Overview
Huygens was designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface. When the mission was planned, it was not yet certain whether the landing site would be a mountain range, a flat plain, an ocean, or something else, and it was hoped that analysis of data from Cassini would help to answer these questions.
Based on pictures taken by Cassini at 1,200 km away from Titan, the landing site appeared to be, for lack of a better word, shoreline. Assuming the landing site could be non-solid, the Huygens probe was designed to survive the impact and splash-down on a liquid surface on Titan and send back data for several minutes on the conditions there. If that occurred it was expected to be the first time a human-made probe would land in an extraterrestrial ocean. The spacecraft had no more than three hours of battery life, most of which was planned to be taken up by the descent. Engineers only expected to get at best 30 minutes of data from the surface.
The Huygens probe system consists of the 318 kg probe itself, which descended to Titan, and the probe support equipment (PSE), which remained attached to the orbiting spacecraft. Huygens' heat shield was 2.7 m in diameter; after ejecting the shield, the probe was 1.3 m in diameter. The PSE included the electronics necessary to track the probe, to recover the data gathered during its descent, and to process and deliver the data to the orbiter, from which it transmitted or "downlinked" to the ground.
The probe remained dormant throughout the 6.7-year interplanetary cruise, except for bi-annual health checks. These checkouts followed preprogrammed descent scenario sequences as closely as possible, and the results were relayed to Earth for examination by system and payload experts. Navigation to Saturn, and specifically to Titan, was a very complicated process in and of itself, and was coordinated by the Jet Propulsion Laboratory (NASA JPL), with astrometric navigation frames provided by various institutions such as the United States Naval Observatory Flagstaff Station.
Prior to the probe's separation from the orbiter on December 25, 2004, a final health check was performed. The "coast" timer was loaded with the precise time necessary to turn on the probe systems (15 minutes before its encounter with Titan's atmosphere), then the probe detached from the orbiter and coasted in free space to Titan in 22 days with no systems active except for its wake-up timer.
The main mission phase was a parachute descent through Titan's atmosphere. The batteries and all other resources were 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. The probe's radio link was activated early in the descent phase, and the orbiter "listened" to the probe for the next 3 hours, including the descent phase, and the first thirty minutes after touchdown. Not long after the end of this three-hour communication window, Cassini's high-gain antenna (HGA) was turned away from Titan and towards Earth.
Very large radio telescopes on Earth were also listening to Huygens' 10-watt transmission using the technique of very long baseline interferometry and aperture synthesis mode. At 11:25 CET on January 14, the Robert C. Byrd Green Bank Telescope (GBT) in West Virginia detected the carrier signal from the Huygens probe. The GBT continued to detect the carrier signal well after Cassini stopped listening to the incoming data stream. In addition to the GBT, eight of the ten telescopes of the continent-wide VLBA in North America, located at Pie Town and Los Alamos, New Mexico; Fort Davis, Texas; North Liberty, Iowa; Kitt Peak, Arizona; Brewster, Washington; Owens Valley, California; and Mauna Kea, Hawaii, also listened for the Huygens signal.
The signal strength received on Earth from Huygens was comparable to that from the Galileo probe (the Jupiter atmospheric descent probe) as received by the VLA, and was therefore too weak to detect in real time because of the signal modulation by the (then) unknown telemetry. Instead, wide-band recordings of the probe signal were made throughout the three-hour descent. After the probe telemetry was finished being relayed from Cassini to Earth, the now-known data modulation was stripped off the recorded signal, leaving a pure carrier that could be integrated over several seconds to determine the probe frequency. It was expected that through analysis of the Doppler shifting of Huygens' signal as it descended through the atmosphere of Titan, wind speed and direction could be determined with some degree of accuracy. A determination of Huygens' landing site on Titan was found with exquisite precision (within one km – one km on Titan measures 1.3' latitude and longitude at the equator) using the Doppler data at a distance from Earth of about 1.2 billion kilometers. The probe landed on the surface of the moon at 10°12′S 192°24′W / 10.2°S 192.4°W / -10.2; -192.4. A similar technique was used to determine the landing site of the Mars exploration rovers by listening to their telemetry alone.
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