Editing Viking program (section)

==Viking orbiters==
The primary objectives of the two Viking orbiters were to transport the landers to Mars, perform reconnaissance to locate and certify landing sites, act as communications relays for the landers, and to perform their own scientific investigations. Each orbiter, based on the earlier [[Mariner 9]] spacecraft, was an [[octagon]] approximately {{cvt|2.5|m|ft}} across. The fully fueled orbiter-lander pair had a [[mass]] of {{cvt|3527|kg|lb}}. After separation and landing, the lander had a mass of about {{cvt|600|kg|lb}} and the orbiter {{cvt|900|kg|lb}}. The total launch mass was {{cvt|2328|kg|lb}}, of which {{cvt|1445|kg|lb}} were propellant and [[Spacecraft attitude control|attitude control]] gas. The eight faces of the ring-like structure were {{cvt|0.457|m|in|0}} high and were alternately {{cvt|1.397|and|0.508|m|in|0}} wide. The overall height was {{cvt|3.29|m|ft}} from the lander attachment points on the bottom to the launch vehicle attachment points on top. There were 16 modular compartments, 3 on each of the 4 long faces and one on each short face. Four solar panel wings extended from the [[Coordinate axis|axis]] of the orbiter, the distance from tip to tip of two oppositely extended solar panels was {{cvt|9.75|m|ft|0}}.

===Propulsion===
The main [[Spacecraft propulsion|propulsion]] unit was mounted above the [[satellite bus|orbiter bus]]. Propulsion was furnished by a [[bipropellant]] ([[monomethylhydrazine]] and [[nitrogen tetroxide]]) liquid-fueled [[rocket engine]] which could be [[gimbal]]led up to 9&nbsp;[[degree (angle)|degree]]s. The engine was capable of {{convert|1323|N|lbf|abbr=on|lk=on}} thrust, providing a [[delta-V|change in velocity]] of {{cvt|1480|m/s|mph}}. [[Spacecraft attitude control|Attitude control]] was achieved by 12 small compressed-nitrogen jets.

===Navigation and communication===
An acquisition [[Sun sensor]], a cruise Sun sensor, a [[Canopus]] [[star tracker]] and an inertial reference unit consisting of six [[gyroscope]]s allowed three-axis stabilization. Two [[accelerometers]] were also on board.

Communications were accomplished through a {{Nowrap|20 W}} [[S-band]] (2.3 [[gigahertz|GHz]]) [[transmitter]] and two {{Nowrap|20 W}} [[TWTA]]s. An [[X-band|X band]] {{Nowrap|(8.4&nbsp;GHz)}} [[downlink]] was also added specifically for [[Doppler effect|radio science]] and to conduct communications experiments. [[Uplink]] was via S band {{Nowrap|(2.1&nbsp;GHz).}} A two-axis steerable [[Directional antenna|parabolic dish antenna]] with a diameter of approximately 1.5 m was attached at one edge of the orbiter base, and a fixed low-gain antenna extended from the top of the bus. Two tape recorders were each capable of storing 1280 [[megabit]]s. A 381-[[MHz]] relay radio was also available.{{citation needed|date=December 2023}}

===Power===
The power to the two orbiter craft was provided by eight {{cvt|1.57|×|1.23|m|in}} [[solar panels]], two on each wing. The solar panels comprised a total of 34,800 solar cells and produced 620&nbsp;W of power at Mars. Power was also stored in two [[nickel-cadmium]] 30-[[ampere hour|A·h]] [[battery (electricity)|batteries]].

The combined area of the four panels was {{convert|15|m2|ft2|abbr=off|sp=us}}, and they provided both regulated and unregulated direct current power; unregulated power was provided to the radio transmitter and the lander.

Two 30-amp·hour, nickel-cadmium, rechargeable batteries provided power when the spacecraft was not facing the Sun, during launch, while performing correction maneuvers and also during Mars occultation.<ref name="jpl-factsheet-viking">{{Cite web |title=Viking Fact Sheet |url=https://www.jpl.nasa.gov/news/fact_sheets/viking.pdf |url-status=dead |archive-url=https://web.archive.org/web/20120310184911/https://www.jpl.nasa.gov/news/fact_sheets/viking.pdf |archive-date=March 10, 2012 |access-date=March 27, 2012 |publisher=[[Jet Propulsion Laboratory]] }}</ref>

