Sir Patrick Moore: Why is it important? Because it may tell us how life was kick-started

On 15 October 1997, the Cassini spacecraft began a journey of an entirely new kind. It was destined for the outer solar system, and to deposit a small probe, Huygens, on the surface of Saturn's largest satellite, Titan.

On 15 October 1997, the Cassini spacecraft began a journey of an entirely new kind. It was destined for the outer solar system, and to deposit a small probe, Huygens, on the surface of Saturn's largest satellite, Titan.

Very little was known about Titan, because its surface was hidden by a dense atmosphere rich in nitrogen. If Huygens did land, would it come down on a layer of ice, solid rock, slush, or would it splash down in a chemical ocean? Of course, Titan is very cold, with a surface temperature of minus 180 degrees centigrade, but nobody knew quite what to expect. Well, the mission has been a triumphant success. Huygens made a perfect landing, and went on sending back data after arrival for well over an hour and a half, much longer than the planners dared to hope. It is a major step forward, partly because of the complexity of the whole operation but also because it shows that we can explore worlds far beyond the inner part of the Sun's family.

Titan is a weird place. The images sent back show channels which must have been cut by liquid, possibly methane; the landing was made upon a surface of about the consistency of wet sand, and there are "boulders", a few inches across at most. No chemical lakes have been found, but of course the pictures have only just been received, and examining them properly will take many weeks. Why is Titan so important? Mainly because it is the only planetary satellite to have a substantial atmosphere, and it may in some ways be not unlike the Earth of around 4,000 million years ago. It does contain all the ingredients needed for life, and could tell us a great deal about how life may have been "kick-started" in a hostile environment. This in turn may be of great value to medical science as a whole. We may also learn more about how planetary bodies evolve.

Huygens is dead: its work has been well done. But the "mother ship" Cassini, which carried Huygens and released it on Christmas Day, is very active. It relayed Huygens' signals back to Earth, and is now well into its own programme of studying the ring system and also Saturn's other satellites. Cassini has much to tell us over the next year. But all in all, it is Titan which has been the most vital part of the mission. This has been one of the greatest triumphs of the space age, and nobody concerned will ever forget "Titan Day".