roof-top solar arrays would still make sense, solar power satellites could potentially replace all our coal, gas, oil, and nuclear power plants with a much cleaner alternative.
Unfortunately, price is an issue. If all the components of a solar power satellite must be launched from Earth the price is far too high. However, most of a solar power satellite is metal for the structure and silicon for the solar cells. Metals and silicon are available on the Moon in very large quantities. Launching material from the Moon is much easier than from Earth because the Moon's gravitational attraction is much less and there is no atmosphere to speak of. Orbital space colonies will be well positioned to mine the moon and build solar power satellites to supply Earth with electricity at a tidy profit.
Sending vast quantities of power to Earth by microwave immediately brings to mind a lot of pre-cooked birds. However, the microwave oven in your kitchen is precisely tuned to excite water molecules, which is why it's so good at cooking. Solar power satellites can use a slightly different frequency. These frequencies pass through the atmosphere with few loses, and are more suited to power transmission. These frequencies do not excite water molecules nearly as much as microwave ovens. That said, the receiving antennas on the ground should avoid major bird flyways.
Although great wealth can come from power and materials, the greatest wealth generator is not physical. It's knowledge. Henry Ford didn't have any materials or energy that anyone else didn't have, but he did figure out how to build cars on an assembly line. Generations later his family is still extremely rich.
Knowledge is created by people. The more people there are the more knowledge can be created. Part of the reason for the explosion of knowledge in the last 50 years is the increase in the human population. All else being equal, ten trillion people in a solar-system wide society will produce far more knowledge than six billion on Earth.
Putting people in new situations also creates new knowledge. This is why the largely European colonizers of the Americas have created so much of value -- airplanes, cars, refrigerators, computers, the Internet, and so on. People living in orbit will be faced with the need to create everything required for a good life out of literally nothing, the vacuum of space. The space program has already created a great deal of useful knowledge, but the colonization of space will put this process on steroids.
So there you have it. Do we want to be limited to this beautiful, but rather small Earth? Fighting in the streets, the courtroom, and the battlefield over limited resources? Waiting for the next asteroid to destroy us? Forced to limit our numbers to avoid over-running the Earth with too many people?
Or would we rather forsake fighting for building orbital space colonies? Creating new land for ten trillion people or more? Diverting the killer asteroids to our ends? Make Gaia truly alive by reproducing? Gaining enormous power? Generating vast wealth? Making the most important move since life began on this planet?
How can we even ask?
[Lewis 1996a] J. S. Lewis, Mining the Sky - Untold Riches from the Asteroids, Comets, and Planets, Helix Books, Addison-Wesley Publishing Company, Inc.
[Lewis 1996b] J. S. Lewis, Rain of Iron and Ice, The Very Real threat of Comet and Asteroid Bombardment, Helix Books, Addison-Wesley Publishing Company, Inc.
[Lovelock 1988] James Lovelock, The Ages of Gaia, a Biography of our Living Earth, Bantam 1988.
[Willoughby and McGuire 1995] Allan J. Willoughby and Melissa L. McGuire (1995), "Adroitly Avoiding Asteroids! Clobber, Coax or Consume?" Space Manufacturing 10, Pathways to the High Frontier, Proceedings of the Twelfth SSI-Princeton Conference, 4-7 May 1995, edited by Barbara Faughnan, AIAA, pages 103-113