Huygens ( ) was an atmospheric entry robotic space probe that landed successfully on Saturn's moon Titan in 2005. Built and operated by the European Space Agency (ESA), and launched by NASA, it was part of the Cassini–Huygens mission and became the first spacecraft to land on Titan and the farthest landing from Earth a spacecraft has ever made. The probe was named after the 17th-century Dutch astronomer Christiaan Huygens, who discovered Titan in 1655.
The combined Cassini–Huygens spacecraft was launched from Earth on 15 October 1997. Huygens separated from the Cassini orbiter on 25 December 2004, and landed on Titan on 14 January 2005 near the Adiri region. Huygenss landing is so far the only one accomplished in the outer Solar System and on a moon other than Earth's.
Huygens 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.
Overview Huygens was designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory 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 thought that analysis of data from Cassini would help to answer these questions.
Based on pictures taken by Cassini above Titan, the landing site appeared to be a shoreline. Assuming the landing site could be non-solid, Huygens was designed to survive the impact, splash down on a liquid surface on Titan, and send back data for several minutes under these conditions. 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 used during the descent. Engineers expected to get at most only 30 minutes of data from the surface.
The Huygens probe system consists of the probe itself, which descended to Titan, and the probe support equipment (PSE), which remained attached to the orbiting spacecraft. Huygens heat shield was in diameter. After ejecting the shield, the probe was 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 where it was transmitted or "downlinked" to the Earth.
The probe remained dormant throughout the 6.7-year interplanetary cruise, except for semiannual 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.
Prior to the probe's separation from the orbiter on 25 December 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 three hours, including the descent phase, and the first thirty minutes after touchdown. Not long after the end of this three-hour communication window, Cassinis 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 14 January, the Robert C. Byrd Green Bank Telescope (GBT) in West Virginia detected the carrier signal from Huygens. 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 position of Huygens's landing site on Titan was found with precision (within one km — one km on Titan measures 1.3 arcminutes of latitude and longitude at the equator) using the Doppler data at a distance from Earth of about . The probe landed on the surface of the moon at . A similar technique was used to determine the landing site of the Mars exploration rovers by listening to their telemetry alone.
Findings Huygens landed at around 12:43 UTC on January 14, 2005 with an impact speed similar to dropping a ball on Earth from a height of about . It made a dent deep, before bouncing onto a flat surface, and sliding across the surface. It slowed due to friction with the surface and, upon coming to its final resting place, wobbled back and forth five times. Huygens sensors continued to detect small vibrations for another two seconds, until motion subsided about ten seconds after touchdown. The probe kicked up a cloud of dust (most likely organic aerosols which drizzle out of the atmosphere) which remained suspended in the atmosphere for about four seconds by the impact.
At the landing site there were indications of pebbles of water ice scattered over an orange surface, the majority of which is covered by a thin haze of methane. Early aerial imaging of Titan from Huygens was consistent with the presence of large bodies of liquid on the surface. The initial photos of Titan before landing showed what appeared to be large drainage channels crossing the lighter colored mainland into a dark sea. Some of the photos suggested islands and a mist shrouded coastline. Subsequent analysis of the probe's trajectory indicated that, in fact, Huygens had landed within the dark 'sea' region in the photos. The photos from the surface of a dry lakebed-like landscape suggest that while there is evidence of liquid acting on the surface recently, hydrocarbon lakes and/or seas might not currently exist at the Huygens landing site. Further data from the Cassini Mission, however, definitely confirmed the existence of permanent liquid hydrocarbon lakes in the polar regions of Titan (see Lakes of Titan). Long-standing tropical hydrocarbon lakes were also discovered in 2012 (including one not far from the Huygens landing site in the Shangri-La region which is about half the size of Utah's Great Salt Lake, with a depth of at least ). The likely supplier in dry desert areas is probably underground aquifers; in other words, the arid equatorial regions of Titan contain "oases".
The surface was initially 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 (that is, a hard surface covering a sticky mud-like subsurface). Subsequent analysis of the data suggests that surface consistency readings were likely caused by Huygens pushing a large pebble into the ground as it landed, and that the surface is better described as a "sand" made of ice grains or snow that has been frozen on top. The images taken after the probe's landing show a flat plain covered in pebbles. The pebbles, which may be made of hydrocarbon-coated water ice, are somewhat rounded, which may indicate the action of fluids on them. The rocks appear to be rounded, size-selected and size-layered as though located in the bed of a stream within a dark lakebed, which consists of finer-grained material. No pebbles larger than across were spotted, while rocks smaller than are rare on the Huygens landing site. This implies large pebbles cannot be transported to the lakebed, while small rocks are quickly removed from the surface.
The temperature at the landing site was and pressure of , implying a methane abundance of 5 ± 1% and methane relative humidity of 50% near the surface. Therefore, ground fogs caused by methane in the neighborhood of the landing site are unlikely. Thermometers indicated that heat left Huygens so quickly that the ground must have been damp, and one image shows light reflected by a dewdrop as it falls across the camera's field of view. On Titan, the feeble sunlight allows only about one centimetre of evaporation per year (versus one metre of water on Earth), but the atmosphere can hold the equivalent of about of liquid before rain forms vs. only a few centimeters on Earth. So Titan's weather is expected to feature torrential downpours causing flash floods, interspersed by decades or centuries of drought.
Huygens found the brightness of the surface of Titan (at time of landing) to be about one thousand times dimmer than full solar illumination on Earth (or 500 times brighter than illumination by full moonlight)—that is, the illumination level experienced about ten minutes after sunset on Earth, approximately late civil twilight. The color of the sky and the scene on Titan is mainly orange due to the much greater attenuation of blue light by Titan's haze relative to red light. The Sun (which was comparatively high in the sky when Huygens landed) would be visible as a small, bright spot, one tenth the size of the solar disk seen from Earth, and comparable in size and brightness to a car headlight seen from about . It casts sharp shadows, but of low contrast as 90% of the illumination comes from the sky.
Detailed Huygens activity timeline
Huygens separated from Cassini orbiter at 02:00 UTC on 25 December 2004 in Spacecraft Event Time. Huygens entered Titan's atmosphere at 10:13 UTC on 14 January 2005 in SCET, according to ESA. The probe landed on the surface of Titan at about 10.6°S, 192.3°W around 12:43 UTC in SCET (2 hours 30 minutes after atmospheric entry).(1.)
There was a transit of the Earth and Moon across the Sun as seen from Saturn/Titan just hours before the landing. Huygens entered the upper layer of Titan's atmosphere 2.7 hours after the end of the transit of the Earth, or only one or two minutes after the end of the transit of the Moon. However, the transit did not interfere with the Cassini orbiter or Huygens probe, for two reasons. First, although they could not receive any signal from Earth because it was in front of the Sun, Earth could still listen to them. Second, Huygens did not send any readable data directly to Earth. Rather, it transmitted data to the Cassini orbiter, which later relayed to Earth the data received.
Instrumentation