On March 21, 2005, the Cassini spacecraft conducted a close flyby of Titan, Saturn's largest moon, passing within about 1,577 kilometers and using radar to peer through the moon's dense nitrogen-methane atmosphere. The data returned that day contributed to the growing picture of Titan as a world with liquid hydrocarbon lakes and a complex weather cycle, findings later confirmed dramatically by the Huygens probe's January 2005 landing and subsequent Cassini passes.
Titan had fascinated astronomers since Christiaan Huygens discovered it in 1655, but its thick, smog-orange atmosphere kept its surface hidden from ordinary telescopes and even from the Voyager probes that flew past in 1980 and 1981. Cassini, which entered Saturn orbit in 2004, carried a suite of instruments specifically designed to cut through that haze: infrared mapping spectrometers and, crucially, a synthetic aperture radar that could bounce signals off the surface and back.
The March 21, 2005, flyby was one of dozens Cassini would make of Titan over its 13-year mission, part of a systematic radar-mapping campaign that gradually built up a picture of a moon startlingly similar to Earth in some ways: rain, rivers, dunes, and seas, but composed of liquid methane and ethane rather than water, at temperatures around minus 179 degrees Celsius.
This particular pass added to data streaming in just weeks after the European Huygens probe had parachuted through Titan's atmosphere and landed on its surface in January, the most distant landing ever achieved by a human-made spacecraft at that time. Together, these observations transformed Titan from a hazy orange mystery into the solar system's most Earth-like world, a place scientists now consider one of the most promising sites to search for exotic forms of life.