The Huygens probe was an atmospheric entry probe carried to Saturn’s moon Titan as part of the Cassini–Huygens mission. The probe was supplied by the European Space Agency (ESA) and named after the Dutch 17th century astronomer Christiaan Huygens.
The combined Cassini–Huygens spacecraft was launched from Earth on October 15, 1997. Huygens separated from the Cassini orbiter on December 25, 2004, and landed on Titan on January 14, 2005 near the Xanadu region. This was the first landing ever accomplished in the outer solar system. It touched down on land, although the possibility that it would touch down in an ocean was also taken into account in its design. The probe was designed to gather data for a few hours in the atmosphere, and possibly a short time at the surface. It continued to send data for about 90 minutes after touchdown. It remains the most distant landing of any craft launched from Earth.
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 flatplain, 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 toward 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 recorded signal was processed against a telemetry template, enabling signal integration over several seconds for determining 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 10.2°S, 192.4°W. A similar technique was used to determine the landing site of the Mars exploration rovers by listening to their telemetry alone.
Early imaging of Titan from the Cassini mission was consistent with the presence of large bodies of liquid on the surface. The photos showed what appeared to be large drainage channels crossing the lighter coloured mainland into a dark sea. Some of the photos suggested islands and mist shrouded coastline. On January 18 it was reported that Huygens landed in “Titanian mud”, and the landing site was estimated to lie within the white circle on the picture to the left. Mission scientists also reported a first “descent profile”, which describes the trajectory the probe took during its descent. Subsequent work done on the probe’s trajectory indicated that, in fact, it landed within the dark ‘sea’ region in the photos. Photos of a dry landscape from the surface suggested that while there was evidence of liquid acting on the surface recently, hydrocarbon lakes and/or seas might not be present on Titan. Further data from the Cassini Mission, however, definitely confirmed the existence of liquid hydrocarbon lakes in the polar regions of Titan.
At the landing site there were indications of chunks of water ice scattered over an orange surface, the majority of which is covered by a thin haze of methane. The instruments revealed “a dense cloud or thick haze approximately 18-20 kilometers from the surface”. The surface itself was reported to be a clay-like “material which might have a thin crust followed by a region of relative uniform consistency.” One ESA scientist compared the texture and colour of Titan’s surface to a crème brûlée, but admitted this term probably would not appear in the published papers.
Subsequent analysis of the data suggests that surface consistency readings were likely caused by Huygens displacing a large pebble as it landed, and that the surface is better described as a “sand” made of ice grains. The images taken after the probe’s landing show a flat plain covered in pebbles. The pebbles, which may be made of water ice, are somewhat rounded, which may indicate the action of fluids on them.