Huygen's descent to Titan's surface

During the journey to Saturn, the probe remained clamped to the orbiter by pyrotechnic release bolts. These explosive bolts were "fired" during the approach to Titan, releasing the probe from the orbiter under the action of three springs, each exerting a force of 500 newtons (112 lbs). A curved track and roller system ensured that the probe spun up to approximately 7 revolutions per minute, or rpm, (for stabilization) and that it left the orbiter with a relative velocity of 0.3 meter per second (about 0.6 mph).

The probe was released from the orbiter on Dec. 24, 2004 PST and was targeted for a 10 degrees South latitude landing site on the "day" side of Titan. The probe entered Titan's atmosphere on Jan. 14, 2005 at a velocity of 6.1 km per second (13,725 mph). The entry phase lasted about 3 minutes, during which the probe's velocity fell to about 400 meters per second (about 895 mph).

Three parachutes were used during the probe's descent. When the onboard accelerometers detected a speed of Mach 1.5 near the end of the deceleration phase, the 2-meter (6.6-foot)-diameter pilot parachute deployed, pulling off the aft cover. This was followed immediately by deployment of the 8.3-meter (27 foot) main parachute. The pilot parachute's ejection device, which induced a load of 13,500 newtons (1.5 tons) on the probe's upper platform for about 5 milliseconds, was a primary "driver" of the probe structural design.

Artists concept: Huygens probe at Titan

About 30 seconds after deployment of the main chute, the probe's velocity dropped from Mach 1.5 to Mach 0.6. The front heat shield was then released, and the probe descended slowly below the main parachute for about 15 minutes while initial scientific measurements were made. The main parachute then separated from the probe and released a smaller 3-meter (9.8 foot) drogue parachute, which allowed the probe to descend faster, and ensured it would reach the surface before the batteries expired. It arrived at the surface in 2.5 hours, with an impact velocity of about 7 meters per second (15 mph).

The probe's entry into Titan's atmosphere -- which is mostly nitrogen with some methane -- caused a shock wave to form in front of the 2.7-meter (8.9-foot)-diameter front heat shield. The plasma in the shock, just forward of the shield, reached a temperature of around 12,000 degrees Celsius (21,632 degrees Fahrenheit). Simultaneously, the deceleration force on the probe reached its maximum of around 16 Gs. The high temperature and deceleration pressure were design drivers for most of the probe structure. The outer shell of the probe was able to withstand the extreme cold (-200 degrees Celsius or -392 degrees Fahrenheit) of Titan's atmosphere without buckling.

Inner Structure Subsystem:

The inner structure of the probe consisted of two aluminum honeycomb platforms and an aluminum shell. It was linked to the front heat shield and the aft cover by fiberglass struts and pyrotechnically operated release mechanisms. The central equipment platform carried, on both its upper and lower surfaces, the boxes containing the electrical subsystems and the science experiments. The upper platform carried the parachute (when stowed) and the antennas needed for communication with the orbiter.

Thermal Control Subsystem:

Huygen's Probe with cover

At different times during the mission the probe was subjected to extreme thermal environments requiring a variety of passive controls to maintain the required temperature conditions. For instance, during the two Venus flybys the solar heat input was very high. The probe gained some protection from the shadow of the high-gain antenna, or HGA, and when the orbiter (and thus the HGA) was off Sun-point for maneuvers or communication, the probe was protected by multi-layer insulation that burned off during the later atmospheric entry.

The probe was at its coldest just after it separated from the orbiter. To ensure that none of the equipment fell below its storage-temperature limits, the probe carried a number of radioisotope (i.e., radioactive) heater units that generated 1 watt (thermal) each.

As described above under the Entry Subsystem, the front heat shield protected the probe during initial atmospheric entry. The front shield was covered with Space Shuttle-like tiles made of a material known as AQ60, developed by Aerospatiale. This material acted essentially as a low-density "mat" of silica fibers. The tile thickness on the front shield was calculated to ensure that the structure would not exceed 150 degrees Celsius (302 Fahrenheit), which is below the melting temperature of lead. The rear side of the probe reached much lower temperatures, so a spray-on layer of "Prosial" silica foam material was used on the rear shield. The overall mass of the Thermal Protection System was more than 100 kilograms (220 pounds), or almost one third of the probe's entire mass.

Electrical Power Subsystem:

During probe checkout activities, the probe obtained power from the orbiter via the umbilical cable. After separation, the orbiter continued to supply power to the probe support equipment, but power for the probe itself was provided by five lithium sulphur-dioxide (LiSO2) batteries. Some of the battery power was used to power the timer for the 20 days of "coasting" to Titan. The higher current needed for probe mission operations was only required for the descent duration of 2.5 hours. The Electrical Power Subsystem was designed to survive the loss of one of its batteries and still support a complete mission.

Command and Data Management Subsystem (CDMS):

This subsystem provided the monitoring and control of all probe subsystem and payload (i.e., science instrument) activities. Specifically, the CDMS performed the following functions:

• Timed the 20-day "coast" phase to Titan and switched the probe "on" just prior to atmospheric entry.
• Controlled the activation of deployment mechanisms during the descent to Titan's surface.
• Distributed telecommands to the engineering subsystems and science instruments.
• Distributed to the science instruments a Descent Data Broadcast providing a timeline of conditions on which the instruments based the scheduling of mode changes and other operations.
• Collected scientific and housekeeping data and forwarded the data to the orbiter via the umbilical cable (during the cruise phase) or the Probe Data Relay Subsystem (during descent).

Probe Data Relay Subsystem (PDRS):

The PDRS provided the one-way probe-to-orbiter communications link and included equipment on both the probe and the orbiter. The elements that were part of the probe support equipment (i.e., on the orbiter) were the Probe System Avionics, or PSA, and the Radio Frequency Electronics, or RFE, the latter included an Ultra-Stable Oscillator (USO) and a low-noise amplifier. The probe carried two redundant S-band transmitters, each with its own antenna. The telemetry in one link was delayed by about six seconds with respect to the other link in order to avoid data loss if there were brief transmission outages. Reacquisition of the probe signal would normally occur within this interval.