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The Huygens Probe was named after Christiaan Huygens, a Dutch astronomer who in 1655 discovered Titan, Saturn's largest moon. The probe was designed by the European Space Agency (ESA), to perform an in-depth study of the clouds, atmosphere, and surface of Titan.

The Huygens probe will be plunging into a planetary atmosphere farther away from Earth than any other deep space probe has gone before.

Artist rendition of Huygens Probe landing on Titan.

Traveling onboard the Cassini orbiter throughout the seven-year journey to Saturn, the probe will undergo a series of in-flight tests and health checks to ensure that all of its instruments are working properly. This is essential, because the distance from Earth is too great to provide signals and commands. This means that the programming of the probe must be precise and work automatically so that valuable data can be communicated back to the orbiter and then back to Earth.

The 319-kilogram (703-pound) Huygens probe will separate from the Cassini orbiter in December of 2004, and will begin a 22-day coast phase toward Titan. Remaining on the Cassini orbiter will be the probe support equipment (PSE), which includes the electronics necessary to track the probe and to recover the data gathered during its descent. Then, in January of 2005, just 45 minutes before reaching the atmosphere of Titan, timers will wake up the Huygens probe.

As it finally enters Titan's atmosphere, three sets of parachutes will slow down the probe and provide a stable platform for scientific measurements. The fully instrumented robotic laboratory will reach the mysterious Titan's surface about two and half hours later.

The Huygens probe will usher in 2005 with its landmark mission at Titan. After a 7 year journey strapped to the side of the Cassini Orbiter, Huygens will be set free on December 25, 2004. The Probe will coast for 22 days enroute to Titan.

Probe Separation and Transit to Titan

Prior to the probe's separation from the orbiter, the "coast" timer will be loaded with the precise time necessary to turn on the probe systems (15 minutes before the initial encounter with Titan's atmosphere). Then the probe will separate from the orbiter and coast to Titan for 22 days with no systems active except for its wake-up timer. Huygens will separate from Cassini at 30 centimeters per second and a spin rate of 7 revolutions per minute to ensure stability during the coast and entry phase. 5 days following Probe release, Cassini will perform a deflection maneuver. This will place the Orbiter in the proper geometry to collect the data during the Probe Mission. The probe will continue in this mode until it reaches the top of Titan's atmosphere.

Titan's nitrogen-rich atmosphere extends ten times further into space than Earth's atmosphere. This means the outer fringes of Titan's atmosphere reach almost 600 kilometers into space. When the probe detects this region of Titan's atmosphere, the sleep timer will go off, awakening the probe's science instruments.

Huygens is equipped with 6 science instruments designed to study the content and dynamics of Titan's atmosphere and collect data and images on the surface. Information on the probe's science instruments and their objectives can be found in the spacecraft section.

Descent Through Titan's Atmosphere

Huygens will make a parachute-assisted descent through Titan's atmosphere, collecting data as the parachutes slow the probe from super sonic speeds. 5 batteries onboard the probe are sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus at least 3 additional minutes (and possibly a half hour or more) on Titan's surface. These batteries are capable of generating 1800 Watt-hours of electricity.

The probe's radio link will be activated early in the descent phase, and the orbiter will "listen" to the probe for the next 3 hours, which includes the descent plus 30 minutes after impact. Not long after the end of this three-hour communication window, Cassini's high-gain antenna (HGA) will be turned away from Titan and toward Earth.

The peak heat -flux is expected in the altitude range below 350 kilometers down to 220 kilometers, where Huygens rapidly decelerates from about 6 kilometers per second to 400 meters per second (Mach 1.5) in less than 2 minutes. At this speed, the parachute deployment sequence initiates, starting with a mortar pulling out a pilot chute which, in turn, pulls away the aft cover and deploys the main chute. After inflation of the 8.3 meter diameter main parachute, the front shield is released to fall from the Descent Module. Then, after a 30 second delay built into the sequence to ensure that the shield is sufficiently far away to avoid instrument contamination, the Gas Chromatograph Mass Spectrometer (GCMS) and Aerosol Collector and Pyrolyser (ACP) inlet ports open and the Huygens Atmospheric Structure Instrument (HASI) boom deploy. The Descent Imager/Spectral Radiometer (DISR) cover is ejected 2 minutes later. The main parachute is sized to pull the Descent Module safely out of the front shield. It is jettisoned after 15 minutes to avoid a protracted descent and a smaller 3.03 meter diameter parachute is deployed. The descent will last between 2 hours and 2 hours 30 minutes.

During its descent, Huygens' camera will capture more than 1,100 images, while the Probe's other 5 instruments will directly sample Titan's atmosphere and determine its composition.

Data from Huygens will be relayed to the Cassini Orbiter passing overhead. The data will be stored onboard Cassini's solid state recorders (SSR) for playback to Earth.

Huygens is managed by the European Space Agency. Complete details on the mission objectives and science can be found on the ESA Huygens Site.

Source: NASA