On Dec. 25, 2004, NASA’s Cassini spacecraft dropped a lander named Huygens at Titan, Saturn’s largest moon.

Huygens was a European spacecraft that hitched a ride to the Saturn system with Cassini. Huygens was asleep for the seven-year trip, but its internal timer told it to wake up right before it entered Titan’s atmosphere. After it parted with Cassini, the descent to Titan’s surface took three whole weeks. It spent two and a half hours parachuting through Titan’s atmosphere and sending data to Cassini, which beamed that data back to Earth. After the touchdown, it transmitted data for another hour and a half before its batteries died. The data and images from Huygens revealed that Titan is one of the most Earth-like places in the solar system. It has a thick atmosphere made of nitrogen and methane, and it's covered in liquid methane lakes. Huygens even found organic molecules on Titan. Scientists now think Titan may be the best place to look for life in the solar system.