View source for Huygens (spacecraft)

{{Use mdy dates|date=May 2020}}
{{DISPLAYTITLE: ''Huygens'' (spacecraft)}}
{{Short description|European reconnaissance lander sent to Saturn's moon Titan}}
{{Infobox spaceflight
| name = ''Huygens''

<!--image of the spacecraft/mission-->| image = [[Image:Huygens probe model.jpg|300px]]
| image_caption = A full-size replica of the ''Huygens'' probe, {{convert|1.3|metres|feet|abbr=off}} across
| image_size = 300px

<!--Basic details-->
| mission_type = [[Titan (moon)|Titan]] lander
| operator = [[European Space Agency|ESA]]{{\}}[[Italian Space Agency|ASI]]{{\}}[[NASA]]
| COSPAR_ID = 1997-061C
| SATCAT = 
| website = [http://www.esa.int/SPECIALS/Cassini-Huygens/ ''Huygens'' home page]
| mission_duration = <!--How long the mission lasted-->

<!--Spacecraft properties-->
| manufacturer = Thales Alenia Space (then [[Aérospatiale]])<ref>{{cite web|url=https://sci.esa.int/web/cassini-huygens/-/47052-huygens|title=HUYGENS}}</ref>
| BOL_mass = {{Convert|320|kg|abbr=on}}
| landing_mass = <!--Mass after landing (recovered spacecraft only)-->
| dimensions = <!--body dimensions and solar array span-->
| power = 1800 Wh total

<!--Launch details-->
| launch_date         = {{start date text| October 15, 1997, 08:42}}&nbsp;UTC
| launch_rocket       = [[Titan IV| Titan IV(401)B]]/ [[Centaur (rocket stage)|Centaur-T]] B-33
| launch_site         = [[Cape Canaveral Air Force Station|Cape Canaveral]] [[SLC-40]]
| launch_contractor   = [[Lockheed Martin]]
| deployment_from     = [[Cassini-Huygens|Cassini]]
| deployment_date     = December 25, 2004

<!--end of mission-->
| disposal_type       = <!--Whether the spacecraft was deorbited, decommissioned, placed in a graveyard orbit, etc-->
| last_contact        = {{end date|2005|01|14|13|37|7=Z}}
| landing_date        = 12:43, January 14, 2005 (UTC)
<!--
    The following template should be used for ONE of the three above fields "end_of_mission", "decay" or "landing" if the spacecraft is no longer operational.
    If it landed intact, use it for the landing time, otherwise for the date it ceased operations, or the decay date if it was still operational when it re-entered.
    {{end date|YYYY|MM|DD|hh|mm|ss|TZ=Z}} (for Zulu/UTC) or {{end date|YYYY|MM|DD}} (if time unknown) 
-->
| interplanetary = <!--Infobox spaceflight/IP can be called multiple times for missions with multiple targets or combined orbiter/lander missions, etc-->
 {{Infobox spaceflight/IP
   |type                = lander
   |object              = [[Titan (moon)|Titan]]
   |arrival_date        = 12:43, January 14, 2005 ([[Spacecraft event time|SCET]] UTC)
   |location            = {{coord|10.573|S|192.335|W|globe:Titan|name=Huygens probe}}<ref>{{cite journal|url=https://www.researchgate.net/publication/229344990|title=Titan's new pole: Implications for the Huygens entry and descent trajectory and landing coordinates|journal=Advances in Space Research|volume=47|issue=9|pages=1622–1632|first1=Bobby|last1=Kazeminejad|date=May 2011|access-date=January 4, 2018|bibcode=2011AdSpR..47.1622K|doi=10.1016/j.asr.2011.01.019}}</ref>
 }}

<!--transponder parameters-->| trans_band = <!--Transponder frequency bands-->
| trans_frequency = <!--specific frequencies-->
| trans_bandwidth = <!--bandwidth-->
| trans_capacity = <!--capacity of the transponders-->
| trans_coverage = <!--area covered-->
| trans_TWTA = <!--TWTA output power-->
| trans_EIRP = <!--equivalent isotropic power-->
| trans_HPBW = <!--half-power beam width-->
| insignia = Huygens insignia.png
| insignia_caption = ESA quadrilateral mission insignia for ''Huygens''
| insignia_alt = ''Huygens'' mission insignia
| insignia_size = 150x150px
| programme = '''[[European Space Agency Science Programme#Horizon 2000|Horizon 2000]]'''
| previous_mission = [[Cluster (spacecraft)|Cluster]]
| next_mission = [[XMM-Newton|XMM-''Newton'']]
}}

'''''Huygens''''' ({{IPAc-en|ˈ|h|ɔɪ|ɡ|ən|z}} {{respell|HOY|gənz}}) was an [[atmospheric entry]] robotic  [[space probe]] that landed successfully on [[Saturn]]'s moon [[Titan (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.<ref>{{cite news|url=https://news.bbc.co.uk/1/hi/sci/tech/4175099.stm|title=Huygens sends first Titan images|work=BBC News|first1=Paul|last1=Rincon|date=January 15, 2005|access-date=August 30, 2016}}</ref> The probe was named after the 17th-century Dutch astronomer [[Christiaan Huygens]],<ref name=Intro>{{cite web|title=Solstice Mission Overview|url=http://saturn.jpl.nasa.gov/mission/introduction|archive-url=https://web.archive.org/web/20081217142207/http://saturn.jpl.nasa.gov/mission/introduction/|url-status=dead|archive-date=December 17, 2008|publisher=NASA|access-date=January 21, 2013}}</ref> who discovered Titan in 1655.

