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Cassini/Huygens: Approaching Saturn - "the Lord of the Rings"

ILA/Berlin, Friedrichshafen, 10 May 2004

The insertion of the joint US/European spacecraft duo, Cassini/Huygens, into orbit round Saturn on 1 July 2004 marks the arrival of "Made in Ottobrunn" technology at the "Lord of the Rings". The final phase of one of the most spectacular planetary missions in space history will begin when the Cassini/Huygens spacecraft reaches Saturn: For the first time the ringed planet and some of its moons are to be studied in detail and a probe is to land on the moon Titan. On December 25, 2004, if all goes according to plan, the European Huygens spacecraft will separate from the mother ship and drift towards Titan. In January 2005, the lander will enter Titan's atmosphere, descend to the surface by parachute and provide unparalleled images and measured data of this extraordinary moon. Thus, a "European" will be the first to explore Titan . Huygens was built for the European Space Agency, ESA, by a European consortium with EADS Astrium responsible for the development and manufacture of the thermal subsystem. Final assembly and testing of the lander were also conducted by EADS Astrium. Cassini is a NASA probe.

With a launch mass of 5.6 tons, Cassini/Huygens is the heaviest interplanetary probe ever built. In comparison: the two Voyager spacecraft, so far the only spaceships to have "visited" Saturn, in 1980 and 1981 respectively, weighed only 815 kg each. Cassini is 6.70 metres high and has a diameter of approximately four metres; Huygens weighs 340 kg and its heat shield has a diameter of 2.70 metres.

On October 15, 1997, Cassini/Huygens was launched into space on a Titan IV/Centaur from Cape Canaveral. Although this is the most powerful U.S. launcher, it did not have enough thrust to catapult this giant probe direct to Saturn. To do this, additional help was needed from nature.

Similar to a stone thrown from a carousel, the spacecraft gained Earth's orbital speed and, at the same time, the Sun's gravity drew the spacecraft into an orbit. In the years that follow, Cassini/Huygens travelled on a spiral orbit ‘against’ the gravity of the sun to the outer reaches of the planetary system. This was only possible by swinging the spacecraft through the gravity fields of three planets. Cassini/Huygens first flew towards the Sun and orbited Venus on 27 April 98 and then again on 24 June 99 after a second orbit around the Sun. Two months later, on August 18, 1999, it passed the Earth at a distance of only 1,000 kilometres. Then, from the gravity field of Earth, the spacecraft duo was catapulted into the outer planetary system. On 30 December 2000 it reached the giant planet of Jupiter, swung round it and finally made its way to Saturn.

During these swing-by or slingshot manoeuvres, Cassini/Huygens gained energy from the gravitational pull and rotation of each of the planets and flew away in a slightly different direction at a much higher speed than before. For example, the probe was accelerated by 20,000 kilometres per hour by the swing-by of Earth alone.

The scientists gained extra benefit from the fly-by of Jupiter by being able to test the instruments on board Cassini and obtain valuable data. From October 2000 to March 2001, the two onboard cameras captured more than 25,000 images of the huge gas planet and some of its moons. They could also observe the thin ring surrounding Jupiter.

After its journey of 3.2 billion kilometres, taking almost seven years, the mission scientists are keen to use the spacecraft to explore the ringed planet Saturn and its moon Titan.
Titan – A strange moon with an atmosphere

In the solar system, all planets – with the exception of Mercury and Venus – are orbited by one or more moons, but only one of these has a dense atmosphere – the Saturn moon Titan.

To be able to build up and maintain an atmosphere in the solar system, a body must have a mass large enough to retain the gaseous envelope. It must not be too hot, as the atmosphere would evaporate. Titan meets both these requirements: With a diameter of 5,150 kilometres it is larger than the planet Mercury and its temperature is about minus 180 degrees Celsius.

Compared to Earth, the Titan atmosphere is totally incapable of sustaining human life. At ground level, it has an atmospheric pressure of 1.6 atmospheres and is made up of roughly 95% Nitrogen and 5% Methane, commonly known as ‘mine’ or ‘marsh gas’. The sunlight has triggered chemical reactions in the upper atmosphere which have resulted in the formation of a thick layer of smog surrounding the atmosphere which obscures the surface. Only in the infrared spectrum can astronomers see through this veil of haze and distinguish the blurred images of differently shaded surfaces.

Many planetary researchers envisage some peculiar surface formations on Titan. At temperatures of approximately minus 180 degrees Celsius, the methane can be gaseous, solid or liquid. Thus, on Titan, methane plays the same role as water on Earth. It can condense into oceans and form clouds in the atmosphere. Therefore, it seems possible that methane oceans wash around solid continents while caustic substances, such as acetylene and smog particles, permanently trickle down and gather in thick sludge layers on the bottom of the oceans. Recently, evidence of water ice has been found on the surface. Huygens will find out whether these predictions are true.

Furthermore, the researchers assume that Earth's primordial atmosphere had a similar composition to that of Titan. The scientists are therefore eager to know whether, despite the negative temperatures, complex organic compounds exist on this moon which could be considered the building blocks for the formation of life.

Another interesting question, for example, is why this moon has an atmosphere when Ganymed, the Jupiter moon, which is approximately the same size as Titan and is also extremely cold, does not .
Smooth landing on Saturn's moon

During the transit flight, the instruments on Huygens remain dormant, except for occasional conditional checks. Only one hour before reaching the upper atmospheric layers does an internal timer activate the probe; then, in January 2005 comes the moment of truth. The complete landing sequence must be fully automatic as radio signals take more than one hour to reach Saturn from Earth, 1.3 billion kilometres away.

