Although Mars is the most earth-like of the planets in our solar system, it is not similar. It is very cold, its atmosphere is thin and mostly made up of carbon dioxide, which is poisonous to humans. There is virtually no oxygen.

There is some gravity - about a third of that on Earth. While this is preferable to zero gravity, it will still create problems. Getting around will be difficult. Astronauts need to be trained to move in a different way. On Mars, walking is the most costly activity in terms of oxygen and energy, while running is easier. Scientists hope to develop computer-controlled space suits which will be able to help astronauts move around, to tell them when to speed up and slow down so as to conserve oxygen.

Other problems are even more threatening. Humans on Mars would be exposed to solar flares, which emit powerful radiation which can cause cancers, cataracts and damage to the central nervous system. Radiation also damages T-cells, which fight infections. Immunologists fear that viruses or bacteria which live within us could be harmful if our immune systems broke down. The threat from the enemy within may be the greatest of all.

Immune systems are vulnerable to other threats too. Stress could be a killer. From a psychological perspective, spending up to three years - or longer - in space could be highly problematic. Separation from loved ones, fear and boredom will all take their toll. Astronauts will have to live at close quarters, without any real contact with anyone else, for a very long time.

The distance between Earth and Mars would make for a time lapse of about 20 minutes in communication. E-mail would be the only way of keeping in touch.

Could we colonise Mars?

Given the huge problems of living in space, colonisation does not seem likely. But it is being considered. Mars Surveyor 2001, a probe which will be launched later this year, will be carrying out experiments designed to investigate what technologies would be needed to support human colonisation. It will be also investigating whether there are minerals which humans could mine. But before humans can spend any length of time on Mars, vast leaps in medical science will have to be made.

Devon Island

One way of investigating the problems which humans might have to face on Mars is through the creation of a Mars space station on Earth. This is what the Mars Society, a private group dedicated to the study and exploration of the Red Planet, has done on Devon Island, one of the Queen Elizabeth islands in the Canadian Arctic.

Pascal Lee, a NASA engineer and leader of the Haughton Mars Project on Devon Island, says: 'By setting ourselves up in this polar desert, we will experience an environment that closely resembles the surface of Mars.' Daytime temperatures on Devon Island are similar to those of a summer day on Mars. In summer, it is largely free of snow and ice.

And it has another feature which enhances its suitability: the Haughton meteorite impact crater, 20 kilometres (12 miles) in diameter, the highest-latitude terrestrial impact crater on Earth. Formed 23 million years ago, the Haughton crater is a polar desert environment which is as close to the conditions of Mars as we likely to find on Earth.

On Devon Island, a group of scientists from the Mars Society have built a life-size space module such as would be used to travel to Mars. Here, astronauts and researchers can practise a Mars mission. As the Mars Society says: 'The Haughton-Mars Project offers a unique opportunity for innovative engineering in geographic and planetary exploration.'

But the Devon Island project also has a political objective. Robert Zubrin, president of the Mars Society, says that the project is about 'pushing Mars to the top of the space exploration agenda'.

Humans in space | Humans on Mars

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