The two Viking orbiters carried the two landers into orbit around Mars, observed the planet to certify the landing sites, released the landers for the landings, and subsequently served as telemetry relays for the lander data. In addition, they conducted scientific investigations using two cameras, an infrared radiometer for temperature measurements, an infrared spectrometer for water vapor measurements, and the radio communication system. The nature and extent of the orbiter observations have been influenced by the requirements for lander support, the capabilities of various orbiter subsystems, and the visibility of the planet from the orbits. All the orbiter scientific experiments are continuing.