That’s No Moon, It’s … Oh, Wait, It’s a Moon

PIA 12570 — Mimas, the Death Star MoonSo the Death Star joke has been done to death, and Cassini’s 2010 photo of Mimas has become pretty much the standard picture for the second smallest of the planemo (planetary-mass object) moons around Saturn, which has enough moons that astronomers haven’t finished naming them all.

JPL explains the famous Death Star picture:

Herschel Crater is 130 kilometers, or 80 miles, wide and covers most of the right of this image. Scientists continue to study this impact basin and its surrounding terrain (see PIA12569 and PIA12571).

Cassini came within about 9,500 kilometers (5,900 miles) of Mimas on Feb. 13, 2010.This mosaic was created from six images taken that day in visible light with Cassini’s narrow-angle camera on Feb. 13, 2010. The images were re-projected into an orthographic map projection. This view looks toward the area between the region that leads on Mimas’ orbit around Saturn and the region of the moon facing away from Saturn. Mimas is 396 kilometers (246 miles) across. This view is centered on terrain at 11 degrees south latitude, 158 degrees west longitude. North is up. This view was obtained at a distance of approximately 50,000 kilometers (31,000 miles) from Mimas and at a sun-Mimas-spacecraft, or phase, angle of 17 degrees. Image scale is 240 meters (790 feet) per pixel.

Pac Man MimasMimas is fascinating even without its striking resemblance to a giant space station. The Herschel Crater, named after eighteenth-century astronomer William Herschel, who discovered Mimas with a forty-foot telescope, spans eighty-one miles; proportionately, such a crater on Earth would be larger than Australia.

Additionally, thermal images taken by the Composite Infrared Spectrometer (CIRS) aboard Cassini show a curious distribution of daytime temperatures, resulting in a “Pac Man” appearance.

Moreover, Mimas has a complex orbital relationship with Saturn’s rings. The Cassini Division is a gap between the A and B rings, perhaps best explained by Kirsten Larson’s 2007 paper for the College of Wooster. A more comprehensible explanation can be found at Wikipedia, which condenses the situation thus:

A number of features in Saturn’s rings are related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn’s two widest rings, the A Ring and B Ring. Particles in the Huygens Gap at the inner edge of the Cassini division are in a 2:1 resonance with Mimas. They orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap. The boundary between the C and B ring is in a 3:1 resonance with Mimas. Recently, the G Ring was found to be in a 7:6 co-rotation eccentricity resonance with Mimas; the ring’s inner edge is about 15,000 kilometres (9,300 mi) inside Mimas’s orbit.

And we would be remiss to omit that Mimas also has a 2:1 resonance with Tethys, and a 2:3 resonance with Pandora.

There is a lot going on ’round Saturn, and Mimas, despite its diminutive size, seems quite important to the workings.


Really. Astronomers have logged sixty-one moons around Saturn, though one of them, Titan, comprises 96% of the mass orbiting the second-largest planet in our solar system. That’s right. The other sixty moons and the famous rings comprise all of four percent of Saturn’s known orbital satellite mass. Oh, and it gets so complicated that there are also two mythical moons, Chiron and Themis, claimed in 1861 and 1905 respectively, but never confirmed.

There are two Saturnian “Pac Man” moons known so far, with Tethys being the other.


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