On December 24th, 2004, at 7:08 PM Pacific Standard Time, the Cassini spacecraft will release a probe that has hitched a ride all the way from Earth out to Saturn. The Huygens Probe, named after the Dutch scientist Christiaan Huygens, who discovered Titan and Saturn's rings in the 17th century and built by the European Space Agency, will spend 22 days traveling to its rendezvous with Saturn's mysterious moon Titan on January 14th. Titan is one of the remaining puzzles of the solar system - while Cassini's imaging cameras and radar instrument have begun to reveal the details of its surface, the Huygens probe will be the first spacecraft to venture beneath Titan's thick clouds

Since the Huygens probe doesn't have its own means of propulsion, its release point will be carefully timed so that its trajectory will take it right to Titan. The Cassini spacecraft will change course slightly so that it flies past Titan, but doesn't impact it. Once Huygens is released, it will be in a coasting configuration, with its scientific instruments turned off except for a timer that will wake up the spacecraft just before it reaches Titan.

Huygens' first encounter with Titan will be when it reaches the top of Titan's atmosphere. Due to Titan's low gravity, its atmosphere is ten times deeper than Earth's - the outer limits are at 600 km above Titan's surface! The probe will wake up when it first detects the outer fringes of atmosphere. As Huygens descends on its parachutes through Titan's thick atmosphere, it will use a group of six instruments to study the mysterious moon. Huygens has enough battery power to survive for between 2.5 and three hours, which includes a descent time to the surface between 2 and 2.5 hours, plus at least a few minutes (and up to half an hour) of operating time on the surface, assuming it survives the landing.

The descent through Titan's atmosphere is carefully choreographed, with a series of parachutes deploying at certain intervals to slow the probe's downward trajectory. Once the velocity is slow enough and temperatures are safe, the heat shield at the bottom of the spacecraft will be jettisoned and the scientific instruments it was protecting can begin to take data. These instruments include the Gas Chromatograph Mass Spectrometer (GCMS) and the Aerosol Collector and Pyrolyser (ACP), both of which will measure the composition of Titan's atmosphere.

The Huygens Atmospheric Structure Instrument (HASI) will deploy on a boom to measure the structure of Titan's atmosphere, including its density, pressure, and temperature at various altitudes, and the Descent Imager / Spectral Radiometer (DISR) camera will begin taking panoramic images as the spacecraft spins on its parachute. DISR is planned to take more than 1100 images during the descent. Shortly before landing, DISR will switch on a special lamp to measure the reflection spectrum of Titan's surface, to help determine its chemical composition. Also during Huygens' descent, the Doppler Wind Experiment (DWE) will use the shifts in the radio signals sent from Huygens back to Cassini to determine the wind direction and magnitude at various points in Titan's atmosphere.

Once Huygens reaches Titan's surface, the Surface Science Package (SSP) will make surface measurements with nine different sensors. Since we do not even know if Huygens will land on a solid or liquid surface, the sensors are designed to take measurements of a wide variety of materials, including both solid and fluid properties. These measurements include such basic physical properties as conductivity, thermal properties, acoustic properties, fluid permittivity, fluid density, and refractive index.

The Cassini spacecraft has a three-hour window to listen to the signals transmitted from Huygens; after that time has elapsed, Cassini's trajectory will have carried it out of sight of the probe (and the probe's battery will be used up soon after). Cassini will then turn its antenna toward Earth and start transmitting what it has received from Huygens. Stay tuned on January 14th and beyond for some amazing discoveries!