Editing Viking program (section)

==Viking landers==
[[File:NASM-A19790215000-NASM2016-02690.jpg|thumb|Proof test article of the Viking lander]]
[[File:Sagan large.jpg|thumb|Astronomer [[Carl Sagan]] stands next to a model of a ''Viking'' lander to provide scale]]
Each lander comprised a six-sided aluminium base with alternate {{convert|1.09|and|0.56|m|in|abbr=on}} long sides, supported on three extended legs attached to the shorter sides. The leg footpads formed the vertices of an equilateral triangle with {{convert|2.21|m|ft|abbr=on}} sides when viewed from above, with the long sides of the base forming a straight line with the two adjoining footpads. Instrumentation was attached inside and on top of the base, elevated above the surface by the extended legs.<ref>{{cite magazine|magazine=Popular Mechanics|author=Hearst Magazines|title=Amazing Search for Life On Mars|url=https://books.google.com/books?id=aOIDAAAAMBAJ&pg=PA66|date=June 1976|publisher=Hearst Magazines|pages=61–63}}</ref>

Each lander was enclosed in an [[aeroshell]] heat shield designed to slow the lander down during the entry phase. To prevent contamination of Mars by Earth organisms, each lander, upon assembly and enclosure within the aeroshell, was enclosed in a pressurized "bioshield" and then [[Sterilization (microbiology)|sterilized]] at a temperature of {{convert|111|°C|sp=us}} for 40 hours. For thermal reasons, the cap of the bioshield was jettisoned after the Centaur upper stage powered the Viking orbiter/lander combination out of Earth orbit.<ref name="science-19760827">{{Cite journal |last1=Soffen |first1=G. A. |last2=Snyder |first2=C. W. |date=August 27, 1976 |title=The First Viking Mission to Mars |url=https://www.science.org/doi/10.1126/science.193.4255.759 |url-status=live |journal=[[Science (journal)|Science]] |volume=193 |issue=4255 |pages=759–766 |doi=10.1126/science.193.4255.759 |pmid=17747776 |bibcode=1976Sci...193..759S |archive-url=https://web.archive.org/web/20230211150701/https://www.science.org/doi/10.1126/science.193.4255.759 |archive-date=February 11, 2023 |access-date=December 21, 2023 |url-access=subscription }}</ref>

Astronomer [[Carl Sagan]] helped to choose landing sites for both ''Viking'' probes.<ref name="britannica-carl-sagan">{{Cite encyclopedia |last=Kragh |first=Helge |title=Carl Sagan |encyclopedia=[[Encyclopædia Britannica]] |url=https://www.britannica.com/biography/Carl-Sagan |access-date=August 9, 2022 |archive-url=https://web.archive.org/web/20231108075739/https://www.britannica.com/biography/Carl-Sagan |archive-date=November 8, 2023 |url-status=live }}</ref>

===Entry, Descent and Landing (EDL)===
Each lander arrived at Mars attached to the orbiter. The assembly orbited Mars many times before the lander was released and separated from the orbiter for descent to the surface. Descent comprised four distinct phases, starting with a [[Deorbit#Deorbit and re-entry|deorbit burn]]. The lander then experienced [[atmospheric entry]] with peak heating occurring a few seconds after the start of frictional heating with the Martian atmosphere. At an altitude of about {{convert|6|km|mi|abbr=off|sp=us}} and traveling at a velocity of 900 kilometers per hour (600&nbsp;mph), the parachute deployed, the aeroshell released and the lander's legs unfolded. At an altitude of about 1.5 kilometers (5,000 feet), the lander activated its three retro-engines and was released from the parachute. The lander then immediately used [[retrorockets]] to slow and control its descent, with a [[soft landing (rocketry)|soft landing]] on the surface of Mars.<ref name="astro.if.ufrgs.br-viking">{{Cite web |title=Viking |url=http://astro.if.ufrgs.br/solar/viking.htm |url-status=live |archive-url=https://web.archive.org/web/20230813090441/http://astro.if.ufrgs.br/solar/viking.htm |archive-date=August 13, 2023 |website=astro.if.ufrgs.br }}</ref>
{{Wide image|Mars_Viking_12a001.png|800px|First "clear" image ever transmitted from the surface of Mars – shows [[List of rocks on Mars#Viking|rocks]] near the ''[[Viking 1]]'' lander (July 20, 1976).}} At landing (after using rocket propellant) the landers had a mass of about 600&nbsp;kg.

