Following launch using a
Titan/
Centaur launch vehicle on August 20, 1975, and an 11-month cruise to Mars, the orbiter began returning global images of Mars about five days before orbit insertion. The
Viking 1 Orbiter was inserted into Mars orbit on June 19, 1976, and trimmed to a 1,513 x 33,000 km, 24.66 h site certification orbit on June 21. Landing on Mars was planned for July 4, 1976, the
United States Bicentennial, but imaging of the primary landing site showed it was too rough for a safe landing. The landing was delayed until a safer site was found, The lander separated from the orbiter at 08:51
UTC and landed at
Chryse Planitia at 11:53:06 UTC. It was the first attempt by the United States at landing on Mars.
Orbiter The instruments of the
orbiter consisted of two
vidicon cameras for imaging, an infrared spectrometer for water vapor mapping, and infrared radiometers for thermal mapping. The orbiter primary mission ended at the beginning of
solar conjunction on November 5, 1976. The extended mission commenced on December 14, 1976, after solar conjunction. Operations included close approaches to
Phobos in February 1977. The
periapsis was reduced to 300 km on March 11, 1977. Minor orbit adjustments were done occasionally over the course of the mission, primarily to change the walk rate — the rate at which the areocentric longitude changed with each orbit — and the periapsis was raised to 357 km on July 20, 1979. On August 7, 1980,
Viking 1 Orbiter was running low on
attitude control gas and its orbit was raised from 357 × 33,943 km to 320 × 56,000 km to prevent impact with Mars and possible contamination until the year 2019. Operations were terminated on August 17, 1980, after 1,485 orbits. A 2009 analysis concluded that, while the possibility that
Viking 1 had impacted Mars could not be ruled out, it was most likely still in orbit. More than 57,000 images were sent back to Earth.
Lander The lander and its
aeroshell separated from the orbiter on July 20 at 08:51 UTC. At the time of separation, the lander was orbiting at about . The aeroshell's retrorockets fired to begin the lander de-orbit maneuver. After a few hours at about altitude, the lander was reoriented for atmospheric entry. The aeroshell with its ablative
heat shield slowed the craft as it plunged through the
atmosphere. During this time, entry science experiments were performed by using a retarding potential analyzer, a
mass spectrometer, as well as pressure, temperature, and density sensors. The
Viking 1 lander touched down in western
Chryse Planitia ("Golden Plain") at On the day after the landing the first color picture of the surface of Mars (displayed below) was taken. The seismometer failed to uncage, and a sampler arm locking pin was stuck and took five days to shake out. Otherwise, all experiments functioned normally. The lander had two means of returning data to Earth: a relay link up to the orbiter and back, and by using a direct link to Earth. The orbiter could transmit to Earth (S-band) at 2,000 to 16,000 bit/s (depending on distance between Mars and Earth), and the lander could transmit to the orbiter at 16,000 bit/s. The data capacity of the relay link was about 10 times higher than the direct link. The lander operated for 2,245
sols (about 2,306 Earth days or 6 years) until November 11, 1982 (sol ), when a faulty command sent by ground control resulted in loss of contact. The command was intended to uplink new battery charging software to improve the lander's deteriorating battery capacity, but it inadvertently overwrote data used by the antenna pointing software. Attempts to contact the lander during the next four months, based on the presumed antenna position, were unsuccessful. In 2006, the
Viking 1 lander was imaged on the Martian surface by the
Mars Reconnaissance Orbiter. == Mission results ==