The combined ''Cassini–Huygens'' spacecraft was launched from [[Earth]] on 15 October 1997.<ref name=Intro/> ''Huygens'' separated from the ''Cassini'' [[orbiter]] on 25 December 2004, and landed on Titan on 14 January 2005 near the [[Adiri (Titan)|Adiri region]].<ref>{{cite web |title=Cassini-Huygens |url=http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/The_mission |publisher=European Space Agency |access-date=March 22, 2019}}</ref> ''Huygens''{{'}}s landing is so far the only one accomplished in the outer [[Solar System]] and on a moon other than Earth's.<ref>{{cite web |title=Cassini-Huygens Mission Facts |url=http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Cassini-Huygens_mission_facts |publisher=European Space Agency |access-date=March 22, 2019}}</ref>

''Huygens'' touched down on land, although the possibility that it would touch down in an [[Lakes of Titan|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'' {{cvt|1200|km|mi}} 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.

[[File:Huygens cutaway.jpg|thumb|Cutaway image of ''Huygens'']]
The ''Huygens'' probe system consists of the {{cvt|318|kg|lb}} probe itself, which descended to Titan, and the {{cvt|30|kg|lb}}  probe support equipment (PSE), which remained attached to the orbiting spacecraft.  ''Huygens''{{'}} heat shield was {{cvt|2.7|m|ft}} in diameter. After ejecting the shield, the probe was {{cvt|1.3|m|ft}} 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.<ref>{{cite web|title=Cassini–Huygens Mission |url=http://huygensgcms.gsfc.nasa.gov/Mission.htm |publisher=NASA |access-date=January 30, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130219190048/http://huygensgcms.gsfc.nasa.gov/Mission.htm |archive-date=February 19, 2013 }}</ref>  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&nbsp;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&nbsp;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, ''Cassini''{{'}}s high-gain antenna (HGA) was turned away from Titan and towards Earth.

Very large [[radio telescope]]s 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 [[Very Long Baseline Array|VLBA]] in North America, located at [[Pie Town, New Mexico|Pie Town]] and [[Los Alamos, New Mexico]]; [[Fort Davis, Texas]]; [[North Liberty, Iowa]]; [[Kitt Peak National Observatory|Kitt Peak, Arizona]]; [[Brewster, Washington]]; [[Owens Valley|Owens Valley, California]]; and [[Mauna Kea|Mauna Kea, Hawaii]], also listened for the ''Huygens'' signal.<ref>{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Radio_astronomers_confirm_Huygens_entry_in_the_atmosphere_of_Titan |title=Radio astronomers confirm Huygens entry in the atmosphere of Titan |date=January 14, 2005 |website=European Space Agency |access-date=March 22, 2019 }}</ref>

The signal strength received on Earth from ''Huygens'' was comparable to that from the [[Galileo probe|''Galileo'' probe]] (the Jupiter atmospheric descent probe) as received by the [[Very Large Array|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 1.2 billion kilometers. The probe landed on the surface of the moon at {{coord|10.573|S|192.335|W|globe:Titan|name=Huygens probe}}. 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 {{cvt|1|m|ft|sigfig=1}}. It made a dent {{cvt|12|cm}} deep, before bouncing onto a flat surface, and sliding {{cvt|30|to|40|cm}} 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.<ref name=jpl>{{cite web|title=Bounce, Skid, Wobble: How Huygens Landed on Titan|work=www.jpl.nasa.gov|date=2012|url=http://www.jpl.nasa.gov/news/news.php?release=2012-317|access-date=January 19, 2015}}</ref>

[[File:Drainage channels and shoreline on Titan, by Huygens probe.jpg|thumb|left|The first image released, taken from an altitude of {{cvt|16|km|mi}}, showing what are speculated to be drainage channels flowing to a possible shoreline. The darker areas are flat plains, while the lighter areas represent high ground.]]
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 {{cvt|1|m|ft|sigfig=1}}). The likely supplier in dry desert areas is probably underground [[aquifers]]; in other words, the arid equatorial regions of Titan contain "[[oases]]".<ref name=caltech>{{cite web|title=Tropical Methane Lakes on Saturn's Moon Titan|work=saturntoday.com|date=2012|url=http://www.saturntoday.com/news/viewpr.html?pid=37429|access-date=June 16, 2012|archive-url=https://web.archive.org/web/20121103072650/http://saturntoday.com/news/viewpr.html?pid=37429|archive-date=November 3, 2012|url-status=usurped}}</ref>

