The Huygens Probe System Design

The Huygens Probe is the ESA-provided element of the joint NASA/ESA Cassini/Huygens mission to Saturn and its largest moon Titan. Huygens is an entry probe designed to enter Titan's atmosphere and descend under parachute down to the surface. The Probe is carried to Titan on board the Cassini Saturn Orbiter. Huygens is dormant for 7.2 years, during the interplanetary journey and during the first 6 months around Saturn. It is activated about every 6 months for an in-flight checkout to verify and monitor its health and to perform a periodic maintenance and calibration of the payload instruments. The Probe will be targeted to Titan and released from the Orbiter about 3 weeks before the Titan encounter on the third Orbit around Saturn. During the 3-week coast phase the Probe is 'OFF', except a timer unit that has the task to awaken Huygens before it enters Titan's atmosphere. The Probe's aeroshell will decelerate it in less than 2 minutes from the entry speed of about 6 km s(-1) to 400 m s(-1) (Mach 1.5) at an altitude of 150-180 km. From that point onwards, a pre-programmed sequence will trigger the parachute deployment and the heat-shield ejection. The main part of the scientific mission will then start, lasting for a descent of 2-2(1/2) hours. The Orbiter will listen to the Probe for a total duration of at least 3 hours, which includes time to receive data from the surface, should the Probe continue to transmit data after touchdown. Huygens' transmissions are received and stored aboard the Orbiter for later retransmission to the Earth. This paper presents a technical description of the elements of the Huygens Probe System. The reader is invited to refer to the companion paper (Lebreton and Matson, 2002) for further background information about the Huygens mission, and the payload. The early in-flight performance of the Probe is briefly discussed. During in-flight testing in 2000, a technical anomaly was found with the Probe-to-Orbiter telecommunication system that required a change in the Huygens mission scenario designed before launch. It required also a change in the Orbiter trajectory during the Probe mission. This change was achieved by modifying the initial Cassini/Huygens orbits around Saturn. At the time of writing, details of the implementation of the revised Huygens mission scenario are still being worked.