FLUG REVUE September 2004: Cassini-Huygens arrives at Saturn

CASSINI PROBE ARRIVES AT SATURN

By Matthias Gründer

On 1 July 2004, after a flight lasting almost seven years and four gravity-assist swing-by manoeuvres past different planets in the solar system, the space probe made a precision turn into orbit around Saturn. According to the mission plan, over the next four years the cosmic scout will circle the giant 75 times and add significantly to our present knowledge of the Saturn system.

Cassini probe arrives at Saturn

Cassini has already been taking pictures of Saturn, its ring system and moons since commencing its approach at the beginning of February. The resolution of these images has exceeded all the research material previously available to astronomers on Earth. The scientific community is enthralled, and even the public has been captivated by the impressive images. Forgotten now are the squabbles which once beset the mission run-up and the launch itself.

What triggered the strife between space aviation experts, politicians and anxious environmental conservationists was the fact that, due to the long distance from the sun, the 5.5 tonne spacecraft cannot use solar panels to generate its onboard energy supply. Instead, it has three radionuclide generators on board, whose fissionable material – over 32 kilograms of plutonium – deliver enough electrical power for all the manoeuvres and the scientific equipment.

This fact even occasioned a special debate in the German Bundestag on 8 October 1997, during which MPs wanted to know what would happen if the rocket were to explode during a failed launch or if the probe were to burn up during a swing-by manoeuvre too close to Earth. The whole discussion was ignited further by revelations from a scientist in the USA who described horror scenarios but was not prepared to release the basis for his calculations. The gutter press seized the subject gratefully. Yet in the event, after several weather-induced delays the launch of the Titan IVB space booster on 15 October 1997 went off smoothly, so that soon the world could divert its attention to other exciting topics.

USING GRAVITY AS A BOOSTER

Once Cassini-Huygens had been successfully launched, the public in fact paid little further attention to it during the mission, and only the scientists tracked the complicated course of the spacecraft to its destination. The gravity-assist swing-by manoeuvres mentioned above were necessary because the launch vehicle was not sufficiently powerful to transport the space probe, the biggest one ever to have been built by human hand, on a direct route to Saturn. It was therefore necessary to execute selective flybys of other planets, as the resultant exchange of energy would then not only alter the trajectory but also boost the speed of the space vehicle.

The first such change of trajectory occurred on 26 April 1998, when Cassini was accelerated around Venus at an altitude of only 287km, and naturally the scientists took the opportunity to scrutinise the planet it was flying past and to calibrate all the equipment. Thus, since the first Venus passage we now know, amongst other things, that there are no storms in the dense clouds that envelop the planet. A second flyby of Venus occurred on 24 June 1999, this time at an altitude of 603km, and on 18 August of the same year Cassini came within 1,180km of Earth.

However, there are no pictures of this passage because the main antenna had to be constantly aligned with the sun during the flight in the inner solar system, in order to protect the onboard equipment from overheating. This meant that the camera was also shielded and could transmit only a few photos of the moon to Earth.

A fourth and final flyby occurred precisely at the turn of the century 2000/2001, when the spacecraft received one last gravity assist from Jupiter. Over the next 14 weeks, Cassini, the Jupiter probe Galileo and the astronomical satellites Hubble and Chandra joined forces to deliver a vast wealth of data and photos of this giant planet. After that, Cassini-Huygens turned onto a direct path to Saturn, and nothing further very exciting happened, at least in the eyes of the public, so the spacecraft was almost forgotten. But the participating experimenters were thrown into utter confusion during one of their regular system checks.

ELEGANT SOLUTION TO SIGNAL PROBLEMS

What had happened? Examination of the Huygens onboard systems in the summer of 2000 revealed problems in the decoding of data, which meant that while Huygens was being set down on the surface of the moon Titan, Cassini would not be able to receive all the signals it was transmitting. Yet originally the space probe was supposed to approach the moon at a speed of over 21,000km per hour and to eject Huygens on a course bound for Titan rather like a cannon. The problem was that the frequency of the signals emitted by the lander would have shifted to a point beyond the bandwidth of Cassini's receiver.

To prevent total loss, the mission controllers decided right on the first approach to abandon the plan for Cassini to fly towards Titan. Instead, they instructed the space probe to execute an additional orbit around Saturn right at the beginning of the mission. The effect of this is that its trajectory will no longer take it directly towards Titan but instead it will fly past the moon with a first approach at 60,000km. This will be sufficient to receive the Huygens signals in an adequate quality. Only then should the approach occur. The space vehicle is now set to approach the moon for the first time on 26 October at an altitude of 1,200km.

So far have there have been no other major problems, and the approach to Saturn was a spectacular success. The precision of all the trajectory manoeuvres to date is illustrated, for example, by the way Cassini penetrated the ring system virtually in its centre, the area known as the Enck's Division, and only then turned off into its orbit. Before that, the space probe had approached to within 20,000km of the uppermost cloud boundary, and a braking manoeuvre executed by the onboard engine then ensured that Cassini was caught up in the gravitational field of the planet.

The first orbit around Saturn lasted 118 days. A slow change in trajectory was then initiated to ensure that in the summer of next year the spacecraft will fly precisely in the equatorial plane of the planet. During this manoeuvre, it will once again send thousands of pictures and a huge quantity of data back to Earth. During the first photo shoot on 6 February, the whole planet with its entire ring system fitted into the field of view. The next stage will be to zoom in on the detail. New, as yet undiscovered moons could even be discovered.

From page 74 of FLUG REVUE 9/2004

Last updated 16 August 2004

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