===Main findings===
[[File:Mars Valles Marineris.jpeg|thumb|Mars image mosaic from the ''Viking 1'' orbiter]]
By discovering many geological forms that are typically formed from large amounts of water, the images from the orbiters caused a revolution in our ideas about [[water on Mars]]. Huge river valleys were found in many areas. They showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and travelled thousands of kilometers. Large areas in the southern hemisphere contained branched stream networks, suggesting that rain once fell. The flanks of some volcanoes are believed to have been exposed to rainfall because they resemble those caused on Hawaiian volcanoes. Many craters look as if the impactor fell into mud. When they were formed, ice in the soil may have melted, turned the ground into mud, then flowed across the surface. Normally, material from an impact goes up, then down. It does not flow across the surface, going around obstacles, as it does on some Martian craters.<ref name="Kieffer1992">{{Cite book |url=https://archive.org/details/marsspacescience00unse |title=Mars |publisher=[[University of Arizona Press]] |year=1992 |isbn=978-0-8165-1257-7 |editor-last=Kieffer |editor-first=Hugh H. |lccn=92010951 |access-date=March 7, 2011 |editor-last2=Jakosky |editor-first2=Bruce M. |editor-last3=Snyder |editor-first3=Conway W. |editor-last4=Matthews |editor-first4=Mildred S. }}</ref><ref name="raeburn-1998">{{Cite book |last=Raeburn |first=Paul |url=https://archive.org/details/marsuncoveringse00raeb |title=Mars: Uncovering the Secrets of the Red Planet |publisher=[[National Geographic Society]] |year=1998 |isbn=0-7922-7373-7 |editor-last=Mulroy |editor-first=Kevin |lccn=98013991 }}</ref><ref name="moore-1990">{{Cite book |last1=Moore |first1=Patrick |last2=Hunt |first2=Garry |last3=Nicolson |first3=Iain |last4=Cattermole |first4=Peter |url=https://archive.org/details/atlasofsolarsyst0000unse_o5j4 |title=The Atlas of the Solar System |publisher=[[Mitchell Beazley]] |year=1990 |isbn=0-86134-125-2 |editor-last=Garlick |editor-first=Judy }}</ref> Regions, called "[[Martian chaos terrain|Chaotic Terrain]]," seemed to have quickly lost great volumes of water, causing large channels to be formed. The amount of water involved was estimated to ten thousand times the flow of the [[Mississippi River]].<ref name="morton-2002">{{Cite book |last=Morton |first=Oliver |url=https://archive.org/details/mappingmarsscien00mort_0 |title=Mapping Mars: Science, Imagination, and the Birth of a World |publisher=[[Picador (imprint)|Picador]] |year=2002 |isbn=0-312-24551-3 }}</ref> Underground volcanism may have melted frozen ice; the water then flowed away and the ground collapsed to leave chaotic terrain.

{{Clear}}
{{Gallery
|align=center
|title = '''Viking mosaics'''
|width=160
|File:Streamlined_Islands_in_Maja_Valles.jpg|alt1=Streamlined islands show that large floods occurred on Mars. (location: Lunae Palus quadrangle). |<small>Streamlined islands show that large floods occurred on Mars. <br />(''[[Lunae Palus quadrangle]]'')</small>
|File:Chryse Planitia Scour Patterns.jpg|alt2=Scour patterns were produced by flowing water. [[Dromore (crater)|Dromore]] crater is at bottom. <br />(''[[Lunae Palus quadrangle]]'') |<small>Scour patterns were produced by flowing water. [[Dromore (crater)|Dromore]] crater is at bottom. <br />(''[[Lunae Palus quadrangle]]'')</small>
|File:Detail of Maja Valles Flow.jpg|alt3=Large floods of water likely eroded the channels around the crater. (''[[Lunae Palus quadrangle]]'') |<small>Large floods of water likely eroded the channels around [[Dromore (crater)|Dromore]] crater. <br />(''[[Lunae Palus quadrangle]]'')</small>
|File:Viking Teardrop Islands.jpg|alt4=Tear-drop shaped islands carved by flood waters from [[Ares Vallis]]. (''[[Oxia Palus quadrangle]]'') |<small>Tear-drop shaped islands carved by flood waters from [[Ares Vallis]]. <br />(''[[Oxia Palus quadrangle]]'')</small>
|File:Vedra,_Maumee,_and_Maja_valles.jpg|alt5=Image of three valleys: [[Vedra Valles]], [[Maumee Valles]], and [[Maja Valles]]. (''[[Lunae Palus quadrangle]]'') |<small>Image of three valleys: [[Vedra Valles]], [[Maumee Valles]], and [[Maja Valles]]. <br />(''[[Lunae Palus quadrangle]]'')</small>
|File:Flow from Arandas Crater.jpg|alt6=[[Arandas (crater)|Arandas]] crater may be on top of large quantities of water ice, which melted when the impact occurred leaving a mud-like [[ejecta]]. (''[[Mare Acidalium quadrangle]]'') |<small>[[Arandas (crater)|Arandas]] crater may be on top of large quantities of water ice, which melted when the impact occurred, producing a mud-like ejecta. <br />(''[[Mare Acidalium quadrangle]]'')</small>
|File:Alba Patera Channels.jpg|alt7=Channels running through [[Alba Mons]]. (''[[Arcadia quadrangle]]'') |<small>Channels running through [[Alba Mons]]. <br />(''[[Arcadia quadrangle]]'')</small>
|File:Branched Channels from Viking.jpg|alt8=Branched channels in [[Thaumasia quadrangle]], possible evidence of past rain on Mars. |{{small|Branched channels in [[Thaumasia quadrangle]] provide possible evidence of past rain on Mars.}}
|File:Dissected Channels, as seen by Viking.jpg|alt9=These branched channels provide possible evidence of past rain on Mars. (''[[Margaritifer Sinus quadrangle]]'') |{{small|These branched channels provide possible evidence of past rain on Mars. (''[[Margaritifer Sinus quadrangle]]'')}}
|File:Ravi Vallis.jpg|alt10=[[Ravi Vallis]] was possibly formed from extreme flooding. <br />(''[[Margaritifer Sinus quadrangle]]'') |<small>[[Ravi Vallis]] was possibly formed from extreme flooding. <br />(''[[Margaritifer Sinus quadrangle]]'')</small>
}}