===Propulsion===

Propulsion for deorbit was provided by the [[monopropellant]] [[hydrazine]] (N<sub>2</sub>H<sub>4</sub>), through a rocket with 12 [[Rocket engine nozzles|nozzle]]s arranged in four clusters of three that provided {{convert|32|N|lk=in|sp=us}} thrust, translating to a [[delta-V|change in velocity]] of {{convert|180|m/s|abbr=on}}. These nozzles also acted as the control [[Spacecraft propulsion|thrusters]] for [[translation (geometry)|translation]] and [[rotation]] of the lander.

Terminal [[descent (aircraft)|descent]] (after use of a [[parachute]]) and [[landing]] used three (one affixed on each long side of the base, separated by 120&nbsp;degrees) monopropellant hydrazine engines. The engines had 18 [https://artsandculture.google.com/asset/_/fgFN2s2GG-OfQA nozzles] to disperse the exhaust and minimize effects on the ground, and were [[throttle]]able from {{convert|276|to|2667|N|lk=in|sp=us}}. The hydrazine was purified in order to prevent contamination of the Martian surface with Earth [[microbes]]. The lander carried {{convert|85|kg|abbr=on}} of propellant at launch, contained in two spherical [[titanium]] tanks mounted on opposite sides of the lander beneath the RTG windscreens, giving a total launch mass of {{convert|657|kg|abbr=on}}. Control was achieved through the use of an [[inertial reference unit]], four [[gyroscope|gyros]], a [[radar altimeter]], a terminal descent and landing [[radar]], and the control thrusters.

===Power===
Power was provided by two [[radioisotope thermoelectric generator]] (RTG) units containing [[plutonium-238]] affixed to opposite sides of the lander base and covered by wind screens. Each Viking RTG<ref>{{cite web |title=SNAP-19 Radioisotope Thermoelectric Generator Fact Sheet by Energy Research & Development Administration (ERDA) Diagram 2 - The Energy Research and Development Administration |url=https://artsandculture.google.com/asset/_/twFyqw9QR3uOoQ |website=Google Arts & Culture |access-date=9 August 2022 |language=en}}</ref> was {{convert|28|cm|abbr=on}} tall, {{convert|58|cm|abbr=on}} in diameter, had a mass of {{convert|13.6|kg|abbr=on}} and provided 30&nbsp;watts of continuous power at 4.4&nbsp;volts. Four [[wet cell]] sealed nickel-cadmium 8 [[ampere hours|Ah]] (28,800&nbsp;[[coulomb]]s), 28&nbsp;volt [[rechargeable battery|rechargeable batteries]] were also on board to handle peak power loads.

===Payload===
[[File:Viking2lander1.jpg|thumb|right|Image from Mars taken by the ''Viking 2'' lander]]

==== Communications ====
Communications were accomplished through a 20-watt S-band transmitter using two [[traveling-wave tube]]s. A two-axis steerable high-gain parabolic antenna was mounted on a boom near one edge of the lander base. An [[omnidirectional antenna|omnidirectional]] low-gain S-band antenna also extended from the base. Both these antennae allowed for communication directly with the Earth, permitting Viking 1 to continue to work long after both orbiters had failed. A [[UHF]] {{Nowrap|(381 MHz)}} antenna provided a one-way relay to the orbiter using a 30&nbsp;watt relay radio. Data storage was on a 40-Mbit tape recorder, and the lander computer had a 6000-[[Word (data type)|word]] memory for command instructions.

