For decades, the most popular destination for migrants the world over has been the United States. It was in America that the downtrodden and the footloose of this world saw their destiny. But America's ability to accommodate such people has always been finite. Billions of poverty-stricken people today crave the comfort and the affluence of a better world and almost none of them can have it. The increase in global population now exceeds the entire population of the US every five years; if migration is the solution to the problems of mankind then we're going to have to find a different planet.
With this in mind, the Americans have just launched their very latest rocket, the Ares 1. Fully developed, Ares will be able to carry four people to the moon.
It doesn't sound like much but it's a start, and quite a few other nations – including the Chinese and the Indians – are planning missions of this kind. For a generation weaned on the adventures of Jean-Luc Picard in Star Trek, the idea of moving out into outer space doesn't sound quite as preposterous as it used to do. The big question is, where could we go to and what would it be like when we got there?
Our species has evolved to live on the surface of our own little planet about 93 million miles from the Sun. Sadly, most of the nine planets in our solar system. are freezing balls of gas, and only about four are rocky worlds like the Earth.
The first rock from the Sun, Mercury, is pretty tiny and ludicrously hot. In fact, talk of colonising Mercury is largely fanciful and we should probably move on to a promising little planet by the name of Venus. Venus has enough going for it to make it stand out as the planet with most potential for recreating our own environment.
Venus is almost exactly the same size as the Earth and this gives it a similar gravitational field. Since gravity is practically the only thing we couldn't simulate on another planet, this is a big plus for Venus. On the downside, surface pressures would crush us to death in an instant and even if you did survive, surface temperatures would fry you even faster. Establishing an acceptable environment on Venus would be a colossal challenge, and most serious space explorers have long since discarded the option.
Which leaves us with a difficult decision. Should we go for our own Moon or for Mars? Before we get too excited by the merits of one world over another, it's worth remembering that the Moon has a land surface area about the same as the African continent, whereas Mars has a land surface area similar to the dry land on our own world. Even assuming there might be a way for us to get there in large numbers, current global population growth could saturate our Moon to the same level as Africa within 10 years.
Furthermore, life on the Moon isn't likely to be pleasant. Without falling back on any kind of imaginary science, the creation of large pressurised cities is at least possible, but gravity on the Moon is only one sixth of the Earth – that's why Neil Armstrong is always bouncing around so much in the archive film. Worse still, the Moon has a four week day-night cycle, meaning the nights go on for 14 Earth days. Since most plans for space colonisation rely on solar power, the arrival of nightfall would be a big problem. The only good thing about the Moon is that if things go wrong in your spacecraft (and things do go wrong in spacecraft) you could beat a hasty retreat to the Earth in less than a week.
As far as the next 100 years goes, Mars is the only realistic world we could build on, but the early stages of colonisation are likely to be tough. The first manned expedition to Mars is likely to take six months. Given that the return journey would take six months too, the crew would have to spend several weeks on the surface of the planet just to justify the journey time.
Freed from gravity for six months, osteoporosis is a big problem and the risk that the first man on Mars might slip and break his femur the moment he gets off the spacecraft is a very real one. Simulating gravity in the outbound spacecraft would be possible by rotation, but a luxury of this kind is unlikely to be available to a pioneering crew and the early astronauts will probably spend all day dosed up on the kind of drugs we currently give to the elderly.
The capsule will be cramped. Sexual tensions between crew members are likely to be unbearable and the first expeditions will probably be all-male. If the spacecraft arrives at the planet and finds itself over an impenetrable sandstorm, well, this wouldn't surprise anyone who's worked at NASA so far. Mars has a habit of kicking up a tantrum just when you least want it to. Having said that, an atmosphere can be useful for slowing a spacecraft down, and every vehicle to land on Mars so far has deployed parachutes.
As surface pressures are much too low for oxygen masks, the crew would have to wear full pressure suits out on the surface. Mars is vast. Meaningful exploration would require a nuclear-powered buggy. It's hard to imagine any government funding a one-off manned trip to Mars, so the crew would lay the foundations of a future base there. The day-night cycle is almost identical to that on Earth and the option of pressurised greenhouses on Mars has been widely discussed. Plants are good for eating, but they can also turn carbon dioxide into breathable oxygen. Recycling would be essential for any long-term settlement on Mars.
There doesn't seem to be a lot of water on the surface, but there's heaps of the stuff near the polar ice caps and in time the astronauts would presumably trek out there in search of this crucial commodity. Water is good for drinking and for irrigation, but it contains the basic ingredients of rocket fuel too and the discovery of water in the equatorial permafrost would transform the credibility of a future colony.
Right now, there are people on our own planet dreaming up all kinds of weird and wonderful ways to cut the CO2 and arrest global warming. Maybe, just maybe, some of these techniques will give us the option of turning the planet Venus into an Earth-like world. But even if they do, the idea of Venus as a place worth visiting is still centuries away. If we Earthlings are looking for a place to establish our first colonies in space, it is surely Mars.
Steven Cutts's book about Mars, Viking Village, is available in December 2009 (Pen Press)