At 8.51am UT, July 20, 1976, the Viking 1 lander separated from its orbiting counter-part. In just over two hours it would become the the first spacecraft to successfully land on Mars and carry out its mission.
After 10 months of interplanetary travel, Viking 1 entered orbit around the planet Mars on June 21, 1976 and began preparing to send its lander down on the Fourth of July. However, closer inspection revealed a landing site too treacherous to safely put down on so Mission Control was forced to delay the descent for over two weeks while a new one was found. On July 20th, the new site had been confirmed, the lander separated from the Orbiter portion, and America's first attempt to put a machine on Mars was underway.
From a starting speed of 4km/s (a href="http://www.csgnetwork.com/tornadotable.html">28x the top wind speed of an F5 tornado) the lander first used rockets aboard the protective aeroshell to position itself for deorbiting and slowly drop to a height of 300km . The lander then prepared for atmospheric entry using its heat shield to slow the fall. As it fell, the lander took readings using a retarding potential analyser, a mass spectrometer, and a variety of pressure and temperature sensors.
By the time the lander hit 6km above the surface it was still rocketing along at 250m/s (top speed of the Messerschmitt p.1111). To further slow its roll, the lander deployed a set of 16m wide parachutes, jettisoned the heat shield and extended its landing legs. In less than a minute it had slowed to 60m/s (top speed of the Nemesis Electric Car). At 1.6km up, another set of retrorockets on the lander itself ignited and, once it slowed to 2.4m/s (top speed of me), the lander arrived in western Chryse Planitia with a light jolt thanks to its honeycomb aluminium shock absorbers.
"The Viking team didn't know the Martian atmosphere very well, we had almost no idea about the terrain or the rocks, and yet we had the temerity to try to soft land on the surface," recalls Gentry Lee, Solar System Exploration chief engineer at NASA's Jet Propulsion Lab in Pasadena. "We were both terrified and exhilarated. All of us exploded with joy and pride when we saw that we had indeed landed safely."
And unlike the Soviet Mars-3 lander that arrived on the Red Planet back in 1971, but failed to operate after the first 14 seconds, the Viking 1 didn't stop to wait for a welcoming party once it hit Martian soil - within 25 seconds it was recording images of the surroundings while activating the rest of its systems, including two Plutonium 238-powered radioisotope thermal generators. It raised a high-gain antenna for direct communication with mission control (rather than having to relay it through the Orbiter) then it took this picture:
Originally designed to function for three months, the Viking spacecraft collected data until November 13, 1982, six years and 116 days, and earning it the record for the second longest Mars surface mission behind the Twin Rovers, Spirit and Opportunity. During that time, the lander took 4500 up-close images of the Martian surface while the orbiter grabbed more than 50,000 images as it mapped 97 per cent of the Red Planet.