Credit: Manufacturer Image
Status: Operational 1997. First Launch: 1997-10-15. Last Launch: 1997-10-15. Number: 1 .
NASA NSSDC Master Catalog Description
Huygens is an atmospheric probe designed to make in situ observations of the Saturnian satellite Titan. ESA's contribution to the Cassini mission, Huygens' objectives are to: (1) determine the physical characteristics (density, pressure, temperature, etc.) of Titan's atmosphere as a function of height; (2) measure the abundance of atmospheric constituents; (3) investigate the atmosphere's chemistry and photochemistry, especially with regard to organic molecules and the formation and composition of aerosols; (4) characterize the meteorology of Titan, particularly with respect to cloud physics, lightning discharges, and general circulation; and, (5) examine the physical state, topography, and composition of the surface.
Huygens consists of two pieces of apparatus: the probe and the probe support equipment (PSE). The probe itself consists of two elements as well: the aeroshell, which protects the instruments during the high-velocity entry into Titan's atmosphere, and the descent module, which contains the scientific instrumentation. The descent module is enclosed in the aeroshell. They are mechanically attached to each other at three locations.
The aeroshell is comprised of two parts: a front shield and a back cover. The front shield is a 79 kg, 2.75 m diameter, 60 degree half-angle coni-spherical surface. Tiles of "AQ60" ablative material (a felt of phenolic resin reinforced by silica fibers) provide protection against the heat of entry into Titan's atmosphere. The supporting structure is a carbon fiber reinforced plastic honeycomb shell, also designed to protect the descent module from the heat generated during entry. The tiles were attached to the supporting structure by an adhesive. A suspension of hollow silica spheres in a silicon elastomer (Prosial) was sprayed directly on the aluminum structure of the rear surface of the front shield to further insulate the surface. The back cover, which experiences far less heating during atmospheric entry, carries multi-layer insulation to protect the probe during the cruise phase to Saturn and during the coast phase. A hole in the cover allowed for depressurization during launch and allows for repressurization during entry. It is an 11.4 kg stiffened aluminum shell protected by a 5 kg coating of Prosial.
The descent module consists of a forward dome and an after cone which surround the experiment platform. A top platform completes the enclosure. The forward dome and the top platform contain a variety of ports to permit experiment sensors access to the atmosphere and to provide a means for deployment of the parachutes.
The PSE, although a part of the Huygens system, remains attached to the Cassini orbiter. Its purpose is to support the probe and provide power to the probe prior to separation and to provide communications between the probe and orbiter both prior to and after separation. It also provides the spin given to the probe during the separation process.
Power for the Huygens probe after separation is provided with five LiSO2 batteries capable of storing 1600 W-h of energy and can supply about 250 W of power for the planned three hours of probe operation. For thermal control, the probe uses multiple layers of insulation and about 35 W of radioisotope heater units. A power conditioning distribution unit (PCDU) handles the distribution and conversion of orbiter energy and probe battery energy to all experiments and sub-systems of the probe. It also provides arming and firing functions of pyro lines. Prior to separation, all power to the probe is provided by the Cassini orbiter.
Probe events are controlled via both software and hard-wired sequences, including a triply-redundant wake-up timer and a g-switch to detect deceleration of the probe by Titan's atmosphere. Redundant radar altimeters measure altitude from 20 km down, each transmitting 60 mW of power at 15.4 or 15.8 GHz via a 125 x 162 mm planar slot antenna.
During the cruise phase to Saturn, Huygens has been attached to Cassini and, in the main, dormant. Periodic checks of its status have been carried out and its health remains solid.
The first two orbits around Saturn were designed to set up the necessary trajectory for deployment of the Huygens probe on the third orbit. A maneuver took place in December 2004 that placed the paired spacecraft on an intersect course with Titan. The probe was released from the orbiter at 02:00 UTC on December 25, 2004. The two spacecraft separated with a relative velocity of 0.3-0.4 m/s but will remain in the same orbit for about three weeks. At that time, Cassini will execute a deflection maneuver to enable it to fly by Titan at an altitude of 60,000 km and positioning it to receive transmissions from Huygens as it enters Titan's atmosphere, some 2.1 hours prior to Cassini's closest approach. Entry into the atmosphere is expected to occur on January 14, 2005.
The aeroshell surrounding the descent module will decelerate it from 6 km/s at arrival to 400 m/s in about two minutes. A parachute will then be deployed and the aeroshell jettisoned. The probe will float down through the atmosphere making measurements. A swivel on the parachute harness enables the module to spin during descent to provide scan for its camera. Beginning at an altitude of around 40 km, about two hours after entry, the parachute will be released and the probe will free-fall through the lower atmosphere. The duration of the parachute descent is estimated at 120-150 minutes with 3-30 minutes of operation on the surface of Titan, depending on battery power.
The total data return from the probe is anticipated to be 500 MB.