April 25, 1999
In the time it took you to read this, the Earth moved another thousand miles through space. You are not standing still. You never have been.
Mimas: Small Moon with a Big Crater
Birthday · April 25, 1999
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APOD published April 25, 1999. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
Mimas is one of the smaller moons of Saturn but shows one of the largest impact craters. In fact, if the impact had been much greater, it would have disrupted the entire satellite. The large crater has been named Herschel after the 1789 discoverer of Mimas, Sir William Herschel. Mimas' low mass produces a surface gravity just strong enough to create a spherical body but weak enough to allow such relatively large surface features. Mimas is made of mostly water ice with a smattering of rock - so it is accurately described as a big dirty snowball. Voyager 1 flew by in 1980 and took the above picture.