Around the world, there is renewed interest in sending a manned mission to other planets in our Solar System. What conditions await future astronauts? Space science provides many clues. Before leaving Earth, scientists want to use robotic spacecraft to find out more about the conditions that human travellers will face once they reach some far-off destination. A flotilla of planetary exploration missions is already providing us with invaluable scientific data about other worlds.

Stepping onto other planets to perform scientific investigations yourself is an old dream. However, most science done today uses robotic missions. Huge distances in space, harsh environmental conditions, and the status of current technology put strong limitations on the human exploration of space. Of the nine planets in our Solar System, Earth is the only one that is 'habitable', meaning that human beings can breathe the atmosphere and move around in reasonable temperatures. Of the other planets, the best we can hope for is that they are 'hospitable'. None of them possesses the special mixture of gases that make up the Earth's air, nor its mild temperature.

The giant planets of the outer Solar System, that is, Jupiter, Saturn, Uranus, and Neptune are all gaseous. Lacking solid surfaces, they have clouds all the way down to their deep, compacted cores. Although they all have moons on which you could land, these are too far away to be within the current reach of astronauts. The nearest planet to Earth is Venus, but this world is a boiling mixture of noxious gases with a surface temperature hotter than a kitchen oven and a pressure 600 times higher than on Earth. The planet Mercury is too close to the Sun. So neither Venus nor Mercury are 'hospitable' by human standards.

That leaves Mars and the Moon. Both are hospitable, with temperature ranges that are bearable and an atmosphere that is absent in the case of the Moon and not too corrosive in the case of Mars. Astronauts with proper spacesuits would be able to get by in these conditions. Both the Moon and Mars are also relatively close by to Earth, requiring journey times of less than a year. In the case of the Moon, about three days, for Mars, about nine months. Apart from these large worlds, there are also a number of smaller destinations within reach. For example, there are very many so-called near-Earth asteroids that are the leftovers of planet formation. Going there may reveal clues to how the Earth and other planets formed.

Current scientific missions to other worlds in the Solar System are not only important for their absolute scientific value, but also because the information we gather prepares us for future visits by human beings. Astronauts will have to endure many hardships living and working in these places. For example, there are gravity differences compared to Earth, the lack of atmosphere to shield them from harmful solar radiation and, on Mars, the tremendous dust storms that regularly engulf the planet. Understanding as much as possible about these places is essential before we launch any human missions.

ESA's current planetary missions are perfect starting points to learn about new worlds. For example, Mars Express and SMART-1 will provide vital data about the presence and the distribution of water and ice on Mars and the Moon, respectively. Another mission, Rosetta, will even drop a lander on a comet. The experience we gain with Rosetta will be invaluable for us to perfect techniques to land on near-Earth asteroids in the future. Other projects such as Mars Express, Venus Express, BepiColombo, which will go to Mercury, and Huygens, which will go to Titan, will play their part in refining environmental measurements on other worlds also.

In 2001, ESA began preparing the Aurora programme, Europe's bold roadmap towards the eventual human exploration of the Solar System. While the purely scientific investigation of the Solar System continues, ESA's newest exploration programme, Aurora, will add another dimension. It will launch a number of robotic missions focused on clarifying those scientific aspects that we need to understand to make human exploration possible.