• In operation
• Launched 1997
• Arrived at Saturn July 2004
• Huygens probe landed on Titan 14 January 2005

Cassini-Huygens is the first mission to make a long-term study of Saturn, its moons, rings and complex magnetic environment. A joint NASA/European Space Agency (ESA)/Italian Space Agency project, Cassini-Huygens involves UK scientists on both the orbiter (Cassini) and probe (Huygens).

Since entering Saturn’s orbit, the spacecraft has transformed our understanding of the ringed planet. Achievements include the discovery of new rings and several new moons. Cassini has also witnessed a massive hurricane-like storm and found evidence that the planet’s rotation appears to be slowing. The spacecraft recently sent back views of the planet from high above and below these rings – a perspective never seen before.

Remarkable discoveries have been found among Saturn’s moons. The tiny moon, Enceladus, has spectacular jets of ice particles erupting from its south pole. Another moon, Phoebe, has turned out to be older than Saturn itself! The ‘black and white’ moon, Iapetus, was found to have a ridge along its equator higher than any mountain on Earth. The latest moon – the 60th – has been nicknamed ‘Frank’. It was spotted by Carl Murray from Queen Mary, University of London in collaboration with the Cassini imaging team.

Saturn’s largest moon, Titan, is a major focus of the mission. Titan has a very thick atmosphere - similar to Earth's when it was a very young planet. By studying Titan, scientists hope to gain an insight into how life might have first become established on Earth.

In January 2005, the Huygens probe descended by parachute through Titan’s atmosphere and survived for several hours on the surface. No-one knew whether to expect a hard or soft landing. In fact it was somewhere in between.

In March 2007, instruments on board Cassini found evidence of seas in the northern parts of Titan that might be filled with liquid methane or ethane. Results based on data from Huygens also suggest there is liquid methane rain on the planet.

Mission facts

• The Cassini spacecraft is named after the Italian-French astronomer Jean-Dominique Cassini (1625 - 1712) who discovered four of Saturn's moons: Iapetus, Rhea, Tethys and Dione.
  
  
• Huygens is named after the Dutch scientist Christiaan Huygens (1629 - 1695) who explained the nature of Saturn's rings and discovered its largest moon, Titan, in 1655.
  
  
• Cassini-Huygens is as tall as a two-storey house and weighed 5,574 kg when it left the Earth.
  
  
• The spacecraft travelled 3.5 billion km to reach its destination. Due to its weight and the distance it had to travel, Cassini-Huygens used a series of ‘gravity assists’ or ‘fly-bys’ on route to Saturn.
  
  
• Gravity assist is a manoeuvre in which a spacecraft flies passed a planet. It works because of the mutual gravitational pull between the moving planet and a spacecraft. The planet pulls on the spacecraft but the spacecraft's own mass also pulls on the planet. This permits an exchange of energy. The fly-bys with Venus (twice) and Earth saved the equivalent of 68,000 kg of rocket fuel.

Technology

Cassini-Huygens has a total of 18 instruments equipped to investigate many different aspects of the Saturn system.

Cassini

The Cassini Plasma Spectrometer (CAPS) measures the energy and electrical charge of particles like electrons and protons, to help us understand the nature of Saturn's magnetic field.

The Cosmic Dust Analyser (CDA) determines which elements make up the small dust particles around Saturn and how they interact with the planet's rings, moons and magnetic field.

The Composite Infrared Spectrometer (CIRS) provides vital information about Saturn and Titan's atmospheres. It has also been helping scientists identify the molecular composition of Titan's surface.

The Ion and Neutral Mass Spectrometer (INMS) is used to study Saturn's magnetic field and find out how gases around the planet's moons interact with the solar wind - the stream of charged particles coming from the Sun.

The Imaging Science Subsystem (ISS) has two optical cameras that have sent back tens of thousands of spectacular images.

The Dual Technique Magnetometer, or MAG, measures the interior structure and internal magnetic fields of Saturn and its moons, showing us how they interact with the particles that make up the solar wind.

The Magnetospheric Imaging Instrument (MIMI) has been taking images of Saturn’s hot plasmas: gases made up of ions, electrons and neutral particles.

The Radio Detection and Ranging Instrument (RADAR) is being used to create a map of Titan.

The Radio and Plasma Wave Science (RPWS) instrument measures radio and plasma waves given off by the solar wind as it comes into contact with Saturn's atmosphere and magnetic field. This is another experiment that helps scientists build up a clear picture of the planet, and what effect the Sun has on it.

The Radio Science Subsystem (RSS) measures how radio signals from Cassini change as they are sent through objects, such as Saturn's rings. This will give scientists detailed information on the structure of these objects.

The Ultraviolet Imaging Spectrograph (UVIS) measures the ultraviolet light being emitted from, and reflected off, Saturn's atmosphere, surfaces and rings. It will also tell scientists which elements make up the planet's atmosphere.

The two cameras on the Visible and Infrared Mapping Spectrometer (VIMS) are helping scientists determine the weather patterns on Saturn and Titan as well as the composition of the rings and moons.

Huygens

As Huygens reached Titan, it switched itself on, activated its radio link to Cassini and began its descent into the atmosphere at around 20,000 km per hour.

As the first readings were collected, three sets of parachutes deployed to control its descent. Two and a half hours later, the first man-made object touched the Titanian surface.

Instruments on board Huygens measured the physical and electrical properties of the atmosphere and surface while a camera sent back images of the alien landscape.

Once it had landed, the probe was only designed to last around half an hour (at the most) before its batteries ran out. However, it is a testament to its construction that the Earth-based Parkes radio telescope was still receiving a signal more than three hours later.

UK involvement

The UK has been at the forefront of the design, engineering and science of Cassini-Huygens.

The Huygens Surface Science Package is led by the Open University (OU) with contributions from the STFC Rutherford Appleton Laboratory (RAL) and Southampton University.

The very first part of Huygens to touch Titan’s surface was a British-built sensor. The OU also has prime responsibility for the instrument that measured the probe’s deceleration through the upper atmosphere.

Imperial College London led the construction of the Dual Technique Magnetometer and analyses scientific data from the instrument, with input from Leicester University.

Queen Mary, University of London, helped in the design of the Imaging Science Subsystem and has a key role in the analysis of the images it is returning.

Oxford University was heavily involved in developing the hardware for the Composite Infrared Spectrometer, and is helping to analyse the data.

Cardiff University and again Queen Mary, University of London, helped to develop the infrared filters on the Composite Infrared Spectrometer.

University College London worked with RAL to develop part of the Cassini Plasma Spectrometer.

UK industry has also played a significant role with software provided by Logica and electronic testing and procurement co-ordinated by IGG component Technology.

IRVIN-GQ worked on the descent control system under contract to Martin Baker Space Systems. The latter was responsible for the parachute systems and the mechanisms needed to control the probe’s descent.

SciSys developed and maintained Huygens’ mission control system which monitored the probe’s health and controlled it billions of kilometres away on Earth.

All this technology had to operate flawlessly after seven years in the harsh space environment.