Exploring the Solar System There is nothing more exciting than the concept of life in space, whether it be strange beings on distant worlds, or the expansion of our own species into the Solar System and beyond. Humankind stands on the verge of major discoveries and exciting progress in both areas. For example, the recent discoveries of possible life-related evidence in Martian meteorites, a subsurface ocean on Europa (one of the large moons of Jupiter), water vapour in the atmosphere of Earth-like Titan and extrasolar planets, all show how close we are to finding out if the Life which teems on our own planet is unique. Space missions Some increasingly sophisticated space missions are currently underway, such as the Mars Sample Return and the flight of Cassini to the Earth-like Saturnian moon Titan. Other missions will visit some of the comets that appear to carry complex organic molecules within their "dirty snowball" nuclei. Plans to return to the Moon, to orbit Europa, to explore Pluto and to place giant planet-seeking telescopes in orbit beyond Jupiter are not far behind. These and other advances already promise rapid progress during the first decades of this millennium. Future space missions to the inner planets, Venus and Mercury, are not expected to reveal any traces of life there, because of the high temperatures. Nevertheless, there are indications that there may be areas near Mercury's poles where the temperature is quite moderate. And some scientists think that Venus may at some earlier time have been rather different from what it is today. Key questions A substantial and detailed description of our Solar System has come from the 'eyes' of spacecraft and telescopes and has told us what to expect in our neighbourhood. Some of the remaining key questions are: If life exists in other parts of our planetary system, what form might it take? Where are the prebiotic molecules and habitable zones located among the objects in our Sun's family? Could Martian ice-covered lakes host microbes or store the remains of life in the form of fossils? Could the subsurface ocean on Europa be teeming with simple life forms? What is the degree of complexity attained by organic molecules on Titan, the moon with a thick nitrogen-dominated atmosphere? What of the other icy satellites of the giant planets? All these questions are now being extensively explored and space missions are being prepared that will go to the various planets, moons and comets and try to find the answers by means of direct, in-situ investigations.

A composite image with the nine major planets in the Solar System, shown in approximately the correct, relative scale. From left to right: the "terrestrial" planets Mercury, Venus, Earth and Mars, then the "Gas giants" Jupiter, Saturn (with the rings), Uranus and Neptune and finally Pluto. All of them, except Mercury and Venus, are known to have moons. There are also many comets and asteroids in the Solar System, as well as large amounts of dust, mostly near the main plane, the Ecliptic. (Composite image by Bill Arnett, based on photos obtained by NASA spacecraft.)