The Viking project consisted of launches of two separate spacecraft to Mars, Viking 1,
launched on 20 August 1975, and Viking 2, launched on 9 September 1975. Each spacecraft
consisted of an orbiter and a lander. After orbiting Mars and returning images used for
landing site selection, the orbiter and lander detached and the lander entered the martian
atmosphere and soft-landed at the selected site. The orbiters continued imaging and other
scientific operations from orbit while the landers deployed instruments on the surface.
The fully fueled orbiter-lander pair had a mass of 3527 kg. After separation and landing,
the lander had a mass of about 600 kg and the orbiter 900 kg.
Orbiter
The primary objectives of the Viking orbiters were to transport the landers to Mars,
perform reconnaissance to locate and certify landing sites, act as a communications relays
for the landers, and to perform their own scientific investigations. The orbiter, based on
the earlier Mariner 9 spacecraft, was an octagon approximately 2.5 m across. The total
launch mass was 2328 kg, of which 1445 kg were propellant and attitude control gas. The
eight faces of the ring-like structure were 0.4572 m high and were alternately 1.397 and
0.508 m wide. The overall height was 3.29 m 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 axis of the orbiter, the distance from tip to tip of two oppositely extended
solar panels was 9.75 m. The power was provided by eight 1.57 × 1.23 m solar panels, two
on each wing. The solar panels were made up of a total of 34,800 solar cells and produced
620 W of power at Mars. Power was also stored in 2 nickel-cadmium 30-amp-hr batteries.
The main propulsion unit was mounted above the orbiter bus. Propulsion was furnished by
a bipropellant (monomethyl hydrazine and nitrogen tetroxide) liquid-fueled rocket engine (RS-21, RS-2101c)
which could be gimballed up to 9 degrees. The engine was capable of 1323 N thrust,
translating to a delta-V of 1480 m/s. Attitude control was achieved by 12 small
compressed-nitrogen jets. An acquisition Sun sensor, a cruise Sun sensor, a Canopus star
tracker and an inertial reference unit consisting of 6 gyroscopes allowed three-axis
stabilization. Two accelerometers were also on board. Communications were accomplished
through a 20-W S-band (2.3 GHz) transmitter and 2 20-W TWTA's. An X-band (8.4 GHz)
downlink was also added specifically for radio science and to conduct communications
experiments. Uplink was via S-band (2.1 GHz). A 2-axis steerable high-gain 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 Mbits. A 381 MHz relay radio was also available.
Scientific instruments for conducting imaging, atmospheric water vapor, and infrared
thermal mapping were enclosed in a temperature controlled, pointable scan platform
extending from the base of the orbiter. The scientific instrumentation had a total mass of
approximately 72 kg. Radio science investigations were also done using the spacecraft
transmitter. Command processing was done by two identical and independent data processors,
each with a 4096-word memory for storing uplink command sequences and acquired data.
Lander
The lander consisted of a 6-sided aluminum base with alternate 1.09 m and 0.56 m long
sides, supported on three extended legs attached to the shorter sides. The leg footpads
formed the vertices of an equilateral triangle with 2.21 m sides when viewed from above,
with the long sides of the base forming a straight line with the two adjoining footpads.
Instrumentation was attached to the top of the base, elevated above the surface by the
extended legs. Power was provided by two radioisotope thermal generator (RTG) units
containing plutonium 238 affixed to opposite sides of the lander base and covered by wind
screens. Each generator was 28 cm tall, 58 cm in diameter, had a mass of 13.6 kg and
provided 30 W continuous power at 4.4 volts. Four wet-cell sealed nickel-cadmium
8-amp-hour, 28 volt rechargeable batteries were also onboard to handle peak power loads.
Propulsion was provided for deorbit by a monopropellant hydrazine (N2H4) rocket with 12
nozzles arranged in four clusters of three that provided 32 N thrust, giving a delta-V of
180 m/s. These nozzles also acted as the control thrusters for translation and rotation of
the lander. Terminal descent and landing was achieved by three (one affixed on each long
side of the base, separated by 120 degress) monopropellant hydrazine engines. The engines
had 18 nozzles to disperse the exhaust and minimize effects on the ground and were
throttleable from 276 N to 2667 N. The hydrazine was purified to prevent contamination of
the martian surface. The lander carried 85 kg 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 657 kg. Control was achieved through the use of
an inertial reference unit, four gyros, an aerodecelerator, a radar altimeter, a terminal
descent and landing radar, and the control thrusters.
Communications were accomplished through a 20 W S-band transmitter and two 20 W TWTA's.
A 2-axis steerable high-gain parabolic antenna was mounted on a boom near one edge of the
lander base. An omnidirectional low-gain S-band antenna also extends from the base. Both
these antennae allowed for communication directly with the Earth. A UHF (381 MHz) antenna
provided a one-way relay to the orbiter using a 30 W relay radio. Data storage was on a 40
Mbit tape recorder, and the lander computer had a 6000 word memory for command
instructions.
The lander carried instruments to achieve the primary scientific objectives of the
lander mission: to study the biology, chemical composition (organic and inorganic),
meteorology, seismology, 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, temperature sensor, and magnet on the end. A 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 targets, 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
chromatograph mass spectrometer. The X-ray flourescence spectrometer was also mounted
within the structure. A pressure sensor was attached under the lander body. The scientific
payload had a total mass of approximately 91 kg.
