Cassini-huygens Mission To Titan, Saturn's Largest Moon

Joining us from mission control at the Jet Propulsion Lab in Pasadena, California, principal investigator on the Huygens probe, Professor John Zarnecki, talks about the events that will take place on Christmas Day, and on January 14 2005, to provide us with our first glimpse of the surface of Saturn's largest moon, Titan.

John - My beloved space probe Huygens, which has been riding piggy-back with the Cassini spacecraft for the last seven years, is currently in orbit around Saturn. In a few days time, Huygens will be released from Cassini to make its final plunge towards Titan.

Chris - Of the 30 or so moons around Saturn, why choose Titan?

John - Not only is Titan the largest, but it is also a bit of an odd ball. It is the only planetary satellite in the solar system that has an atmosphere, and its atmosphere is thicker than Earth's. It is composed of a very complex mixture of gases, including a range of hydrocarbons. This makes it similar to Earth's very early atmosphere, which was immensely important as it probably provided the conditions from which life formed. Earth's atmosphere today is very different, but Titan's atmosphere seems to have got caught in a deep freeze, as the moon is extremely cold. Titan is therefore important because it is like going back to an early Earth.

Chris - What will happen when the probe is released?

John - Cassini will dispatch the probe on Christmas day by releasing a number of bolts. From there, three springs will gently push the probe away. Huygens will touch the upper atmosphere of Titan on the 14th January, where it will start to decelerate due to atmospheric friction. A dish-shaped shield on the front of the probe will act as protection against the intense heat. Closer to the ground, three parachutes will be released in succession to allow a gentle two and a half hour decent to the surface. We then have two hours, whilst Cassini is in touch with Huygens, before it moves out of reach on in its orbit. This gives us a total mission time of four hours. Despite seeming a very short time for a seven year journey, our data will be unique and should provide some fascinating information.

Chris - How do you protect the probe from high temperatures, and how do you test it before sending it into space?

John - When Huygens enters the atmosphere, it will encounter temperatures around 15000°C over a period of two minutes. It is a huge challenge to provide sufficient thermal protection for the instruments during this time. Once through the upper atmosphere, Huygens faces the opposite problem: a surface temperature of minus 180°C. The probe is kitted out with tiny on-board heaters, thermal insulation for the equipment, and sensors and wires that lose very little energy as heat. All these instruments are tested by dunking them in a vat of liquid nitrogen, which has a temperature comparable to Titan. Some of these instruments failed during initial testing, which meant we had to go back to the drawing board. But by the time we sent Huygens to Saturn, we had tested everything we could think of. It's now in the lap of the gods...