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Artists' impression of Huygens probe descending through Titan's atmosphere. Image courtesy ESL. |
For the last seven years the Cassini-Huygens spacecraft has been travelling to Saturn, one of the largest planets in our solar system. This two-storey high spacecraft is now orbiting the planet and sending back images.
 The Mission
On 15 October 1997, a spacecraft bearing the name of two pioneer astronomers was launched on a seven-year, 3.5 billion-kilometre journey to Saturn. Its mission: to explore the Saturn system and provide a close-up look at the giant gas planet, its rings, and its moons.
The Cassini-Huygens spacecraft is made up of two parts. The Cassini orbiter will spend four years touring the Saturn system and using its twelve scientific instruments to send back pictures and information. The Huygens probe was carried to Saturn on board Cassini and released to descend to the surface of Saturn's largest moon, Titan. Huygens then became the first spacecraft to land on the surface of the moon of another planet.
Cassini-Huygens is an international mission with engineers and scientists from the United States and seventeen European countries. These experts designed, developed, and built the spacecraft and provided mission operations for the journey to Saturn and the exploration of the Saturn system.
 Why Should we explore Saturn?
Studies of star formation indicate that our solar system formed within a giant collection of gases and dust, drawn together by gravitational attraction and condensed over many millions of years into many stars.
Our Sun was one of these stars, and surrounding the Sun, the leftover material flattened into a disc from which the planets formed. The large outer gas giant planets, including Saturn, contain much of the primordial cloud gases not trapped by the Sun.
Saturn, like the other gas giants, has a family of moons creating the equivalent of a miniature solar system. They are believed to have been formed by processes similar to those responsible for our planetary system around the Sun.
Saturn and its ring system provide a model for the disc of gas and dust that surrounded the early Sun and from which the planets formed. Pictures and information returned by Cassini will help us get a better understanding of the Saturn system and add to our knowledge of planetary formation.
Detailed knowledge of the history and processes now occurring on Saturn's moons may provide valuable clues to help understand how the planets in our solar system evolved to their present state.
The Saturn system provides an environment where scientists can look for answers to many fundamental questions about the physics, chemistry, and the evolution of the planets - and the conditions that give rise to life.
 The Spacecraft
The Cassini-Huygens spacecraft is 6.7 metres high and weighs 5712 kilograms. More than half of the spacecraft's weight at launch was the fuel required for its engine.
The main body of the Cassini orbiter is cylindrical and contains the computer and control systems that are required to operate the spacecraft. There are more than twelve kilometres of wires linking the instruments, computers, and mechanical devices.
At the top of the spacecraft is the four-meter diameter antenna, which is used to transmit data and images between the spacecraft and Earth. At the base are the rocket engines, which provide the propulsion for major changes to Cassini's trajectory. Sixteen smaller thrusters are used to control Cassini's orientation and to make small adjustments to the flight path.
Attached about halfway up the spacecraft are the cameras and other scientific instruments and the Huygens probe. Electricity is provided by three small atomic power generators.
Most of the spacecraft is covered with special blankets to protect it against the extremes of heat and cold in space and against damage from small particles that travel at great speeds through space.
Cassini has a cargo of twelve scientific instruments. Cameras take pictures while the other instruments observe and sample the surrounding environment. Travelling at the speed of light, radio signals containing the images and data take 84 minutes to make the journey from Cassini back to engineers and scientists on Earth.
 Why is Titan so Special?
Titan, Saturn's largest moon, presents an environment that appears to be unique in the solar system. It has a thick, hazy atmosphere containing organic (carbon-based) compounds, an organic ocean or lakes, and a rich soil filled with frozen molecules similar to what scientists believe led to the origin of life on Earth.
Titan has been described as having an environment similar to that of Earth before biological activity forever altered the composition of the Earth's atmosphere.
The major difference on Titan, however, is the absence of liquid water and the very low temperatures, which are so cold as to prevent any biological activity. The surface of Titan could not be seen through cameras on the earlier passing Voyager spacecraft. Cassini pictures have shown more detail of the atmosphere.
 The Huygens Descent to Titan
The Huygens probe was bolted on to the side of the Cassini spacecraft and rode along during the nearly seven years' journey to Saturn in 'sleep' mode.
On December 24 2004, the bolts were released and Huygens sprang loose from the mother ship and began its mission to Titan. On 15 January 2005, Huygens arrived at Titan and, travelling at 20,000kph, entered the moon's atmosphere. The probe had an especially designed outer shell to withstand this intense heat.
Soon after Huygens entered the atmosphere, its parachutes opened to slow the descent so the probe could conduct an extensive program of scientific observations all the way down to Titan's surface.
The descent to the surface took two-and-a-half hours and during this time the camera on the probe collected images that gave us our first look at this intriguing world.
The pictures, together with other information telling us about winds and the structure and composition of the atmosphere as well as the surface of Titan, were transmitted to the Cassini spacecraft overhead and then back to Earth.
Huygens landed on the surface and continued sending data for about two hours.
 Cassini's next four years
The Cassini-Huygens mission is a modern day voyage of discovery. The Huygens probe provided our first close-up look at the world of Titan and a great deal of information to help us understand the evolution of our own world.
The Cassini spacecraft has already provided many great pictures and much new scientific information and will continue to add to our knowledge of Saturn, its rings, and its moons, while exploring the Saturn system over the next four years.
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