Cassini/Huygens: Saturn Orbiter with Huygens Titan Probe

J.-P. Lebreton

Huygens homepage http://www.estec.esa.nl/spdwww/huygens/html

Cassini homepage http://www.jpl.nasa.gov/cassini

Cassini/Huygens is a joint NASA/ESA planetary mission designed to explore the Saturnian system in general and Titan in particular. The Cassini spacecraft comprises the NASA-provided Saturn Orbiter (with a significant Italian contribution under a bilateral NASA/ASI agreement) and the ESA-supplied Huygens Titan Probe.

The overall mission is named after the Italian astronomer Giovanni Domenico Cassini, who discovered several of Saturn's satellites and ring features (the Cassini division) during 1671- 1685, while ESA's Probe is named after the Dutch astronomer Christiaan Huygens, who discovered Titan in 1655.

The mission's scientific objectives are to carry out a detailed 4-year exploration of all the elements of the Saturnian system, i.e. Titan, Saturn's atmosphere and interior, the rings, the magnetosphere and most of the planet's 17 known icy satellites. The development of both mission elements proceeded well during 1995-96 as they were readied for the October 1997 prime launch window. The current launch date is 6 October 1997. The Titan 4B/Centaur-G launcher is not powerful enough to send the 6 t spacecraft on a direct trajectory to Saturn.

Gravity assist manoeuvres at Venus (twice), the Earth and Jupiter are required to boost it to Saturn, with arrival in July 2004. Huygens will be separated from the Orbiter in early November 2004 for a coast phase of 3 weeks towards Titan. During Huygens' 2-2 h parachute descent through Titan's atmosphere on 27 November 2004, and possibly afterwards on the surface, the Probe will transmit its data to the Orbiter at 8 kbit/s. The Probe-to-Orbiter link will be maintained for 3 h, including at least 30 min of communication from the surface in case of survival. The four major Huygens mission phases are illustrated in Fig. 3.1.2/1.

Figure 3.1.2/1: Artist's illustration of the four main Huygens mission phases.

The Huygens hardware programme comprises four models: the Structural Thermal and Pyro Model (STPM), the Electrical Model (EM), the balloon drop-test model, and the Flight Model (FM). After completion of the integration and testing activities of the STPM and EM models in Europe, both models were delivered in turn to NASA's Jet Propulsion Laboratory (JPL) for, respectively, mechanical/environmental tests and electrical/functional tests with the Orbiter.

A highly successful drop-test took place from a stratospheric balloon at 38 km altitude over Kiruna, Sweden in 1995. It verified the parachute deployment sequence and provided essential data for the characterisation of the Probe's spin. As a result, a minor modification was required to the spin vanes to meet the spin requirements imposed by the Descent Imager/Spectral Radiometer imaging requirements.

Some delays were experienced with the delivery of the flight experiments but, by November 1996, they were all delivered and integrated. The accommodation of the payload is shown in Fig. 3.1.2/2.

Figure 3.1.2/2: Huygens Probe principal features.

The scientific performances of the payload will be formally reviewed as part of the Huygens Flight Acceptance Review in March 1997. The capabilities of the scientific payload were regularly monitored during the development phase. Intermediate reviews indicated that the Huygens payload will meet all the scientific objectives put forward during the payload selection process. In several areas, the payload capabilities exceed the initial expectations.

The FM Probe will be delivered to Kennedy Space Center in early April 1997 for integration and testing activities with the Orbiter before the mating of the spacecraft with the launcher in July 1997. The Huygens Probe operations will be conducted from the Huygens Probe Operations Centre (HPOC) at ESOC. Huygens will be accessible only by telecommand (through the Orbiter) as long as it remains attached to the Orbiter. The Huygens operations include biannual checkouts during the 7-year cruise, and verification of the Probe configuration prior to separation 3 weeks before its entry into Titan's atmosphere. After separation, the Probe will be fully autonomous. During descent, the data will be received and stored onboard the Orbiter. At the first downlink opportunity thereafter, the Probe data will be sent to NASA's Deep Space Network (DSN), placed in the Cassini data base at JPL and then transferred to the HPOC. The Huygens data will be distributed to the PI teams for display and preliminary near- realtime analysis. Raw data from all checkout sessions and those collected during the descent will be distributed to the PIs on CD-ROM within weeks after being received at ESOC. Plans are being developed to archive the Huygens data as a subset of the Cassini archive at the Planetary Data System (PDS) in the US.

The Project Scientist's work has been characterised by the following activities:

• Routine interaction with the Huygens Project Team on science-related matters;
• Interaction with the Cassini Project Scientist and Cassini Project Team members at JPL, as required, on Huygens science- related matters and on the international cooperation aspect of the Cassini/Huygens mission;
• Providing Huygens Project support for the Phase C/D activities;
• Participation in all the Huygens Project Reviews, and in Huygens instrument reviews when necessary;
• Participation in several Cassini reviews at JPL;
• Chairing the Huygens SWT and participating in the Cassini Project Science Group (PSG) as vice chairman;
• Ensuring liaison between the Project and the science community with regard to the various environmental models required for the design of the Probe;
• Advising ESOC on science operation aspects of Huygens;
• Acting as scientific coordinator for the Huygens Special Publication Huygens: Science, Payload and Mission, which will appear in 1997 (ESA SP-1177);
• Acting as an advisor for Huygens-related public relations (video, brochure, poster, signature CD-ROM, etc);
• Presentation of Huygens at several conferences to inform the wide scientific community about the mission.

Reference
Lebreton, J.-P. (Ed).Huygens: Science, Payload and Mission, ESA SP-1177, 1997.