Artists Concept: Huygens probe at Titan

The European Space Agency's Huygens probe ushered in 2005 with its landmark mission at Titan. After a seven-year journey bolted to the side of the Cassini Orbiter, Huygens was set free on Dec. 25, 2004. The Probe coasted for 21 days en route to Titan.

Probe Separation and Transit to Titan
Prior to the probe's separation from the orbiter, the triplicate "coast" timer, or Mission Timer Unit (MTU) was loaded with the precise time necessary to turn on the probe systems (about 4 hours before the initial encounter with Titan's atmosphere). Then the probe separated flawlessly from the orbiter. Cassini turned and imaged Huygens repeatedly as it set out on its 21-day coast to Titan, with no systems active except for its wake-up timer.

Huygens separated from Cassini at 30 centimeters (about 12 inches) per second and a spin rate of seven revolutions per minute to ensure stability during the coast and entry phase. This rate was confirmed by the Magnetometer instrument on the Cassini Orbiter. Five days following the release of the probe, Cassini performed a deflection maneuver. This placed the orbiter on the proper trajectory - missing Titan instead of impacting - to collect Huygens' data during the probe mission.

Titan's nitrogen-rich atmosphere extends 10 times further into space than Earth's atmosphere. This means the outer fringes of Titan's atmosphere reach almost 600 kilometers (almost 400 miles) into space. When the probe detected this region of Titan's atmosphere, the deceleration set off a sequence of events leading to its perfect parachute descent.

Huygens was equipped with six science instruments designed to study the content and dynamics of Titan's atmosphere and collect data and images on the surface.

Artists Concept: Huygens probe separating to enter Titan's atmosphere

Descent Through Titan's Atmosphere
Huygens made a parachute-assisted descent through Titan's atmosphere, collecting data as the parachutes slowed the probe from super sonic speeds. Five batteries onboard the probe were originally sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus a half hour or more on Titan's surface. In fact, they lasted much longer than that. These batteries were capable of generating a total of 1800 Watt-hours of electrical power.

The probe's radio link was activated early in the descent phase, during time the orbiter was flying overhead, "listening" for the probe. Not long after the end of this four-hour communications window, Cassini's high-gain antenna (HGA) turned away from Titan and pointed toward Earth.

Huygens Probe Descent Profile

Mission Timeline Facts All times are given in Spacecraft Event Time (SCET), Universal Coordinated Time (UTC) Probe Release: December 25, 2004 02:00 UTC Probe entry at Titan: January 14, 2005 11:04 UTC Speed at Entry: 6 kilometers per second Impact Speed: 5 meters per second Mission Duration: 2 hr, 27 min, 13 sec descent, plus 1 hr, 12 min, 9 sec surface Altitude of Cassini during the Huygens Mission: 60,000 kilometers Data rate to Cassini Orbiter: 8 kilobits per second Total Battery Capacity of Probe = 1800 Watt-hours

The peak heat-flux was expected in the altitude range below 350 kilometers (217 miles) down to 220 kilometers (137 miles), where Huygens rapidly decelerated from about 21,600 kilometers (13,424 miles) per hour to 1,440 kilometers (895 miles) per hour in less than two minutes.

At this speed, the parachute deployment sequence initiated, starting with a mortar pulling out a Pilot Parachute which, in turn, pulled away the aft cover and deployed the Main Parachute. After inflation of the 8.3 meter (27.2 foot) diameter main parachute, the front shield was released to fall from the Descent Module. Then, after a 30 second delay built into the sequence to ensure that the shield was sufficiently far away to avoid instrument contamination, the Gas Chromatograph Mass Spectrometer (GCMS) and Aerosol Collector and Pyrolyser (ACP) inlet ports opened and the Huygens Atmospheric Structure Instrument (HASI) boom deployed. The Descent Imager/Spectral Radiometer (DISR) cover was ejected two minutes later.

The main parachute was sized to pull the Descent Module safely out of the front shield. It was jettisoned after 15 minutes to avoid a protracted descent, and a smaller 3-meter (10-foot) diameter parachute was deployed. The descent lasted about two and a half hours.

During its descent, Huygens' camera returned more than 750 images, while the Probe's other five instruments sampled Titan's atmosphere to help determine its composition and structure. Huygens collected 2 hours, 27 minutes, 13 seconds of descent data, and 1 hour, 12 minutes, 9 seconds of surface data, which turned out to be far more surface data than was ever expected.

Every bit of data from Huygens was successfully relayed to the Cassini Orbiter passing overhead, with the exception of a redundant stream called "Chain A." Chain A's radio frequency was based on Huygens' Ultra Stable Oscillator, designed for the Doppler Wind Experiment. While this signal was not received aboard Cassini, it was received on Earth, thanks to Radio Scientists using Earth-based radio telescopes at Green Bank and Parkes. They were actually able to capture the tiny signal, now being called "Chain C," directly from Titan! Telemetry data from Huygens was 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.