{{multiple image
| align             = right
| image1            = Huygens surface color.jpg
| width1            = 200
| alt1              = 
| caption1          = 
| image2            = Huygens surface color sr.jpg
| width2            = 200
| alt2              = 
| caption2          = 
| footer            = ''In situ'' image of Titan's surface from ''Huygens'' (left and right images have different image processing). Globules (probably made of water ice) 10–15 cm in size lie above darker, finer-grained substrate in a variable spatial distribution. Brightening of the upper left side of several rocks suggests solar illumination from that direction, implying a southerly view, which agrees with preliminary evidence from other data sets. A region with a relatively low number of rocks lies between clusters of rocks in the foreground and the background and matches the general orientation of channel-like features in the low-altitude images taken from under {{cvt|7|km|mi}} altitude.
}}
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<ref>[https://news.bbc.co.uk/2/hi/science/nature/4417503.stm Titan probe's pebble 'bash-down'], BBC News, April 10, 2005.</ref> or snow that has been frozen on top.<ref name=jpl/> 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.<ref>[http://www.planetary.org/news/2005/0115_The_Planetary_Societys_Huygens_Weblog.html New Images from the Huygens Probe: Shorelines and Channels, But an Apparently Dry Surface] {{webarchive|url=https://web.archive.org/web/20070829220423/http://www.planetary.org/news/2005/0115_The_Planetary_Societys_Huygens_Weblog.html |date=August 29, 2007 }}, Emily Lakdawalla, January 15, 2005, verified March 28, 2005</ref> 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 {{cvt|15|cm|in}} across were spotted, while rocks smaller than {{cvt|5|cm|in}} 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.<ref name=nature/>

The temperature at the landing site was {{cvt|93.8|K|lk=in}} and pressure of {{cvt|1467.6|mbar|atm}}, 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.<ref name=nature/> 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 {{cvt|10|m|ft|sigfig=1}} 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.<ref>{{cite journal |doi=10.1038/scientificamerican0310-36 |title=The Moon That Would Be a Planet |date=2010 |last1=Lorenz |first1=Ralph |last2=Sotin |first2=Christophe |journal=Scientific American |volume=302 |issue=3 |pages=36–43 |pmid=20184181|bibcode=2010SciAm.302c..36L }}</ref>

''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 {{cvt|150|m|ft|sigfig=1}}. It casts sharp shadows, but of low contrast as 90% of the illumination comes from the sky.<ref name=nature>{{cite journal|title=Rain, winds and haze during the Huygens probe's descent to Titan's surface|volume=438|issue=7069|pages=765–778|journal= Nature|date=2005|doi=10.1038/nature04126|pmid=16319829|last1=Tomasko|first1=M. G.|last2=Archinal|first2=B.|last3=Becker|first3=T.|last4=Bézard|first4=B.|last5=Bushroe|first5=M.|last6=Combes|first6=M.|last7=Cook|first7=D.|last8=Coustenis|first8=A.|last9=De Bergh|first9=C.|last10=Dafoe|first10=L. E.|last11=Doose|first11=L.|last12=Douté|first12=S.|last13=Eibl|first13=A.|last14=Engel|first14=S.|last15=Gliem|first15=F.|last16=Grieger|first16=B.|last17=Holso|first17=K.|last18=Howington-Kraus|first18=E.|last19=Karkoschka|first19=E.|last20=Keller|first20=H. U.|last21=Kirk|first21=R.|last22=Kramm|first22=R.|last23=Küppers|first23=M.|last24=Lanagan|first24=P.|last25=Lellouch|first25=E.|last26=Lemmon|first26=M. |last27=Lunine |first27=Jonathan I. |last28=McFarlane|first28=E.|last29=Moores|first29=J.|last30=Prout|first30=G. M.|display-authors=29|bibcode=2005Natur.438..765T|s2cid=4414457}}</ref>
{{Clear}}

==Detailed ''Huygens'' activity timeline==
{{See also |Cassini–Huygens timeline#Detailed timeline of Huygens mission |label 1=Detailed timeline of Huygens mission}}
[[File:Animation of Huygens trajectory.gif |thumb |right |Animation of ''Huygens''{{'s}} trajectory from December 25, 2004 to January 14, 2005<br />{{legend2|magenta|  ''Huygens''}}{{·}}{{legend2| Lime |[[Titan (moon)|Titan]]}}{{·}}{{legend2| RoyalBlue|[[Saturn]]}}]]
* ''Huygens'' separated from ''Cassini'' orbiter at 02:00 [[Coordinated Universal Time|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&nbsp;hours 30&nbsp;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&nbsp;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==
''Huygens'' had six instruments aboard that took in a wide range of scientific data as the probe descended through Titan's atmosphere. The six instruments are:

