 Did you know that Mars is the only planet entirely inhabited by robots? Yeah, that's right. There's two robots on the surface of Mars right now, and one of them just passed a very important milestone. On February 16, 2018, NASA's Opportunity rover celebrated its 5,000th Martian day on the red planet, and there's no plans to slow down. Opportunity and its twin rover Spirit landed on opposite sides of Mars in early 2004. The mission was planned to be just 90 days long, and the goal was to look for rocks that were formed in wet environments. Essentially, looking for evidence of past rivers, lakes, and oceans on Mars. After a bouncy landing, Opportunity, also known as OPI, immediately hit Paydirt. It found these little round rocks called spherules. They're about one centimeter in size, and they're created when trickling water flows through the gaps of larger rocks and leaves minerals behind that become little round rocks. So, mission accomplished. Mars has a wet history, and after that, the rest of the 90-day mission flew by. But Opportunity was still working at peak capacity. NASA wasn't ready to cut the mission. So, off OPI went, out into the wilderness of Meridiani Planum, a vast expanse of plains on the equator of Mars. Its first destination was a small crater called Endurance, about one kilometer from the landing site. After that was Victoria Crater, and then came the Big Road Trip, a 12 kilometer drive to Endeavour Crater. Now, at an average cruising speed of one centimeter per second, that trip took three years to complete. Now, currently, Opportunity is scouting the rim of Endeavour Crater, continuing its mission to understand a new world. During its drive to Endeavour Crater, Opportunity broke the off-world distance record for a rover. Back in 2014, it completed 40 kilometers. And then, in 2015, it completed the first ever Martian marathon for 42.2 kilometers. To celebrate its 5,000th Martian day, Opportunity took its first ever selfie. So now, after 14 years, over 45 kilometers, there's no plans to slow down. Opportunity is one of the most successful space missions ever.