Every year, on July 20, humans celebrate Space Exploration Day, a day to remember all the missions to space, as well as a day to look forward to future missions that will take us ever farther out into the universe.

This year's Space Exploration Day, though, is even more special: it also marks the 40th anniversary of the Viking 1 landing on Mars, the first American spacecraft to ever do so. The Viking 1 also made history by becoming the first spacecraft to stay long-term on the planet.

Viking 1 launched into space on Aug. 20, 1975 and spent 10 months making its way to Mars. It entered Mars' orbit on June 19, 1976 and then later landed on the Red Planet on July 20. It stayed there for 2,307 days, a record only broken later by the Mars Opportunity rover.

While in orbit, the Viking 1 sent back images of Mars' surface, including its volcanoes, craters and many channels that hinted at ancient floods. It took thousands of more photos during its time on the planet, and its scientific experiments gave the first insights into what the Red Planet was really like, including new information about its soil, its weather and its surface features.

There was, though, much scientific controversy about Viking 1's search for life on Mars. Its instruments took samples of Martian soil and checked it for organic materials but found none. However, since then, other missions have found the opposite. Now, scientists wonder if the tools on the Viking 1 were just not sensitive enough to find organic compounds hidden there all along. A study done in 2010 even suggested that the Viking 1 destroyed those compounds before being able to detect them because its tests involved raising the soil to a high temperature.

The Viking 1 also helped scientists confirm the theory of general relativity by sending radio signals to the lander from Earth and having it send those signals back.

Thanks to the Viking 1, mankind's exploration of Mars fully took off. NASA followed that mission with many others, including the current Mars rover Curiosity and the upcoming Mars 2020 mission.

The anniversary of the Viking 1 landing on Mars also coincides with another special day: on July 20, 1969, Neil Armstrong became the first man to ever set foot on the moon. It truly was "one small step for man, one giant leap for mankind." Without that, Viking 1 — or any of the other missions to space — probably never would have happened.

Happy Space Exploration Day! Here's to the future of mankind exploring the stars.