Viking 1
Following launch and a 304 day cruise to Mars, the orbiter began returning global
images of Mars about 5 days before orbit insertion. The Viking 1 Orbiter was inserted into
Mars orbit on 19 June 1976 and trimmed to a 1513 × 33,000 km, 24.66 hr site certification
orbit on 21 June. Imaging of candidate sites was begun and the landing site was selected
based on these pictures. The orbiter primary mission ended at the beginning of solar
conjunction on 5 November 1976. The extended mission commenced on 14 December 1976 after
solar conjunction. Operations included close approaches to Phobos in February 1977. The
periapsis was reduced to 300 km on 11 March 1977. Minor orbit adjustments were done
occasionally over the course of the mission, primarily to change the walk rate - the rate
at which the planetocentric longitude changed with each orbit, and the periapsis was
raised to 357 km on 20 July 1979. On 7 August 1980 Viking 1 Orbiter was running low on
attitude control gas and its orbit was raised from 357 × 33943 km to 320 × 56000 km to
prevent impact with Mars and possible contamination until the year 2019. Operations were
terminated on 17 August 1980 after 1485 orbits.
The lander and its aeroshell separated from the orbiter on 20 July 08:51 UT. At the
time of separation, the lander was orbiting at about 4 km/s. After separation rockets
fired to begin lander deorbit. After a few hours at about 300 km altitude, the lander was
reoriented for entry. The aeroshell with its ablatable heat shield slowed the craft as it
plunged through the atmosphere. During this time, entry science experiments were
performed. At 6 km altitude at about 250 m/s the 16 m diameter lander parachutes were
deployed. Seven seconds later the aeroshell was jettisoned, and 8 seconds after that the
three lander legs were extended. In 45 seconds the parachute had slowed the lander to 60
m/s. At 1.5 km altitude, retro-rockets were ignited and fired until landing 40 seconds
later at about 2.4 m/s. The landing rockets used an 18 nozzle design to spread the
hydrogen and nitrogen exhaust over a wide area. It was determined that this would limit
surface heating to no more than 1 degree C and that no more than 1 mm of the surface
material would be stripped away. The Viking 1 Lander touched down in western Chryse
Planitia at 22.697 deg N latitude and 48.222 deg W longitude at a reference altitude of
-2.69 km relative to a reference ellipsoid with an equatorial radius of 3397.2 km and a
flatness of 0.0105 (22.480 deg N, 47.967 deg W planetographic) at 11:53:06 UT (4:13 p.m.
local Mars time). Approximately 22 kg of propellants were left at landing.
Transmission of the first surface image began 25 seconds after landing. The seismometer
failed to uncage, and a sampler arm locking pin was stuck and took 5 days to shake out.
Otherwise, all experiments functioned nominally. The Viking 1 Lander was named the Thomas
Mutch Memorial Station in January 1982 in honor of the leader of the Viking imaging team.
It operated until 13 November 1982 when a faulty command sent by ground control resulted
in loss of contact.
Viking 2
Following launch and a 333 day cruise to Mars, the Viking 2 Orbiter began returning
global images of Mars prior to orbit insertion. The orbiter was inserted into a 1500 x
33,000 km, 24.6 hr Mars orbit on 7 August 1976 and trimmed to a 27.3 hr site certification
orbit with a periapsis of 1499 km and an inclination of 55.2 degrees on 9 August. Imaging
of candidate sites was begun and the landing site was selected based on these pictures and
the images returned by the Viking 1 Orbiter. Normal operations called for the
structure connecting the orbiter and lander (the bioshield) to be ejected after
separation, but because of problems with the separation the bioshield was left attached to
the orbiter. The orbit inclination was raised to 75 degrees on 30 September 1976. The
orbiter primary mission ended at the beginning of solar conjunction on 8 November 1976.
The extended mission commenced on 14 December 1976 after solar conjunction. On 20 December
1976 the periapsis was lowered to 778 km and the inclination raised to 80 degrees.
Operations included close approaches to Deimos in October 1977 and the periapsis was
lowered to 300 km and the period changed to 24 hours on 23 October 1977. The orbiter
developed a leak in its propulsion system that vented its attitude control gas. It was
placed in a 302 × 33176 km orbit and turned off on 25 July 1978 after returning almost
16,000 images in 706 orbits around Mars.
The lander separated from the orbiter on 3 September 1976 and landed at Utopia Planitia
at 22:37:50 UT. At the time of separation, the lander was orbiting at about 4 km/s. After
separation rockets fired to begin lander deorbit. After a few hours, at about 300 km
altitude, the lander was reoriented for entry. The aeroshell with its ablatable heat
shield slowed the craft as it plunged through the atmosphere. During this time, entry
science experiments were performed. At 6 km altitude at about 250 m/s the 16 m diameter
lander parachutes were deployed. Seven seconds later the aeroshell was jettisoned, and 8
seconds after that the three lander legs were extended. In 45 seconds the parachute had
slowed the lander to 60 m/s. At 1.5 km altitude, retro-rockets were ignited and fired
until landing 40 seconds later at about 2.4 m/s. The landing rockets used an 18 nozzle
design to spread the hydrogen and nitrogen exhaust over a wide area. It was determined
that this would limit surface heating to no more than 1 degree C and that no more than 1
mm of the surface material would be stripped away.
The Viking 2 Lander touched down about 200 km west of the crater Mie in Utopia Planitia
at 48.269 deg N latitude and 225.990 deg W longitude at a reference altitude of 4.23 km
relative to a reference ellipsoid with an equatorial radius of 3397.2 km and a flatness of
0.0105 (47.967 deg N, 225.737 deg W planetographic) at 22:58:20 UT (9:49:05 a.m. local
Mars time). Approximately 22 kg of propellants were left at landing. Due to radar
misidentification of a rock or highly reflective surface, the thrusters fired an extra
time 0.4 seconds before landing, cracking the surface and raising dust. The lander settled
down with one leg on a rock, tilted at 8.2 degrees. The cameras began taking images
immediately after landing. The Viking 2 Lander operated on the surface for 1281 Mars days
and was turned off on 11 April 1980 when its batteries failed.