===''Huygens'' Atmospheric Structure Instrument (HASI)===
This instrument contains a suite of sensors that measured the physical and electrical properties of Titan's atmosphere. [[Accelerometer]]s measured forces in all three axes as the probe descended through the atmosphere. With the aerodynamic properties of the probe already known, it was possible to determine the density of Titan's atmosphere and to detect wind gusts. The probe was designed so that in the event of a landing on a liquid surface, its motion due to waves would also have been measurable. Temperature and pressure sensors measured the thermal properties of the atmosphere. The Permittivity and Electromagnetic Wave Analyzer component measured the [[electron]] and [[ion]] (i.e., positively charged particle) conductivities of the atmosphere and searched for electromagnetic wave activity. On the surface of Titan, the [[electrical conductivity]] and [[permittivity]] (i.e., the ratio of [[electric displacement field]] to its [[electric field]]) of the surface material was measured. The HASI subsystem also contains a microphone, which was used to record any acoustic events during probe's descent and landing;<ref>{{cite journal |first1=M. |last1=Fulchignoni |first2=F. |last2=Ferri |first3=F. |last3=Angrilli |first4=A. |last4=Bar-Nun |first5=M.A. |last5=Barucci |first6=G. |last6=Bianchini |first7=W. |last7=Borucki |first8=M. |last8=Coradini |first9=A. |last9=Coustenis | display-authors = 8|doi=10.1023/A:1023688607077 |bibcode=2002SSRv..104..395F |date=2002 |title=The Characterisation of Titan's Atmospheric Physical Properties by the Huygens Atmospheric Structure Instrument (Hasi) |journal=Space Science Reviews |volume=104 |issue=1–4 |pages=395–431|s2cid=189778612 }}</ref>

===Doppler Wind Experiment (DWE)===
This experiment used an ultra-stable [[oscillator]]<ref>{{cite web |title=RUBIDIUM ULTRA-STABLE OSCILLATORS AT TITAN: THE HUYGENS DOPPLER WIND EXPERIMENT |url=https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=be263292ae7d4e1292ae9def2fe4052e8897712b}}</ref> which provided a precise S-band [[carrier frequency]] that allowed the Cassini orbiter to accurately determine Huygens's radial velocity with respect to Cassini via the [[Doppler Effect]]. The wind-induced horizontal motion from Huygens would have been derived from the measured Doppler shift measurements, corrected for all known orbit and propagation effects. The swinging motion of the probe beneath its parachute due to atmospheric properties may also have been detected. Failure of ground controllers to turn on the receiver in the ''Cassini'' orbiter caused the loss of this data.<ref name="CommandError">{{cite web |url=https://www.nature.com/news/2005/050117/full/news050117-12.html |title=Huygens: the missing data}}</ref> Earth-based [[radio telescopes]] were able to reconstruct some of it. Measurements started {{cvt|150|km|mi}} above Titan's surface, where ''Huygens'' was blown eastwards at more than {{cvt|400|kph|mph}},<ref>{{cite web |url=https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2005JE002649 |title=Winds on Titan from ground-based tracking of the Huygens probe }}, Figure 5.</ref> agreeing with earlier measurements of the winds at {{cvt|200|km|mi}} altitude, made over the past few years using [[telescopes]]. Between {{cvt|60|and|80|km|mi}}, ''Huygens'' was buffeted by rapidly fluctuating winds, which are thought to be vertical wind shear. At ground level, the Earth-based doppler shift and [[Very Long Baseline Interferometry|VLBI]] measurements show gentle winds of a few metres per second, roughly in line with expectations.

===Descent Imager/Spectral Radiometer (DISR)===
[[File:Huygens descent.ogg|thumb|DISR data visualisation during ''Huygens''{{'}}s descent]]
As ''Huygens'' was primarily an atmospheric mission, the DISR instrument was optimized to study the radiation balance inside Titan's atmosphere. Its visible and infrared [[spectrometer]]s and violet [[photometer]]s measured the up- and downward radiant flux from an altitude of {{cvt|145|km|mi}} down to the surface. Solar aureole cameras measured how scattering by [[aerosol]]s varies the intensity directly around the Sun. Three imagers, sharing the same [[Charge-coupled device|CCD]], periodically imaged a swath of around 30&nbsp;degrees wide, ranging from almost [[nadir]] to just above the horizon. Aided by the slowly spinning probe they would build up a full mosaic of the landing site, which, surprisingly, became clearly visible only below {{cvt|25|km|mi}} altitude. All measurements were timed by aid of a shadow bar, which would tell DISR when the Sun had passed through the field of view. Unfortunately, this scheme was upset by the fact that ''Huygens'' rotated in a direction opposite to that expected. Just before landing a lamp was switched on to illuminate the surface, which enabled measurements of the surface reflectance at wavelengths which are completely blocked out by [[atmospheric methane]] absorption.

DISR was developed at the [[Lunar and Planetary Laboratory]] at the [[University of Arizona]] under the direction of Martin Tomasko, with several European institutes contributing to the hardware. "The scientific objectives of the experiment fall into four areas including (1) measurement of the solar heating profile for studies of the thermal balance of Titan; (2) imaging and spectral reflection measurements of the surface for studies of the composition, topography, and physical processes which form the surface as well as for direct measurements of the wind profile during the descent; (3) measurements of the brightness and degree of linear polarization of scattered sunlight including the solar aureole together with measurements of the extinction optical depth of the aerosols as a function of wavelength and altitude to study the size, shape, vertical distribution, optical properties, sources and sinks of aerosols in Titan’s atmosphere; and (4) measurements of the spectrum of downward solar flux to study the composition of the atmosphere, especially the mixing ratio profile of methane throughout the descent."<ref>M G Tomasko; D Buchhauser; M Bushroe; L E Dafoe; L R Doose; A Eibl; C Fellows; E M Farlane; G M Prout; M J Pringle. The Descent Imager/Spectral Radiometer (DISR) Experiment on the Huygens Entry Probe of Titan. ''Space Science Reviews'', 104, no. 1/2, (2002): 467-549.</ref>

