MISSION BACKGROUND
INTRODUCTION
The Cassini–Huygens Space Mission formally began in 1982 – 15 years before it launched and 22 years before it arrived at Saturn. By 1985 a joint European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) assessment of a Saturn orbiter and Titan probe was completed, and in 1986 ESA’s Science Program Committee gave approval to start the first stages of research on the Saturn orbiter (now named Cassini). In 1987 the Titan probe was renamed Huygens by the ESA.
At this time NASA and ESA announced the opportunity for scientists to propose scientific investigations for the mission.
Then in 1992 a funding cap was placed on the project. The Cassini–Huygens
mission had to be restructured to cut costs and to simplify the mechanical
design of the spacecraft. The designs of Cassini and Huygens were based around
creating the most durable, reliable, and sophisticated spacecraft to date, while
reducing cost and mass. The largest effect of these design choices was on the
large number of moving parts that had to be removed from the original design,
where functions could be performed without them.
Cassini–Huygens was an international collaboration between three space
agencies. Seventeen nations contributed to building the spacecraft. The Cassini
orbiter was built and managed by NASA/CalTech’s Jet Propulsion Laboratory.
The Huygens probe was built by the European Space Agency. The Italian Space
Agency provided Cassini’s high–gain communication antenna. On 15th
October 1997, the Cassini–Huygens Spacecraft was launched sending it on
its way to a meeting with Saturn in July 2004 and Titan in January 2004.
CASSINI–HUYGENS’ MISSION
The Cassini–Huygens spacecraft consisted of two main elements: the Cassini orbiter, named after the Italian–French astronomer Giovanni Domenico Cassini, and the Huygens probe, named after the Dutch astronomer Christiaan Huygens. The spacecraft was launched on 15th October 1997 and entered Saturn’s orbit on 1st July 2004. On 25th December 2004, the probe separated from the orbiter at approximately 02:00 UTC. The probe reached Saturn’s moon Titan on 14th January 2005, where it made an atmospheric descent to the surface.
THE CASSINI ORBITER
It has been planned that the Cassini orbiter will orbit Saturn and its moons for four years. Cassini will conduct various investigations including observations of the atmospheres of Titan and Saturn, and the measurement of the gravitational fields of the planet and its satellites. The Cassini spacecraft, including the orbiter and the Huygens probe, is the largest, heaviest, and most complex interplanetary spacecraft built to date.
THE HUYGENS PROBE
The Huygens probe scrutinised the clouds, atmosphere, and surface of Saturn’s moon Titan in its descent on 15th January 2005. It was designed to enter and brake in Titan’s atmosphere and parachute down to the surface. The Huygens probe system consisted of the probe itself, which descended to Titan, and the Probe Support Equipment (PSE) on Cassini, which remained attached to the orbiting spacecraft. The PSE included the electronics necessary to track the probe, to recover the data gathered during its descent, and to process and deliver the data to the orbiter, from which it was transmitted to Earth.
The Huygens probe had six complex instruments aboard, including the Surface–Science Package (SSP), that took in a wide range of scientific data after the probe descended into Titan’s atmosphere. The SSP contained a number of sensors designed to determine the physical properties of Titan’s surface at the point of impact, whether the surface was solid or liquid. During descent, measurements of the speed of sound gave information on atmospheric composition and temperature, and an accelerometer recorded the deceleration profile at impact, indicating the hardness and structure of the surface. If the surface had been liquid, other sensors would also have measured its density, temperature and light reflecting properties, thermal conductivity, heat capacity, and electrical permittivity.
Huygens was expected to transmit valuable data from the surface of Titan only for between three and thirty minutes. However the Huygens power source lasted much longer than anticipated, and data continued to be transmitted to Cassini for one hour and twenty minutes. Even after that Huygens was still functioning, but Cassini then moved out of communications range and the data could no longer be transmitted to Earth.
Scientists are still busy analysing the 474 Megabits of data they retrieved including 350 images.
On 1st July 2004, the spacecraft flew through a gap in the thin outermost area of Saturn’s rings and achieved orbit, after a seven year voyage. It is the first spacecraft to ever orbit Saturn.
THE MISSION
Cassini had its first distant flyby of Saturn’s largest moon, Titan, on 2nd July 2004, only a day after orbit insertion, when it approached to within 339,000 kilometres (211,000 miles) of Titan and provided the best look at the moon’s surface to date. Images taken through special filters showed south polar clouds thought to be composed of methane and surface features with widely differing brightness. On 27th October 2004 the spacecraft executed the first of the 45 planned close flybys of Titan when it flew a mere 1,200 kilometres above the moon. Almost four gigabytes (4GB) of data were collected and transmitted to Earth, including the first radar images of the moon’s haze–enshrouded surface. Radar imagery observed no conclusive evidence of lakes of liquid hydrocarbons, though it did not dismiss the possibility such lakes could exist. It also revealed the surface of Titan (at least the area covered by radar) to be relatively flat, with topography reaching no more than about 50 meters in altitude.
Cassini released the Huygens probe on 25th December 2004. It entered the atmosphere of Titan on 14th January 2005.
Huygens was designed to enter and brake in Titan’s atmosphere and parachute a fully instrumented robotic laboratory down to the surface. When the mission was planned, it was not yet certain whether the landing site would be a mountain range, a flat plain, an ocean, or something in between. It was hoped that analysis of data from Cassini would help to answer these questions. The Huygens probe was designed to survive the impact with Titan’s surface and for several minutes send back data on the conditions there. The spacecraft had no more than three hours of battery life, most of which was planned to be taken up by the descent. Engineers only expected to get at best 30 minutes of data from the surface.
FINDINGS
The preliminary findings confirm that the targeted region is near the shoreline of a liquid ocean. The photos indicate the existence of drainage channels near the mainland and what appears to be a methane sea complete with islands and a mist–shrouded coastline. There are indications of chunks of water ice scattered over an orange surface, the majority of which is covered by a thin haze of methane. The instruments revealed “a dense cloud or thick haze approximately 18–20 kilometres (11–12 miles) from the surface” which is likely to be due to the reservoir of methane on the surface. The surface itself appears to be clay–like “material which might have a thin crust followed by a region of relative uniform consistency.”