==== Instruments ====
The lander carried instruments to achieve the primary scientific objectives of the lander mission: to study the [[biology]], chemical composition ([[organic compound|organic]] and [[inorganic]]), [[meteorology]], [[seismology]], [[magnetism|magnetic]] properties, appearance, and physical properties of the Martian surface and atmosphere. Two 360-degree cylindrical scan cameras were mounted near one long side of the base. From the center of this side extended the sampler arm, with a collector head, [[Sensor#Thermal|temperature sensor]], and [[magnet]] on the end. A [[Climate of Mars|meteorology]] boom, holding temperature, wind direction, and wind velocity sensors extended out and up from the top of one of the lander legs. A [[seismometer]], magnet and camera [[test target]]s, and magnifying [[mirror]] are mounted opposite the cameras, near the high-gain antenna. An interior environmentally controlled compartment held the [[biology]] experiment and the [[gas chromatography|gas chromatograph]] mass spectrometer. The [[X-ray]] [[fluorescence]] spectrometer was also mounted within the structure. A [[pressure]] sensor was attached under the lander body. The scientific [[Payload (air and space craft)|payload]] had a total mass of approximately {{convert|91|kg|abbr=on}}.

===Biological experiments===
{{Main|Viking biological experiments}}
The Viking landers conducted [[Viking biological experiments|biological experiments]] designed to detect [[life on Mars (planet)|life in the Martian soil]] (if it existed) with experiments designed by three separate teams, under the direction of chief scientist [[Gerald Soffen]] of NASA. One experiment turned positive for the detection of [[metabolism]] (current life), but based on the results of the other two experiments that failed to reveal any [[Organic matter|organic molecules]] in the soil, most scientists became convinced that the positive results were likely caused by non-biological chemical reactions from highly oxidizing soil conditions.<ref name=Beegle >{{Cite journal|title=A Concept for NASA's Mars 2016 Astrobiology Field Laboratory|journal=Astrobiology|date=August 2007|first=LUTHER W.|last=BEEGLE|display-authors=etal |volume=7 | issue = 4|pmid=17723090 |pages=545–577|doi=10.1089/ast.2007.0153|bibcode=2007AsBio...7..545B}}</ref>

{{Multiple image
|direction=vertical
|align=right
|image1=Mars Viking 11a097.png
|image2=Mars Viking 11d128.png
|width=220
|caption1=Dust dunes and a large boulder taken by the ''Viking 1'' lander.
|caption2=Trenches dug by the soil sampler of the  ''Viking 1'' lander.
}}

Although there was a pronouncement by NASA during the mission saying that the Viking lander results did not demonstrate conclusive [[biosignature]]s in soils at the two landing sites, the test results and their limitations are still under assessment. The validity of the positive 'Labeled Release' (LR) results hinged entirely on the absence of an oxidative agent in the Martian soil, but one was later discovered by the [[Phoenix lander|''Phoenix'' lander]] in the form of [[perchlorate]] salts.<ref name="latimes-20080806">{{Cite web |last=Johnson |first=John |date=August 6, 2008 |title=Perchlorate found in Martian soil |url=https://www.latimes.com/la-sci-phoenix6-2008aug06-story.html |url-status=live |archive-url=https://web.archive.org/web/20230419020853/https://www.latimes.com/la-sci-phoenix6-2008aug06-story.html |archive-date=April 19, 2023 |work=[[Los Angeles Times]] }}</ref><ref name="sciencedaily-20080806">{{Cite news |date=August 6, 2008 |title=Martian Life Or Not? NASA's Phoenix Team Analyzes Results |work=Science Daily |url=https://www.sciencedaily.com/releases/2008/08/080805192122.htm |url-status=live |archive-url=https://web.archive.org/web/20231118165426/http://www.sciencedaily.com/releases/2008/08/080805192122.htm |archive-date=November 18, 2023 }}</ref> It has been proposed that organic compounds could have been present in the soil analyzed by both ''Viking 1'' and ''Viking 2'', but remained unnoticed due to the presence of perchlorate, as detected by Phoenix in 2008.<ref>{{Cite news |url=http://www.agu.org/pubs/crossref/2010/2010JE003599.shtml |title=Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars |journal=Journal of Geophysical Research: Planets |volume=115 |issue=E12010 |date=December 15, 2010 |access-date=January 7, 2011 |first=Rafael |last=Navarro–Gonzáles |author2=Edgar Vargas |author3=José de la Rosa |author4=Alejandro C. Raga |author5=Christopher P. McKay |archive-date=January 9, 2011 |archive-url=https://web.archive.org/web/20110109102058/http://www.agu.org/pubs/crossref/2010/2010JE003599.shtml |url-status=dead }}</ref>  Researchers found that perchlorate will destroy organics when heated and will produce [[chloromethane]] and [[dichloromethane]], the identical chlorine compounds discovered by both Viking landers when they performed the same tests on Mars.<ref>{{cite magazine| url=https://news.nationalgeographic.com/news/2012/04/120413-nasa-viking-program-mars-life-space-science/| archive-url=https://web.archive.org/web/20120415072431/http://news.nationalgeographic.com/news/2012/04/120413-nasa-viking-program-mars-life-space-science/| url-status=dead| archive-date=April 15, 2012| magazine=National Geographic| title=Life on Mars Found by NASA's Viking Mission| first=Ker| last=Than| date=April 15, 2012| access-date=April 13, 2018}}</ref>