===Gas Chromatograph Mass Spectrometer (GC/MS)===
[[File:Huygens probe experiment platform (bottom).jpg|left|thumb|A worker in the Payload Hazardous Servicing Facility (PHSF) stands behind the bottom side of the experiment platform for ''Huygens''.]]
This instrument is a gas chemical analyzer that was designed to identify and measure chemicals in Titan's atmosphere.<ref>{{cite journal |first1=H.B. |last1=Niemann |first2=S.K. |last2=Atreya |first3=S.J. |last3=Bauer |first4=K. |last4=Biemann |first5=B. |last5=Block |first6=G.R. |last6=Carignan |first7=T.M. |last7=Donahue |first8=R.L. |last8=Frost |first9=D. |last9=Gautier| display-authors = 8 |doi=10.1023/A:1023680305259 |bibcode=2002SSRv..104..553N |date=2002 |title=The Gas Chromatograph Mass Spectrometer for the Huygens Probe |journal=Space Science Reviews |volume=104 |issue=1 |pages=553–91|hdl=2027.42/43756 |s2cid=1794678 |url=https://deepblue.lib.umich.edu/bitstream/2027.42/43756/1/11214_2004_Article_5106930.pdf |hdl-access=free }}</ref> It was equipped with samplers that were filled at high altitude for analysis. The [[mass spectrometer]], a high-voltage quadrupole, collected data to build a model of the molecular masses of each gas, and a more powerful separation of molecular and isotopic species was accomplished by the [[Gas chromatography|gas chromatograph]].<ref>{{cite journal |doi=10.1038/nature04122 |bibcode=2005Natur.438..779N |title=The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe |date=2005 |last1=Niemann |first1=H. B. |last2=Atreya |first2=S. K. |last3=Bauer |first3=S. J. |last4=Carignan |first4=G. R. |last5=Demick |first5=J. E. |last6=Frost |first6=R. L. |last7=Gautier |first7=D. |last8=Haberman |first8=J. A. |last9=Harpold |first9=D. N.| display-authors = 8 |journal=Nature |volume=438 |issue=7069 |pages=779–84 |pmid=16319830|url=https://deepblue.lib.umich.edu/bitstream/2027.42/62703/1/nature04122.pdf |hdl=2027.42/62703 |s2cid=4344046 |hdl-access=free }}</ref> During descent, the [[GC/MS]] also analyzed pyrolysis products (i.e., samples altered by heating) passed to it from the Aerosol Collector Pyrolyser. Finally, the GC/MS measured the composition of Titan's surface. This investigation was made possible by heating the GC/MS instrument just prior to impact in order to vaporize the surface material upon contact. The GC/MS was developed by [[GSFC|Goddard Space Flight Center]] and the [[University of Michigan]]'s Space Physics Research Lab.

===Aerosol Collector and Pyrolyser (ACP)===
The ACP experiment drew in [[Particulate|aerosol]] particles from the atmosphere through filters, then heated the trapped samples in ovens (using the process of [[pyrolysis]]) to vaporize [[Volatile (astrogeology)|volatiles]] and decompose the complex organic materials. The products were flushed along a pipe to the [[GC/MS]] instrument for analysis. Two filters were provided to collect samples at different altitudes.<ref>{{cite journal |first1=G. |last1=Israel |first2=M. |last2=Cabane |first3=J-F. |last3=Brun |first4=H. |last4=Niemann |first5=S. |last5=Way |first6=W. |last6=Riedler |first7=M. |last7=Steller |first8=F. |last8=Raulin |first9=D. |last9=Coscia | display-authors = 8|date=2002 |title=Huygens Probe Aerosol Collector Pyrolyser Experiment |journal=Space Science Reviews |volume=104 |issue=1–4 |pages=433–68 |doi=10.1023/A:1023640723915 |bibcode=2002SSRv..104..433I|s2cid=54502443 }}</ref>  The ACP was developed by a (French) [[European Space Agency|ESA]] team at the Laboratoire Inter-Universitaire des Systèmes Atmosphériques (LISA).

===Surface Science Package (SSP)===
The SSP contained a number of sensors designed to determine the physical properties of Titan's surface at the point of impact, whether the surface was solid or liquid.<ref>{{cite web |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31193&fbodylongid=740 |title=SSP: Surface Science Package |date=December 7, 2005 |work=ESA Science & Technology |publisher=[[European Space Agency]] |access-date=August 20, 2012}}</ref> An acoustic [[sonar|sounder]], activated during the last {{cvt|100|m|ft|sigfig=1}} of the descent, continuously determined the distance to the surface, measuring the rate of descent and the surface roughness (e.g., due to waves). The instrument was designed so that if the surface were liquid, the sounder would measure the speed of sound in the "ocean" and possibly also the subsurface structure (depth). During descent, measurements of the [[speed of sound]] gave information on atmospheric composition and temperature, and an accelerometer recorded the deceleration profile at impact, indicating the hardness and structure of the surface. A tilt sensor measured [[pendulum]] motion during the descent and was also designed to indicate the probe's attitude after landing and show any motion due to waves. If the surface had been liquid, other sensors would also have measured its [[density]], temperature, [[thermal conductivity]], heat capacity, electrical properties ([[permittivity]] and conductivity) and refractive index (using a critical angle refractometer).  A [[penetrometer]] instrument, that protruded {{cvt|55|mm|in}} past the bottom of the ''Huygens'' descent module, was used to create a penetrometer trace as ''Huygens'' landed on the surface. This was done by measuring the force exerted on the instrument by the body's surface as it broke through and was pushed down into the body by the landing.  The trace shows this force as a function of time over a period of about 400&nbsp;ms. The trace has an initial spike which suggests that the instrument hit one of the icy pebbles on the surface photographed by the DISR camera.

