Viking program
The Viking program consisted of a pair of identical American space probes, Viking 1 and Viking 2 both launched in 1975, and landed on Mars in 1976. The mission effort began in 1968 and was managed by the NASA Langley Research Center. Each spacecraft was composed of two main parts: an orbiter spacecraft which photographed the surface of Mars from orbit, and a lander which studied the planet from the surface. The orbiters also served as communication relays for the landers once they touched down.
The Viking program grew from NASA's earlier, even more ambitious, Voyager Mars program, which was not related to the successful Voyager deep space probes of the late 1970s. Viking 1 was launched on August 20, 1975, and the second craft, Viking 2, was launched on September 9, 1975, both riding atop Titan IIIE rockets with Centaur upper stages. Viking 1 entered Mars orbit on June 19, 1976, with Viking 2 following on August 7.
After orbiting Mars for more than a month and returning images used for landing site selection, the orbiters and landers detached; the landers then entered the Martian atmosphere and soft-landed at the sites that had been chosen. The Viking 1 lander touched down on the surface of Mars on July 20, 1976, more than two weeks before Viking 2s arrival in orbit. Viking 2 then successfully soft-landed on September 3. The orbiters continued imaging and performing other scientific operations from orbit while the landers deployed instruments on the surface. The program terminated in 1982.
The project cost was roughly US$1 billion at the time of launch, equivalent to about $ billion in dollars. The mission was considered successful and formed most of the body of knowledge about Mars through the late 1990s and early 2000s.
Science objectives
- Obtain high-resolution images of the Martian surface
- Characterize the structure and composition of the atmosphere and surface
- Search for evidence of life on Mars
Viking orbiters
Propulsion
The main propulsion unit was mounted above the orbiter bus. Propulsion was furnished by a bipropellant liquid-fueled rocket engine which could be gimballed up to 9 degrees. The engine was capable of thrust, providing a change in velocity of. 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 gyroscopes allowed three-axis stabilization. Two accelerometers were also on board.Communications were accomplished through a S-band transmitter and two TWTAs. An X band downlink was also added specifically for radio science and to conduct communications experiments. Uplink was via S band A two-axis steerable 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 megabits. A 381-MHz relay radio was also available.
Power
The power to the two orbiter craft was provided by eight solar panels, two on each wing. The solar panels comprised a total of 34,800 solar cells and produced 620 W of power at Mars. Power was also stored in two nickel-cadmium 30-A·h batteries.The combined area of the four panels was, 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.
Main findings
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. Regions, called "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. Underground volcanism may have melted frozen ice; the water then flowed away and the ground collapsed to leave chaotic terrain.Viking landers
Each lander comprised a six-sided aluminium base with alternate long sides, supported on three extended legs attached to the shorter sides. The leg footpads formed the vertices of an equilateral triangle with 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.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 sterilized at a temperature of 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.
Astronomer Carl Sagan helped to choose landing sites for both Viking probes.
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 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 and traveling at a velocity of 900 kilometers per hour, the parachute deployed, the aeroshell released and the lander's legs unfolded. At an altitude of about 1.5 kilometers, 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 on the surface of Mars.At landing the landers had a mass of about 600 kg.
Propulsion
Propulsion for deorbit was provided by the monopropellant hydrazine, through a rocket with 12 nozzles arranged in four clusters of three that provided thrust, translating to a change in velocity of. These nozzles also acted as the control thrusters for translation and rotation of the lander.Terminal descent and landing used three monopropellant hydrazine engines. The engines had 18 to disperse the exhaust and minimize effects on the ground, and were throttleable from. The hydrazine was purified in order to prevent contamination of the Martian surface with Earth microbes. The lander carried 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. Control was achieved through the use of an inertial reference unit, four gyros, a radar altimeter, a terminal descent and landing radar, and the control thrusters.