  
Huygens: Robotic Lander
The goals of the Huygens Probe are to make detailed in-situ measurements of the structure, composition and dynamics of the atmosphere of Titan and to characterize the surface of the satellite near the probe's landing site..
Released from the Saturn Orbiter during its first orbit of Saturn, the probe will take 22 days to reach Titan, arriving on November 27, 2004. The probe has no propulsion system and will literally "coast" to its encounter with Titan. The descent through the atmosphere consists of several phases and will take approximately 140 minutes. Images and other remote sensing measurements of the surface will be made during the descent through the atmosphere and from the surface of Titan following impact.
There will be no scientific measurements by the probe prior to its encounter with Titan and for most of the cruise to Saturn, the probe will be switched off. During the 22-day Coast Phase, after separation from the Orbiter, only a timer will operate to activate the probe shortly before predicted entry into Titan's atmosphere. The programming of this time and the activation of the probe's batteries will be the last activities initiated by command from the ground.
Entry Phase
The entry phase begins at 217 miles (350 km) above the surface with the probe traveling at a speed of 13,422 mph (21,596 km/h). A heat shield protects the probe during the descent to an altitude of 137 miles (220 km) during which time the probe decelerates from 13,422 mph (21,596 km/h) to 895 mph (1,440 km/h) undergoing a maximum 16 g force.
Parachute Deployment
At 895 mph (1,440 km/h), the parachute-deployment sequence is initiated, starting with the ejection of a mortar pulling out a pilot chute which itself pulls away the back cover. After inflation of the 26.2 ft (8 m) diameter main parachute, the front heat shield is released forward so that it falls away beneath the descent module. After a delay of 30 seconds, built into the sequence to ensure that the shield is sufficiently far from the descent module to avoid instrument contamination, the inlet ports are opened and the HASI (Huygens Atmospheric Structure Instrument) booms are deployed. The main parachute is sized to pull the descent module safely out of the front heat shield and then it too is jettisoned to avoid too long a descent time. A 8.2 ft. (2.5 m) diameter stabilizer parachute is deployed for the final descent to the surface. It is anticipated that Huygens will strike Titan at a speed of 11 to 15 mph (18 to 25 km/h).
On the Surface of Titan
Huygens is designed to make in-situ measurements for a direct characterization of the surface at the landing site. The probability of the probe's survival and the length of time that it may operate on the surface cannot be estimated since there are too many unknowns. Science measurements will be programmed to go on for at least 30 minutes after impact.
The Surface-Science Package (SSP) is a suite of sensors designed to determine the physical characteristics of the surface at the impact site and to provide information about its composition. The package includes an accelerometer to measure the impact deceleration, and other sensors to measure the index of refraction, the temperature, the thermal conductivity, the heat capacity, the speed of sound and the dielectric constant of the (liquid) material at the impact site.
The Orbiter will listen to Huygens during its 2.5 hour descent phase and for 30 minutes following impact before the high-gain antenna will be turned away from Titan and pointed back at Earth to transmit the data gathered by the probe.
Probe Design
The probe weighs 776 lb. (352 kg) which includes 95 lb. (43 kg) of science experiments and 106 lb. (48 kg) of support equipment on Orbiter. The descent module consists of a heat shield, its support structure, which forms the outer envelope of the probe, the equipment and antenna platforms and the aft cover. Following release from the Orbiter, Huygens is powered by lithium sulfur-dioxide primary batteries which will allow it to transmit data during the 2.5 hour descent phase and up to 1/2 hour on the surface of Titan.
The outer envelope of the probe payload and equipment compartment consists of two conical cylinders which are connected at their outer diameter, closed at the lower end by a thin inner nose cap and at the upper end by the antenna platform. The lower cone interfaces with the equipment platform, where the supporting equipment and scientific payload are located. The cones have cutouts. These ports are closed by ejectable covers, which are jettisoned at the start of the descent phase.
The aft cover is made from heat-resisting material with RF-transparent ports over the antenna dipoles. There is an interface at the center of the dome with the spin-eject device, and the cover is stiffened by ribs to support the ejection and spin-up loads that it will experience during the probe's release from the orbiter.
The heat shield is designed to protect the probe during entry from an altitude of 217 miles (350 km) to 137 miles (220 km) where the probe decelerates from 13,422 mph (21,596 km/h) to 895 mph (1,440 km/h) in less than two minutes. The heat shield is made up of a 4 to 5 mm thick beryllium nose cap, which acts as heat sink during the probe entry phase. It is isolated thermally from the probe's interior compartment. Sections of the nose cap can also be ejected during the descent phase to free the inlets and viewing ports for operation of scientific instruments and their detectors.
At 895 mph (1,440 km/h), the parachute-deployment sequence is initiated. The probe's deployable aerodynamic braking system is used only in the entry phase of the mission to achieve subsonic speed at a high altitude which will allow atmospheric measurement to begin as soon as it enters the atmosphere.
The deployed decelerator looks like a frustum of a cone, with 60 degree half-cone angle and an outer diameter of 9.8 ft (3.3 m). The inner 4.9 ft (1.5 m) diameter has a ring interface with the descent module, which can be removed pyrotechnically when subsonic speed is achieved. The deceleration structure consists of 16 deployable ribs, which support a high-heat-resisting carbon cloth, shortly after the probe's release from the orbiter.
The equipment platform carrying all scientific instrumentation and subsystem equipment is located as low as possible in the outer envelopment probe to keep the overall center of gravity low for added stability. All instrument packages and the communication and data-handling subsystems are located on the top side of the platform; the power subsystem on the bottom. The platform itself consists of an aluminum honeycomb sandwich connected by six struts to the antenna platform. These struts serve as the primary load-carrying structure of the chute-deployment and entry loads.
For the latest developments and news on the mission visit:
NASA - National Aeronautics and Space Administration
ESA - European Space Agency
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