The ''Huygens'' SSP was developed by the Space Sciences Department of the [[University of Kent]] and the Rutherford Appleton Laboratory Space Science Department (now RAL Space) under the direction of Professor [[John Zarnecki]]. The SSP research and responsibility transferred to the [[Open University]] when John Zarnecki transferred in 2000.

==Spacecraft design==
[[File:Huygens thermal multilayer insulation.jpg|thumb|right|Application of [[multi-layer insulation]] shimmers under bright lighting during final assembly. The gold colour of the MLI is due to light reflecting from the [[aluminium]] coating on the back of sheets of amber coloured [[Kapton]].]]

''Huygens'' was built under the Prime Contractorship of [[Aérospatiale]] in its [[Cannes Mandelieu Space Center]], France, now part of [[Thales Alenia Space]]. The heat shield system was built under the responsibility of Aérospatiale near Bordeaux, now part of [[Airbus Defence and Space]].

===Parachute===
[[Martin-Baker]] Space Systems was responsible for ''Huygens''{{'}} [[parachute]] systems and the structural components, mechanisms and pyrotechnics that control the probe's descent onto Titan. [[IRVIN-GQ]] was responsible for the definition of the structure of each of ''Huygens''{{'}} parachutes. Irvin worked on the probe's descent control sub-system under contract to [[Martin-Baker Space Systems]].

==Design flaws==

=== ''Cassini''{{'}}s telemetry relay ===
Long after launch, a few persistent engineers discovered that the communication equipment on ''Cassini'' had a potentially fatal design flaw, which would have caused the loss of all data transmitted by ''Huygens''.<ref name="TitanCalling">{{cite news|title=Titan Calling |first=James |last=Oberg |publisher=[[IEEE Spectrum]] |url=https://spectrum.ieee.org/aerospace/space-flight/titan-calling |archive-url=https://web.archive.org/web/20090713061133/http://spectrum.ieee.org/aerospace/space-flight/titan-calling |url-status=dead |archive-date=July 13, 2009 |date=October 4, 2004 }} (offline as of 2006-10-14, see [https://web.archive.org/web/20041010192803/http://www.spectrum.ieee.org/WEBONLY/publicfeature/oct04/1004titan.html Internet Archive version])</ref><ref>[http://www.thespacereview.com/article/306/1 How Huygens avoided disaster], [[James Oberg]], [[The Space Review]], January 17, 2005.</ref>

Since ''Huygens'' was too small to transmit directly to Earth, it was designed to [[Transmission (telecommunications)|transmit]] the [[telemetry]] data obtained while descending through Titan's atmosphere by [[radio]] to ''Cassini'', which would in turn relay it to Earth using its large {{cvt|4|m|ft}} diameter main antenna. Some engineers, most notably ESA [[ESOC]] employees [[Claudio Sollazzo]] and [[Boris Smeds]], felt uneasy about the fact that, in their opinion, this feature had not been tested before launch under sufficiently realistic conditions. Smeds managed, with some difficulty, to persuade superiors to perform additional tests while ''Cassini'' was in flight. In early 2000, he sent simulated telemetry data at varying power and [[Doppler shift]] levels from Earth to ''Cassini''. It turned out that ''Cassini'' was unable to relay the data correctly.<ref name="TitanCalling"/>

This was because under the original flight plan, when ''Huygens'' was to descend to Titan, it would have accelerated relative to ''Cassini'', causing the [[Doppler effect|Doppler shift]] of its signal to vary. Consequently, the hardware of ''Cassini''{{'}}s receiver was designed to be able to receive over a range of shifted frequencies. However, the [[firmware]] failed to take into account that the Doppler shift would have changed not only the [[carrier signal|carrier frequency]], but also the timing of the [[Payload (computing)|payload]] [[bit]]s, coded by [[phase-shift keying]] at 8192 [[bits per second]].<ref name="TitanCalling"/>

Reprogramming the firmware was impossible, and as a solution the trajectory had to be changed. ''Huygens'' detached a month later than originally planned (December 2004 instead of November) and approached Titan in such a way that its transmissions travelled perpendicular to its direction of motion relative to ''Cassini'', greatly reducing the Doppler shift.<ref name="TitanCalling"/>

The trajectory change overcame the design flaw for the most part, and data transmission succeeded, although the information from one of the two radio channels was lost due to an unrelated error.<!-- no citation is needed: see next section -->

=== Channel A data loss ===
''Huygens'' was programmed to transmit [[telemetry]] and scientific data to the ''Cassini'' orbiter for relay to Earth using two redundant [[S-band]] radio systems, referred to as Channel A and B, or Chain A and B. Channel A was the sole path for an experiment to measure wind speeds by studying tiny frequency changes caused by ''Huygens''{{'}} motion. In one other deliberate departure from full redundancy, pictures from the descent imager were split, with each channel carrying 350 pictures.

