Wish you were here?


The Moon


· Great views of Earth, and it will take just three days to get there in the new crew exploration vehicle currently being designed by Nasa.

· It's made from the same stuff as Earth (it was knocked out of our planet in a violent asteroid collision billions of years ago) and is full of useful minerals that could be used to build shelters, help make fuel and oxygen, and generally sustain life. One useful Moon mineral is ilmenite, which could provide colonists with oxygen, hydrogen and helium.

· Nasa is already scouting around for ideas on how to build a base there in the next few decades. In his speech this week, Stephen Hawking talked about humans having a permanent base on the moon in 20 years' time, and that's theoretically quite possible, although Nasa's plans are in their infancy; because taking stuff into space is so expensive at the moment, scientists want to be ready to build as much as possible on the Moon itself.

· Food is going to be a major issue (along with oxygen) when living on a large, airless, lifeless rock, but Nasa has been working with microbiologist Amy Grunden of North Carolina State University on genetically engineering food crops that can be grown in harsh, off-planet environments.

· You can already plan where to put up your mansion; it's possible to buy real estate there. Pick a decent spot now.


· The unending sea of grey might get depressing, and it must be remembered that much of the moon is permanently in shadow - very gloomy, very cold.

· While talk of people living in domes "a bit like the ones at the Eden project" is all very well, no one has yet succeeded in designing a shelter (or even a way to build a one) that would be safe from being punctured by asteroids while allowing enough light in to remain pleasant enough to live in (and grow plants).

· The lack of gravity will eventually tell. Kevin Fong, a doctor specialising in space medicine at University College London, says that, in the long term, the low gravity would mean bones and muscles wasting away.

· People have been selling land on the moon since the 70s; one Cornwall-based company says it has sold more than £4m-worth. However, the United Nations Office for Outer Space Affairs drew up the Outer Space Treaty in 1967, and that says no nation can claim ownership of any celestial body (including the moon) and that all nations must agree to their peaceful use. This might mean that your deeds to land on the moon will eventually prove worthless.

· And finally: if Stephen Hawking thinks we need to leave Earth to avoid getting wiped out in some major catastrophe, will running to the moon be far enough?



· Stunning scenery, especially if you like red, and great for climbers - it boasts the biggest mountain in the solar system, Olympus Mons, which rises majestically to 26,000m, leaving Everest's 9,000m peak in the shade.

· After the Moon, it's probably our best bet for a colony; this week Hawking talked about us having a base there in 40 years.

· The gravity here - about 40% of that on Earth - is probably enough to keep you healthy for a reasonable amount of time and there's a slim chance that you will stumble across some humble form of native life on the planet, which would win you an instant Nobel prize.

· Unlike the Moon, Mars does have an atmosphere, albeit a rather thin, carbon-dioxide-heavy one, so there's a chance that humans won't always be confined to shelters or space suits. "Terraforming" the planet (which would involve building up the atmosphere to make it more habitable for humans) has already been much discussed; ideas include building some kind of roof to trap an atmosphere, or introducing bacteria and plants to build up useful gases such as oxygen.

· There's plenty of frozen water at the poles and, probably, underground.


· Two words: Beagle 2. Getting stuff to land safely on Mars isn't easy - it's not like landing on the Moon.

· It's an awful lot further from Earth and would take at least six months to commute to. It's also darker than Earth (further from the Sun), and communicating with home would involve 20-minute delays each way.

· As for terraforming ... "All serious studies say it would take tens of thousands of years, certainly on places like Mars," says Ian Crawford, an astronomer at Birkbeck College in London. Getting to the water could be difficult, too - nobody really knows how much there is and melting it would require masses of energy. Fong reckons terraforming is probably unethical anyway: do we really have the right to screw up another planet's ecology?



· On paper, this could be perfect. It's about the same size and age as Earth, not too far away, and made from the same stuff.


· "Venus can be summed up in three words: hot, smelly and windy," says Monica Grady, a planetary scientist at the Open University. In fact Venus is a vision of hell. A runaway greenhouse effect (thanks to a 97% concentration of carbon dioxide in the atmosphere) means the temperatures on the planet's surface can soar to 450C - hot enough to melt lead. Clouds of sulphuric acid blanket the planet in 12-mile-thick layers, the atmospheric pressure is 100 times that on Earth's, there is no oxygen, and any water has long since disappeared. Venus wobbles wildly on its axis, making seasonal fluctuations more intense. And there is no magnetic field, which means the atmosphere is being gradually being leached away by the solar wind, a stream of high- energy particles streaming out from the Sun. "Earth just happens to be the right distance from the right-sized star. It happens to have a stable rotational axis because, miraculously, it's got a big moon that stabilises the axial position," says Richard Taylor, a fellow of the British Interplanetary Society. Venus, unfortunately, isn't and doesn't.



· A better prospect than Venus, despite being closer to the Sun. It's much cooler, for a start - you might find regions around the poles and in the craters at the right temperature. The poles are probably the best places to set up camps similar to the ones on the Moon as there's a source of water nearby.