The question of microbial life on Mars remains unresolved. Nonetheless, on April 12, 2012, an international team of scientists reported studies, based on mathematical speculation through [[complexity analysis]] of the [[Viking biological experiments#Labeled release|Labeled Release experiments]] of the 1976 Viking Mission, that may suggest the detection of "extant microbial life on Mars."<ref name="Bianciardi-2012">{{cite journal |last1=Bianciardi |first1=Giorgio |last2=Miller |first2=Joseph D. |last3=Straat |first3=Patricia Ann |last4=Levin |first4=Gilbert V. |title=Complexity Analysis of the Viking Labeled Release Experiments |journal=IJASS |date=March 2012 |volume=13 |issue=1 |pages=14–26 |doi=10.5139/IJASS.2012.13.1.14 |bibcode=2012IJASS..13...14B |df=mdy-all |doi-access=free }}</ref><ref name="Discovery-20120412">{{cite web |last=Klotz |first=Irene |title=Mars Viking Robots 'Found Life' |url=http://news.discovery.com/space/mars-life-viking-landers-discovery-120412.html |date=April 12, 2012 |publisher=[[Discovery Channel|DiscoveryNews]] |access-date=April 16, 2012 }}</ref> In addition, new findings from re-examination of the Gas Chromatograph Mass Spectrometer (GCMS) results were published in 2018.<ref>{{Cite journal|last1=Guzman|first1=Melissa|last2=McKay|first2=Christopher P.|last3=Quinn|first3=Richard C.|last4=Szopa|first4=Cyril|last5=Davila|first5=Alfonso F.|last6=Navarro-González|first6=Rafael|last7=Freissinet|first7=Caroline|date=2018|title=Identification of Chlorobenzene in the Viking Gas Chromatograph-Mass Spectrometer Data Sets: Reanalysis of Viking Mission Data Consistent With Aromatic Organic Compounds on Mars|journal=Journal of Geophysical Research: Planets|language=en|volume=123|issue=7|pages=1674–1683|doi=10.1029/2018JE005544|bibcode=2018JGRE..123.1674G|s2cid=133854625 |issn=2169-9100|url=https://hal-insu.archives-ouvertes.fr/insu-01820363/file/2018JE005544.pdf |archive-url=https://web.archive.org/web/20201103090059/https://hal-insu.archives-ouvertes.fr/insu-01820363/file/2018JE005544.pdf |archive-date=2020-11-03 |url-status=live}}</ref>

===Camera/imaging system===
[[File:Viking mission - Viking Lander Camera.png|thumb|Viking Lander Camera]]
The leader of the imaging team was [[Thomas A. Mutch]], a geologist at [[Brown University]] in [[Providence, Rhode Island]]. The camera uses a movable mirror to illuminate 12 [[photodiode]]s. Each of the 12 silicon diodes are designed to be sensitive to different frequencies of light.