''Cassini'' never listened to channel A because of an error in the sequence of commands sent to the spacecraft.<ref name="CommandError"/> The receiver on the orbiter was never commanded to turn on, according to officials with the European Space Agency. ESA announced that the error was a mistake on their part, the missing command was part of a command sequence developed by ESA for the ''Huygens'' mission, and that it was executed by ''Cassini'' as delivered.

Because Channel A was not used, only 350 pictures were received instead of the 700 planned. All [[Doppler radar|Doppler]] radio measurements between ''Cassini'' and ''Huygens'' were lost as well. Doppler radio measurements of ''Huygens'' from Earth were made, although they were not as accurate as the lost measurements that ''Cassini'' made. The use of accelerometer sensors on ''Huygens'' and [[Very Long Baseline Interferometry|VLBI]] tracking of the position of the ''Huygens'' probe from Earth  allowed reasonably accurate wind speed and direction calculations to be made.

== Contributions from citizen science projects ==
The fact that ''Huygens'' rotated in the opposite direction than planned delayed the creation of surface mosaics from the raw data by the project team for many months.  On the other hand, this provided an opportunity for some [[citizen science]] projects to attempt the task of assembling the surface mosaics. This was possible because the European Space Agency approved the publication of the DISR raw images and gave the permission for citizen scientists to present their results on the internet.<ref name=LiekensSpiegelOnline>{{cite web |url=http://www.anthony.liekens.net/index.php/Main/Huygens |title=Enthusiast compositions of the Huygens images |work=Anthony Liekens |access-date=February 14, 2022 |quote=In an article of Der Spiegel Online, a spokesperson of ESA confirms that this publication of raw images, to allow open source editing and compositing, is part of a study by ESA to see if the publication of the raws is indeed a good strategy. |archive-date=January 6, 2022 |archive-url=https://web.archive.org/web/20220106163210/http://anthony.liekens.net/index.php/Main/Huygens |url-status=dead }}</ref>
Some of these citizen science projects have received a lot of attention in the scientific community,<ref name=Nature_Peplow>{{cite journal|title= Amateurs beat space agencies to Titan pictures |journal= Nature|date=2005|doi=10.1038/news050117-7|last=Peplow|first=Mark|bibcode= }}</ref> in popular scientific journals<ref name=AmericanScientistHarris>{{cite magazine |last=Harris |first=Roger |date=2005 |title=Improve Your Image: Were planetary scientists scooped by a chat group of amateur enthusiasts? |url=http://www.jstor.org/stable/27858574 |magazine=American Scientist |volume=93|issue=3|pages=215–216|publisher=Sigma Xi, The Scientific Research Society |jstor=27858574 }}</ref><ref name=AstronomyNowMacRobert>{{cite magazine |last=Mac Robert |first=Alan |date=May 2005 |title=Amateurs-just amateurs|magazine=Astronomy Now|pages=67–69|publisher=Pole Star Publications Ltd.|location=UK}}</ref><ref name=Sky&TelescopeMacRobert>{{cite magazine |last=Mac Robert |first=Alan |date=January 2005 |title=Wild, Weird Titan Reveals More Secrets|magazine=Sky & Telescope|publisher=American Astronomical Society|location=United States}}</ref><ref name=Sky&TelescopeGoldman>{{cite magazine |last=Goldman |first=Stuart J. |date=May 2005 |title=Landscapes on Titan: amateurs scoop the pros: spacecraft-imagery openness may lead to amateurs beating planetary scientists to the punch.|magazine=Sky & Telescope|publisher=American Astronomical Society|location=United States}}</ref>
and in the public media.<ref name=Times20050121>{{cite magazine |author=<!--Staff writer(s); no by-line.--> |title=Amateurs who beat Nasa over the moon |magazine=The London Times |location=UK |date=January 21, 2005 |pages=32}}</ref><ref name=LeFigaroGoursac>{{cite magazine |last=de Goursac |first=Olivier |date=July 23, 2005 |title=Titan enfin dévoilé |trans-title=Titan finally unveiled|language=French |magazine=Le Figaro Magazine |location=France |publisher=Socpresse/Dassault}}</ref>
While the media liked to present the story of amateurs outperforming the professionals,<ref name=AmericanScientistHarris /><ref name=Sky&TelescopeGoldman /><ref name=Times20050121 /> most of the participants understood themselves as citizen scientists, and the driving force behind their work was a desire to find out and show as much as possible of the hitherto unknown surface of Titan. Some enthusiasts' projects were the first to publish surface mosaics and panoramas of Titan on the day after ''Huygens'' landed.<ref name=Liekens>{{cite web |url=http://www.anthony.liekens.net/index.php/Main/Huygens |title=Enthusiast compositions of the Huygens images |work=Anthony Liekens |access-date=February 14, 2022 |archive-date=January 6, 2022 |archive-url=https://web.archive.org/web/20220106163210/http://anthony.liekens.net/index.php/Main/Huygens |url-status=dead }}</ref>
Another project worked with the ''Huygens'' DISR data for several months until virtually all images with recognizable structures were able to be assigned to their correct position, resulting in comprehensive mosaics and panoramas.<ref>{{cite web |url=http://www.beugungsbild.de/huygens/huygens.html |title=Panoramic Views and Landscape Mosaics of Titan stitched from Huygens Raw Images|work=René Pascal |access-date=February 14, 2022}}</ref>
A surface panorama from this citizen science project was finally published in the context of a Nature review by Joseph Burns.<ref name=NatureBurns>{{cite journal|title= The four hundred years of planetary science since Galileo and Kepler |volume=466|issue=7306|pages=575–584|journal= Nature|date=2010|doi=10.1038/nature09215|pmid=20671701|last=Burns|first=Joseph A.|bibcode= 2010Natur.466..575B|s2cid=4412744 }}</ref>