· Like the Moon, the surface of Mercury is a desert pockmarked with craters. The temperature ranges from -180C at the poles to 400C at the side facing the Sun. The average surface temperature is around 180C.

· Grady says that the lack of magnetic field (about 1% as strong as Earth's) means the high-energy radiation streaming in from the Sun would make life unbearable on the planet and the lack of any real atmosphere would restrict people to tiny shelters.

The gas giants
(Jupiter, Saturn, Neptune and Uranus)


· Complicated weather systems with multicoloured storms that any Brit worth his salt could spend a lifetime talking about.

· All these planets have beautiful rings: Saturn's are merely the most famous.


· The lack of solid ground on Jupiter and Saturn would mean living in floating homes. Crawford suggests balloons floating in the upper atmosphere with gondolas attached. Fun for an afternoon - but for a lifetime?

· The molten surfaces of Neptune and Uranus are covered in thick layers of ammonia and methane. Neptune also has the highest wind speeds in the solar system with hurricanes often reaching 2,000kph.

· All in all, probably best avoided.



· One of Jupiter's moons, Europa would probably offer fabulous cross-country skiing - the whole moon is covered in a layer of ice about 15 miles thick. "Europa has more water than Earth," says Seth Shostak, senior astronomer at the Seti Institute in California. The ice means easy access to fresh water, and if anyone can work out how to drill through the ice, there's probably plenty of liquid water underneath it all.


· It is right in the middle of Jupiter's magnetic field, which means that lots of nasty radiation is funnelled directly towards the moon's surface.

· Europa captures some of the sulphur dioxide spewed out by the volcanoes of Io, another moon of Jupiter. "You'd never be able to live on the surface," says Grady.

· Living underneath the ice is a possibility but it means never seeing sunlight again. You won't be missing much, though - the strength of the Sun's light at Europa is only 4% of that which reaches Earth.

· There's no atmosphere so, like the Moon or Mercury, any human colony would have to be self-contained. Or else you would have to walk around in a thick, radiation-proof spacesuit for your entire life.



· The largest satellite of Saturn, Titan is the only moon in the solar system with a fully developed atmosphere. Awesome, front-row views of the rings of Saturn.


· "Asthma sufferers, don't go there," says Grady. The Huygens probe sent back detailed images of Titan's thick, smoggy atmosphere in January last year. This is a planet covered largely in nitrogen with some added methane. Clouds of nasty gases reflect away sunlight, leaving the surface at a blisteringly-cold -179C. There is water on Titan, but it's usually mixed with ammonia or trapped deep underground. The final nail in the coffin: there is no oxygen.

Space stations


· The designs and engineering feasibility studies have already been done. In the 1970s, space scientist Gerald O'Neill came up with the idea of 10km-long aluminium tubes orbiting Earth and predicted that, by the 1990s, millions of people would be living in them. The tubes would spin to produce an artificial gravity and, according to Shostak, residents would have a good life. "There are no mosquitoes, the weather's always good, and you can see the neighbours just by looking up," he tells the Guardian. "If you grew up on one of these things you would probably pity the poor people who had to contend with nature in the raw." In theory, we could build as many spinning tubes as we needed.


· None have been built yet and the technology is unproven. The one space station we have has so far cost $100bn (£54bn), can only support up to six people at a time and has, many scientists argue, proved about as useful as a chocolate teapot.

· As with the Moon, Earth-orbiting space stations might not be the best option if you need to get away from some cataclysmic event on Earth.

Another Earth
(Outside the solar system)


· Finding a replica of Earth, or something pretty damn similar, somewhere outside our own solar system would be a brilliant solution. No terraforming needed, no shelters, and no need to worry about any dangerous radiation. "We won't find anywhere as nice as Earth unless we go to another star system," said Hawking in his speech this week.


· Finding one. So far, 150 extrasolar planets have been spotted, but they are all giants. Taylor says that, for humans to survive on a planet, we need something no more than twice the mass of Earth. Any bigger and the gravity becomes extreme, pulling in any gas that happens to float by the planet and creating a planet resembling Neptune or Uranus, neither of which are fit for humans (see above). As our telescopes get better, we will no doubt find more small planets, but we're not there yet. "It is very likely that Earth-like planets are present but we haven't discovered them, so we don't know which stars to aim for," says Crawford.

· There's also an ethical problem. An Earth-like planet with an oxygen-rich atmosphere breathable by humans is already likely to have its own life. "You won't find an oxygen-rich planet without life," says Crawford. "What is being advocated is appropriating somebody else's planet. That will be ethically repugnant."

· Lastly, of course, there's the small issue of distance. Other solar systems are a very, very, very ... very long way away. And until we've cracked the problem of travelling faster than the speed of light, spotting a great planet elsewhere isn't going to do us a scrap of good. The smallest planet discovered so far was spotted in January last year and it is 15 light-years away - that's 90 trillion miles. Taking the space probe Cassini as an example (it recently arrived at Saturn after a seven-year, 2bn mile journey), it would take 315,000 years for us to reach this planet. That's more than three times longer than Homo sapiens has been on Earth.