Why send astronauts to other worlds? After all, some would argue that robots are far better suited to the extremes of space. Automated probes offer a cheaper and safer way to explore the solar system, they say -- just look at the success of NASA's Mars Rovers and the Huygens probe on Titan. Such missions justify their expense by the boost they provide to human knowledge.
By contrast, proponents of human exploration invoke a more romantic argument: the human spirit's desire to tread new land, see new sights or climb virgin peaks, simply "because they are there." Yet there is a more compelling reason to send people. It comes down to a single word: survival.
If we remain on Earth we will surely become extinct, and probably long before an expanding Sun roasts our planet. The fact is that we are vulnerable to the same types of catastrophic events that have wiped out other species on Earth. Geological history shows that such extinction events are routine. Tyrannosaurus rex lasted only a few million years. Mammalian species, on average, last just a couple of million years. Our parent species, Homo erectus, lasted about 1.6 million years, while Neanderthals died out after only 300,000 years. We might have conquered the planet, but it is just a tiny island in the universe, and species confined to a single island are often found on the endangered list.
So what about colonization? The problem is that it is hard to justify the huge expenditure of sending colonists into space -- probably hundreds of billions of dollars -- when we do not know when catastrophe will strike. Yet thanks to the revolutionary ideas of a medieval Polish astronomer, coupled with some simple math, it is possible to get a handle on our future. We can estimate how long it will be before the human race is extinguished. We can even determine how long the human space program can likely be maintained.
The clock is ticking, however. Can we hope to colonize a new world in time?
One thing is certain: About 5 billion years from now, the sun will have swelled up to form a red giant. While this bloated sun may not actually engulf our planet, it certainly will fry it. Yet we will most likely die out long before then. An asteroid or comet strike could cause mass extinctions, as could runaway climate change. Avian flu, AIDS, Ebola, malaria, TB and smallpox also loom, to say nothing of the possibility of a new virus emerging to infect and kill everyone before we know enough about it to respond.
While it is well known that we pose a danger to other species -- just ask the passenger pigeon -- it is not a great leap of reasoning to realize that we also pose a danger to ourselves. Nuclear war, bioterrorism or even nanotechnology gone wrong may yet kill us off. Intelligence of the kind we have, with our aptitude for abstract reasoning and the ability to discern our place in the universe, may turn out to be, as biologist Stephen Jay Gould said, just "one bauble on the Christmas tree of evolution." What's more, Charles Darwin tells us that species usually do not live up to their potential, not least because most species do not leave descendant species.
Not surprising, getting off Earth multiplies our chances of survival. If we were to plant a self-supporting colony on Mars, say, we would become a two-planet species. This could as much as double our long-term survival prospects. Colonies are also a great bargain. Send out a few colonists and they can use indigenous materials to sustain themselves and then increase their numbers, as well as seed other colonies elsewhere.
The first words spoken on the moon were in English not because England sent astronauts but because it planted a colony in North America that did. A colony on Mars might double our chances of ever going to Alpha Centauri, because 1,000 years from now it might be as likely for our descendants on Mars to mount the expedition as for people from Earth to do it.
There is some urgency to this. In the 16th century, the Polish polymath Nicolaus Copernicus suggested that we do not occupy a special place at the center of the universe, but rather inhabit one of a number of planets circling the sun. Since then we have discovered that the sun is an ordinary star in a run-of-the-mill galaxy, in a typical supercluster of galaxies. The Copernican principle -- that our location in the universe is not likely to be special -- has proved one of the most successful hypotheses in the history of science, playing a role in many discoveries. This perspective also allows us to predict how long the human race will survive.
The basic argument is as follows. If our current location in space-time is not special, we can expect to be living at some random point between the first and last moments of the human race. This can be framed using statistical theory to set upper and lower limits with 95 percent confidence -- the standard for scientific predictions. Accordingly, we have a 95 percent chance of being in the middle 95 percent of our species' lifetime (not in the first 2.5 percent and not in the last 2.5 percent). If we find ourselves at the beginning of the 95 percent stretch, then we are 2.5 percent, or 1/40th, of the way through our existence and our species' future lifetime is 39 times as long as the past. At the end of this middle period, our future is only 1/39th times as long as the past. In other words, we can be 95 percent certain that the human species will last between 1/39th and 39 times its lifetime so far.
Our species, Homo sapiens, is about 200,000 years old. That means there is a 95 percent chance that the human race will last at least another 5,100 years, but less than another 7.8 million years. Right now, having no actuarial data on other intelligent species like ourselves, this is arguably the best estimate we can make. Interestingly, this calculation gives us a similar longevity to other mammal and hominid species, yet the prediction is based solely on our past lifetime as an intelligent species -- quite a coincidence, some may think.
Notice that 7.8 million years is a far shorter time than the estimated lifetime of the sun. It is a warning that we may go extinct by some ordinary catastrophe long before our planet's oceans boil. It seems that we do not have billions of years to get off Earth: We have to colonize before we become extinct here. Worse than that, and essential to our survival prospects, is that we have to get off before the human space program folds.
The Copernican principle suggests that you or I are likely to live in a century when the human population is high, at a time in which expensive projects such as the space program are possible. If the population falls catastrophically, expensive projects become more difficult. So it's possible that the human space program will end before the first colonists blast off, leaving our species on Earth, like passengers stranded on the Titanic without a lifeboat.
So when will the human space program end? In 1993, when it was 32 years old, I predicted it would last at least another 10 months, but less than 1,248 years, using the same calculation we applied to the lifetime of our species. It survived those first 10 months. However, the ultimate space race is whether we can colonize another world before the human space program ends.
Another reason to worry is that we are having this debate on Earth. If humankind remains on the planet, that would make you and me typical -- not special at all. Yet if we eventually colonize a billion habitable planets, then you and I are very special: lucky to be on the first planet out of a billion on which humans live. There would be only one chance in a billion of being so lucky.
An alternative statistical technique called Bayesian inference comes to a similar conclusion to the Copernican method. Even if you originally regarded the idea that humans will be stranded on Earth as equally likely as the idea we will colonize a billion planets in the galaxy, then after observing that you are on the Earth -- the first planet inhabited by humans -- you should regard the "colonizing the galaxy" hypothesis as a billion times less likely than the "stranded on Earth" hypothesis.
What does all this tell us? The human space program has been funded for the past 46 years, and we can expect another 46 years of funding (a period uncertain by that factor of 39). During this time, the most significant project that we could undertake would be to establish a colony on Mars.
Mars is a good place for a self-supporting colony capable of growth because the planet has gravity, solar energy and all the chemicals necessary for life, including water. Locating the colony 10 meters underground would protect it from cosmic rays and solar radiation storms. Even on the surface, the Martian atmosphere offers some protection: I estimate that colonists could explore the surface for up to 31 hours a week and still live for about 70 years. As well as offering a good site for a colony, Mars is also very interesting scientifically. If we want to discover whether Mars ever harbored life, having colonists there is one way of finding out.
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