==Landing site==
The probe landed on the surface of Titan at {{coord|10.573|S|192.335|W|globe:Titan}}.
[[File:PIA20713-Titan-SaturnMoon-LabeledFeaturesIAU-June2015.jpg|thumb|center|800px|The red cross marks the landing site of ''Huygens''. The bright region to the right is [[Xanadu (Titan)|Xanadu Region]].]]

== See also ==
{{Portal|Spaceflight}}

* [[List of European Space Agency programmes and missions]]
* [[Cassini retirement|''Cassini'' retirement]]
* [[Europlanet]]
* [[List of missions to the outer planets]]
* [[Titan Mare Explorer]]
* [[Titan Saturn System Mission]]

==References==

===Citations===
{{Reflist}}

===Bibliography===
* [https://www.nature.com/nature/volumes/438/issues/7069 ''Nature'' '''438''', Dec. 2005] - The results analyzed in nine articles, letters to the editor and related media are available with free access online.

== Further reading ==
* {{Cite book |author=Ralph Lorenz |title=NASA/ESA/ASI Cassini-Huygens: 1997 onwards (Cassini orbiter, Huygens probe and future exploration concepts) (Owners' Workshop Manual)|publisher=Haynes Manuals, UK |year=2018 |isbn=978-1785211119}}

== External links ==
{{Sister project links|wikt=no|commons=Category:Huygens (spacecraft)|v=no|q=no|s=no}}

* [http://anthony.liekens.net/index.php/Main/Huygens Amateur compositions of images, preceding NASA and ESA releases] {{Webarchive|url=https://web.archive.org/web/20050117005427/http://anthony.liekens.net/index.php/Main/Huygens |date=January 17, 2005 }}
* [http://arquivo.pt/wayback/20091223004202/http://saturn.esa.int/ European Space Agency ''Cassini–Huygens'' website], including [http://www.esa.int/SPECIALS/Cassini-Huygens/SEMKVQOFGLE_0.html videos of the descent]
* [http://sci.esa.int/science-e/www/area/index.cfm?fareaid=12 ESA ''Huygens'' scientific information]
* [http://www.esa.int/SPECIALS/Cassini-Huygens/SEMD6E2VQUD_0.html Interactive Flash-Animation of ''Cassini'' orbits through 2008] 
* [https://news.google.com/news?q=huygens+probe&hl=en&lr=&sa=N&tab=nn&oi=newsr Latest News on the ''Huygens'' Probe]
* [http://www.nasa.gov/mission_pages/cassini/main/index.html NASA's ''Cassini–Huygens'' page]
* [https://web.archive.org/web/20050420115953/http://www.newscientist.com/channel/space/cassini-huygens ''New Scientist'' &mdash; "Cassini-Huygens: Mission to Saturn"]
* [http://planetary.org/saturn/index.html Planetary Society's Saturn coverage]
* [http://www.beugungsbild.de/huygens/huygens.html Surface Mosaics and extensive Image Processing by an Amateur]
* [https://web.archive.org/web/20041206082039/http://esapub.esrin.esa.it/bulletin/bullet92/b92lebre.htm "The Huygens Probe: Science, Payload and Mission Overview"]
* [http://www.exploratorium.edu/saturn/webcasts.html Exploratorium webcasts about Saturn and Titan]
* [http://www.esa.int/esapub/bulletin/bullet92/b92lebre.htm ESA Bulletin on ''Huygens'']
* [https://web.archive.org/web/20110721163014/http://www.ingenia.org.uk/ingenia/articles.aspx?Index=317 Engineering the parachute and computer systems on the ''Huygens'' probe]


{{Cassinimission}}
{{Saturn spacecraft}}
{{European Space Agency}}
{{Solar System probes}}
{{Orbital launches in 1997}}
{{Extreme motion}}
{{titan}}
{{Christiaan Huygens}}

{{DEFAULTSORT:Huygens Probe}}
[[Category:Cassini–Huygens]]
[[Category:European Space Agency space probes]]
[[Category:Titan (moon)]]
[[Category:Spacecraft launched in 1997]]
[[Category:Derelict landers (spacecraft)]]
[[Category:Attached spacecraft]]
[[Category:Articles containing video clips]]
[[Category:Christiaan Huygens]]
[[Category:Extraterrestrial atmosphere entry]]