The Cassini Mission to Saturn

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.

MISSION EVENT	ORIGINAL DATE	REVISED DATE	DAYS FROM LAUNCH
See also Cassini-Huygens Mission Timeline on Wikipedia.
Launch	15 October 1997	6 October 1997	0
Aphelion 1		1 November 1997	26
Perihelion 1		23 March 1998	169
Venus swingby #1	21 April 1998	21 April 1998	198
Deep space maneuver	2 December 1998	2 December 1998	423
Aphelion 2		4 December 1998	426
HGA Window begins		16 December 1998	436
HGA Window ends		10 January 1999	461
Venus swingby #2	20 June 1999	20 June 1999	622
Perihelion 2		27 June 1999	629
Earth swingby	16 August 1999	16 August 1999	680
HGA permanently deployed		29 January 2000	969
Jupiter swingby	30 December 2000	30 December 2000	1181
Science observations begin	1 June 2004	1 January 2004	2277
Insertion into Saturn orbit	1 July 2004	1 July 2004	2460
First flyby of Titan	27 November 2004	26 October 2004	2577
Second flyby of Titan		13 December 2004	2625
Third flyby of Titan		25 December 2004	2637
Huygens probe release	6 November 2004	25 December 2004	2637
Huygens on Titan	27 November 2004	14 January 2005	2659
End of nominal mission	1 July 2008	1 July 2008	3921

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:

To Open The Sky

The Cassini Mission to Saturn

A personal view

The Mission and the Science

Updated February 2003

Cassini

Huygens