At a speed of 22,000 kilometres per hour, the spacecraft races towards Titan. When descending through the atmosphere, all onboard instruments will briefly be exposed to sixteen times the gravitational acceleration and the ceramic heat shield will heat to up to 1,800 degrees Celsius. Approximately 170 kilometres above the surface, the primary parachute will open and separate at a height of about 100 kilometres. A smaller parachute then unfurls and the probe lands softly on the surface.

The descent is expected to take about two-and-a-half hours and in that time, special 750 watt batteries will provide power to the equipment. Six instruments will measure the physical/chemical condition of Titan's atmosphere and examine the dynamics of clouds and the veils of haze. In addition, a multi-spectral camera will take approximately one thousand pictures.

Strong winds are expected in the atmosphere pushing the probe up to speeds of 250 kilometres per hour. With the help of a Doppler wind experiment it will be possible to determine the direction and intensity of the zonal winds. To do this, the frequency, or Doppler, shift of the radio signal will be relayed by Huygens to the Cassini orbiter for extremely precise measurement These measurements will enable an altitude profile of the wind speed over a range of 160 kilometres to be determined with an accuracy of less than one metre per second. This can only be achievedusing an extremely stable carrier signal from the radio transmitter. The transmitter's frequency stability of 0.4 Hz is ensured by an ultra-stable rubidium oscillator which was developed and built by EADS Astrium. A particular challenge was to build the oscillator so that its stability is not affected, either by the various accelerations or the increase in atmospheric pressure during landing.

A small transmitter on board Huygens will relay the data collected by the instruments to the Cassini orbiter at eight kilobits per second. Received via the primary antenna, the data will be stored on board and subsequently transmitted to Earth. Huygens has been designed so that it can survive the impact and even if it lands in an ocean it can ‘swim’ for some time. Provided that the landing is successful – as is hoped by the scientists – Huygens will send unique data and images from the Titan surface to Cassini until the instruments stop working. At temperatures reaching minus 180 degrees Celsius, the battery can provide power for up to two and a half hours. The landing site can be determined by the researchers with the help of the ultra-stable oscillator.

Originally, Huygens was expected to land on Titan in November of this year, 2004. However, a couple of years back, ESA engineers recognised that Cassini would be moving away from Huygens too fast. This would have resulted in a strong Doppler-shift of the transmitter's radio signals on Huygens, so that they could no longer be received by the Cassini receiver. Therefore, it was decided to change the flight schedule, and hence the landing date.

Cassini at the Lord of the Rings

Even before entering its orbit round Saturn, the spacecraft duo has to survive a stringent test, the traversing of the ring system. Although this will not be through the main visible rings, the existence of dust particles and differently sized rocks cannot, however, be excluded. This is why the probe will cross this area using the primary antenna at the front as a protective shield. Only a few minutes are left after this to turn Cassini/Huygens by 180 degrees so that the engines point in the direction of flight to slow it down. The engines must then burn for at least an hour and a half to bring the spacecraft into a stable orbit around the planet.

The following day, Cassini/Huygens will pass over Titan at an altitude of 340,000 kilometres, i.e. something less than the mean distance between the Earth and the Moon. Then the probe will send the first images of Titan to Earth.

Cassini will orbit the ringed planet for four years and study it in considerable detail. It will perform a total of 48 fly-bys of Titan and visit the moons Iapetus, Mimas, Enceladus, Thethys, Dione, Rhea, Hyperion and Phoebe to study them thoroughly.

The mission's name

The mission was named in honour of two prominent astronomers of the seventeenth century who also, among other things, explored Saturn. The Dutch astronomer Christian Huygens was the first to detect that the Saturn ring is made up of freely floating matter and he also discovered Titan in 1655. The Italian-French astronomer Jean-Domenico Cassini detected four Saturn moons and, in 1675, found a gap in the ring system, the so-called Cassini division.

EADS Astrium is Europe's leading satellite system specialist. Its activities cover complete civil and military telecommunications and Earth observation systems, science and navigation programmes, and spacecraft avionics and equipment. EADS Astrium is a wholly owned subsidiary of EADS SPACE, which is dedicated to providing civil and defence space systems. In 2003, EADS SPACE had a turnover of more than 2.4 billion euros and about 12,000 employees in France, Germany, the United Kingdom and Spain.

EADS is a global leader in aerospace, defence and related services. In 2003, EADS generated revenues of 30.1 billion euros and employed a workforce of more than 100,000.

Munich/Friedrichshafen, May 2004/04010

The European Huygens probe

Total mass: 343 kg
includes instruments at: 48 kg
Number of instruments: 6
Diameter of heat shield: 2.70 m
Power supply: 750 W (max. 153 minutes)

Comparison of Titan v Earth's Moon

Titan Earth's Moon
Diameter 5150 km 3476 km
Orbit duration around planet: 15.95 days 27.32 days
Surface temperature: -180 °C +110/–150°C
Atmosphere: 95% Nitrogen (N 2 )
5% Methane (CH 4 )


The Doppler Wind Experiment

The Doppler Wind Experiment used on Huygens is based on a phenomenon everyone knows from everyday life – the acoustic Doppler effect. The sound of a police car’s siren seems to be higher when the car is approaching than when it is moving away. Assuming that the sound waves of the police siren are a continuous series of wave crests and troughs, then more wave trains per second reach our ear in the case of an approaching car. This means that the frequency of the acoustic wave first increases and then decreases, which is heard as a change in pitch.

This effect, named after the Austrian physicist Christian Doppler, works for every type of electromagnetic wave, such as radio waves or light. As the frequency change directly depends on the relative velocity between the transmitter and the receiver, this method can be used to determine the speed of Huygens and Cassini from the frequency change of the radio waves between them.

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