To Open The Sky
The Front Pages of Christopher P. Winter
The Cassini Mission to Saturn
A personal view
The Mission and the Science
Updated February 2003
Cassini was launched in 1997. Three years later, in the autumn of 2000, engineers discovered a shortcoming in the receiver aboard Cassini that is to pick up the data from the Huygens probe as it descends to Titan. Simply put, the receiver would not have been able to handle the apparent change in frequency (the Doppler shift) of the radio signal from the probe as it slowed down relative to Cassini.
ESA and NASA engineers and scientists formed a joint Huygens Recovery Task Force. After six months of analysis, they came up with a plan to save the mission: Send Cassini cruising past Titan at a larger distance, so the Doppler shift is not so great. The plan, endorsed by senior management from both agencies and by members of the Cassini-Huygens scientific community, also changes the timeline of the Huygens mission. Separation of the probe will be delayed by seven weeks, to 14 January 2005, and Cassini will make three passes by Titan beforehand, instead of the two originally planned.
The timeline below gives both the original and revised dates for the main events of Cassini's mission. Mission elapsed time corresponds to the revised timeline.
Once Cassini reaches Saturn, the plan goes as follows: Science observations begin on 1 June with a close flyby of Phoebe as the first event. Following this, Cassini will decelerate into an orbit aimed for the general neighborhood where Titan will be four months hence. This path gets fine-tuned on 12 September. When Huygens is cut loose, its internal timer is loaded with the exact intervals for its planned sequence of events. Rotating at 7 rpm for stability, Huygens will drift for 22 days, finally entering Titan's atmosphere at a high-latitude point on the sunlit side. Things happen fast after that. Its heat shield will protect it as it barrels into the atmosphere at 20,000 km/hr, slowing to 1,400 km/hr in a mere three minutes. Next, a series of parachutes will complete the deceleration, and the heat shield will drop away. The probe will ultimately come to rest, gathering data from 6 science packages and sending it up to Cassini. Whether Titan's surface is solid or liquid, Huygens is designed to stay upright and afloat. It should complete its mission in under three to four hours.
Cassini, meanwhile, will point its high-gain antenna at Titan for those few hours, storing any data received for later relay to Earth. After this period, Huygens having done its own thing, the mother ship will turn away and begin its four-year mission.
In brief, that mission will be to gather data on the plasma, dust, radio waves and magnetic fields present in the vicinity of Saturn, to observe its rings and icy moons, and to map the geometry of the Saturn system as accurately as possible.
The complements of scientific instruments and their principal investigator (PI) or team leader (TL) are: