On July 20, 1976, Viking 1 touched down on the surface of Mars, becoming the second spacecraft to do so, and the first to function properly upon landing. (The Soviet Mars 2 lander had crash landed in 1971, and Mars 3 had soft-landed but failed shortly after a few weeks later.) Viking 1 landed in Chryse Planitia, and one of its first jobs was to photograph its own footpad to see how dense the Martian soil was. This is what it returned:
Powered by a Pu238 RTG, Viking 1 would go on to function on the Martian surface for a staggering 2245 Martian sols (or 2307 Earth days), a record that it held until surpassed by the Mars Exploration Rover B, “Opportunity”, which as of today (July 20) has been operating on Mars for 4440 sols, and is still going – on solar power, no less, which shows how far battery technology has come since then, enabling Opportunity to keep surviving the cold, dark Martian nights.