FIVE MONTHS AFTER the Cassini orbiter arrives at Saturn after a seven-year, 1.5 billion kilometre journey, a small, cone-shaped craft will be despatched to that planet's largest moon, Titan, on a suicide mission which could throw new light on the origins of life on Earth.
When it arrives in late 2004, the NASA/ European Space Agency (ESA)-built Huygens probe (named after the Dutch discoverer of Titan in 1655) will be the first man-made robot to land on a moon other than our own. If theories about Titan's primitive atmosphere prove correct, it may also be the first space lander to drop into an extraterrestrial "sea" - made not of water, but of liquid methane.
For sheer boldness, the Cassini/Huygens mission rivals any previous interplanetary endeavour. It is also the last of the current series of "great" space-science missions, economic reality having forced an end to the always imaginative and often highly successful string of robotic investigations of our planetary neighbours. From now on, missions will have to be far less complex, and inexpensive, to stand a higher chance of passing the budget cutter's axe.
The first visit to Titan was made by the US Voyager 1 probe in 1980, as part of that craft's spectacular visit to the outer planets. The experiments designed for the Huygens mission are based almost entirely on the data gathered from the Voyager's flyby, which, although minimal, fired enough scientific curiosity to demand a dedicated return mission. "I was very exited about the results from Voyager 1," says French Voyager scientist Daniel Gautier. "It immediately came into my mind that the next mission should be to Titan."
Others felt the same and, in November 1982, the Huygens probe, built by a European team with France's Aerospatiale as prime contractor, was suggested to ESA. The project faced an immediate survival test, however, because of the limitations placed on ESA's ambitious space-exploration plans contained in the original Horizon 2000 programme. The Huygens, which accounts for around $400 million of the $2 billion overall mission cost, came through, although some $100 million is being paid for by the institutions responsible for the experiments.
Launch of the Cassini/Huygens is due aboard the appropriately named Titan IV/Centaur rocket on 6 October, 1997. The flight itself includes no less than four planetary swingbys to gain momentum, using Venus twice, the Earth and, finally, Jupiter. Five months of careful manoeuvring near Saturn are then necessary to position the Huygens correctly for delivery to Titan.
The Huygens will carry six experiments designed to measure atmospheric composition and conditions and, if it survives the landing, the kind of terrain underneath the craft. Up to 3h of battery life have been planned into the mission for post-arrival measurements, but the incredibly hostile Titan environment may take its toll well before then. Even so, a mission which achieves any understanding of the nature of its surface will be judged to have been successful.
DESCENT TO TITAN
During the descent and (it is to be hoped) after landing, the Cassini craft, in an orbit 76,000km away, will relay the Huygens' findings to Earth, continuing to do so until the mother craft disappears below Titan's horizon on 27 November, 2004. The Cassini will then carry on by itself for another four years, gathering new data about Saturn and its rings and moons.
By far the most dramatic part of the mission is the descent by the Huygens into Titan's boiling atmosphere, using a system developed by Martin-Baker. With its battery power strictly limited, the aim is to leave the craft "asleep" for as long as possible, bringing it to life only as it approaches the outer fringes of the moon. Around 1,000km above its surface, the probe will decelerate from its 20,000km/h approach speed. Measurements taken here will reveal the changing density of Titan's outer layers.
Atmospheric heating demands that the Huygens instruments be protected during descent - hence the cone-shaped front shield, which will both protect against 12,000° C temperatures and act as an atmospheric brake, slowing the craft during three fierce minutes to just 1,400km/h at 180km above the surface. The shield is made from silica-fibre tiles, to prevent the temperature exceeding 1,600° C, while a rear shield, which experiences far less heating, is made from small silica spheres sprayed on to stiffened aluminium sheeting.
A pilot parachute will then deploy, pulling out the main parachute and easing the speed to just 300km/h. The protective cone will then fall away, exposing the instruments. If predictions about the density of the atmosphere are correct, the probe will at this point be at a height of 160km, descending through gases at a temperature of -120°C. Finally, to speed up the final descent and make the most of the remaining battery power, the main parachute will be jettisoned after 15min, and replaced by a smaller unit which brakes the craft for its final descent from 120km to the ground. This will take around 2h, the robot taking measurements according to the pressure altitude.
During descent, the deceleration force will have reached up to 16G - placing severe constraints on the design of the measuring instruments which will now come into action. Assuming that the probe and its payload have survived, the scientific action will now begin in earnest. Winds of up to 250km/h are expected to drift the probe sideways, allowing a German experiment to measure the wind speeds by reference to the Cassini Orbiter. At this point, the radar altimeter is expected to begin revealing topographic details of the still-invisible surface.
Detailed weather information will be gathered by several instruments measuring temperature, barometric pressure, electrical properties and radio pulses from lightning flashes. A microphone will listen for thunderclaps.
A French/Austrian experiment will collect atmospheric aerosol particles during the descent, its harvest processed periodically by heating the contents of a filter to 650°C in an oven. The resulting gases will then be supplied to a US-designed mass spectrometer which will sort and count molecules and atoms by weight, while a gas chromatograph will determine what the molecules actually are. Nitrogen is expected to be by far the most dominant substance.
Even tiny traces of other materials will be traced, enabling scientists to develop the most comprehensive picture of extra-terrestrial atmospheric composition ever achieved, pointing the finger at Titan's origin and subsequent evolution. The presence of argon atoms, for example, would help establish whether the atmosphere came from the comet-like bodies which created Saturn's moons, or from within the moon itself.
Closer to the surface, at an altitude of around 50km, the temperature is expected to begin to climb slightly, clearing the haze and allowing the first images of the surface. Volcanoes emitting ammonia and water, oceans of mixed methane and ethane, dominated by brightly coloured "icebergs", or an altogether dry landscape with geysers spouting methane from underground reservoirs are considered to be possible scenarios.
Small vanes around the probe's exterior will cause it to rotate as it closes on Titan's surface, enabling 360° photographic images to be collected by a US-designed experiment. This will provide imagery of the surface as the craft descends through the murky atmosphere. The images will provide detail at three resolution levels, allowing panoramic and medium- and short-range digital photographs of the solid or liquid landing zone to be obtained.
Finally, the probe will either hit the ground or splash into the "sea" at around 20km/h, whereupon a UK-built experiment will come into play. If touchdown is into a liquid, a tiltmeter will detect waves, while an acoustic sounder will measure its depth and density. If the surface is hard, a special penetrating tool will gauge the hardness.
The schedule for the mission is tight, having been laid down in 1990 and calling for delivery of the first flight model to NASA on 1 May, 1997, five months before the launch. Development is on target, according to Hamid Hassan, ESA's project manager for Huygens.
A successful first test has already been carried out to test the vital parachute-deployment system. In May 1995, a 100m-tall helium balloon released a full-scale model of the Huygens 37.5km above the Esrange upper-atmosphere site at Kiruna, Sweden, the probe passing through the entire automatic procedure for release of the three parachutes and the front and rear shields. The drop test could achieve a velocity of only Mach 0.8, however, so windtunnel tests have also been carried out to create the Mach 1.5 velocity at which the pilot chute will be deployed in Titan's atmosphere.
Despite the risks faced by the Huygens, those involved in the programme convey real excitement about this extraordinary attempt to send a human-built object further than ever before to land safely on another world. "I believe it is one of our most ambitious missions ever," says Hassan, "but I have a feeling that it will succeed."
Source: Flight International