Several broad band diodes (designated BB1, BB2, BB3, and BB4) are placed to focus accurately at distances between six and 43 feet away from the lander.<ref name=":1">{{Cite web |title=PDS: Instrument Information |url=https://pds.nasa.gov/ds-view/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=CAM1&INSTRUMENT_HOST_ID=VL1 |access-date=2023-03-28 |website=pds.nasa.gov}}</ref> A low resolution broad band diode was named SURVEY.<ref name=":1" />  There are also three narrow band low resolution diodes (named BLUE, GREEN and RED) for obtaining [[RGB color model|color images]], and another three (IR1, IR2, and IR3)  for [[infrared]] imagery.<ref name=":1" />

The cameras scanned at a rate of five vertical scan lines per second, each composed of 512 pixels. The 300 degree panorama images were composed of 9150 lines. The cameras' scan was slow enough that in a crew shot taken during development of the imaging system several members show up several times in the shot as they moved themselves as the camera scanned.<ref>{{cite book|chapter-url=https://history.nasa.gov/SP-425/ch8.htm|title=The Martian Landscape|author=The Viking Lander Imaging Team|publisher=NASA|date=1978|chapter=Chapter 8: Cameras Without Pictures|page=22}}</ref><ref name="nytimes-19760721">{{Cite news |last=McElheny |first=Victor K. |date=July 21, 1976 |title=Viking Cameras Light in Weight, Use Little Power, Work Slowly |url=https://www.nytimes.com/1976/07/21/archives/viking-cameras-light-in-weight-use-little-power-work-slowly.html |url-status=live |access-date=September 28, 2013 |archive-url=https://web.archive.org/web/20210222145516/https://www.nytimes.com/1976/07/21/archives/viking-cameras-light-in-weight-use-little-power-work-slowly.html |archive-date=February 22, 2021 |newspaper=[[The New York Times]] }}</ref>

=== Mass Breakdown of Viking Landers === 


{| class="wikitable"
|-
! Item !! Mass,kg (lb)<ref name=MarsLanderRetroprop>{{Cite web |title=Mars Lander Retro Propulsion (IAF-99-S.2.02) |url=https://www.researchgate.net/publication/232617757 |access-date=2024-05-14 |website= |language=en |archive-date=|archive-url= |url-status= }}</ref> 
|-
| Structures and Mechanisms 
|| {{cvt|132|kg|lb}}
|-
| Propulsion 
|| {{cvt|56|kg|lb}}
|-
| Pyro and Cabling
|| {{cvt|43|kg|lb}}
|-
| Thermal Control 
|| {{cvt|36|kg|lb}}
|-
| Guidance and Control 
|| {{cvt|79|kg|lb}}
|-

| Power 
|| {{cvt|103|kg|lb}}
|-
| Communications / Telemetry 
|| {{cvt|57|kg|lb}}
|-
| Science Instruments 
|| {{cvt|91|kg|lb}}
|-

| ='''Total Dry Mass'''  || {{cvt|595|kg|lb}} 
|-
| +Landing Propellant (incl. ~{{cvt|15|kg|lb}} residuals)
|| {{cvt|84|kg|lb}}
|-
| +Decelerator (incl.lander deorbit propellant)
|| {{cvt|118|kg|lb}}
|-
| +Aeroshell
|| {{cvt|269|kg|lb}}
|-
| +Bioshield
|| {{cvt|74|kg|lb}}
|-
| +Cap
|| {{cvt|54|kg|lb}}
|-
| ='''Total Launch Mass''' (Lander+Flight Capsule)  || {{cvt|1194|kg|lb}} 
|-

|-

|}



[[File:Viking control room.jpg|thumb|Viking control room at the [[Jet Propulsion Laboratory]], days before the landing of Viking 1.]]