The Mars Exploration Rover Spirit lands on the Red Planet on January 3, 2004. 21 days later, its twin, Opportunity, also arrived safely. In one of the longest and most successful missions in NASA history, Spirit would survey Martian geography for the next seven years, while Opportunity remained active until June of 2018.
The rovers' primary mission was expected to last 90 sols, the term used for Martian days. In March, scientists announced that they had made a momentous discovery: a survey of Martian rocks strongly suggested that water had once flowed there, and analysis of Opportunity's landing site indicated that it had once been the bed of a salty sea. Later in 2004, Opportunity also discovered the first meteorite to be found on Mars.
The rovers continued to explore Mars for several years, with Spirit becoming a "stationary research platform" after getting stuck in sand. Spirit eventually fell out of contact with NASA, which declared its mission over in 2011. Opportunity, however, continued exploring. In 2014, it broke the record for longest distance driven by an off-Earth wheeled vehicle, and the next year NASA celebrated as Opportunity finished a "marathon," having traversed over 26.2 miles. In February 2019, NASA announced the end of the MER mission after Opportunity ceased responding to their communications. The rover had broken several other records, including the highest elevation reached on Mars, and sent back 224,642 images. Having far surpassed its original goals and contributed greatly to human understanding of Mars and its potential to host life, the MER mission had a major impact on mankind's knowledge of our solar system.