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A Propeller in the B-ring

Published Article
American Astronomical Society, DDA meeting
Publication Date
May 27, 2012
SETI Institute


In 2009 July, close to Saturn equinox, an isolated shadow-casting object was imaged in Saturn's B-ring at an orbital radius of about 116910 km. Spitale and Porco (2010, AJ) interpreted it as an embedded satellite, whose diameter, inferred by the length of the shadow, would have been 300 m. Although the object does not appear in any other images, which is not surprising since that radius/co-rotating longitude was only imaged once during the equinox period, it was assigned a provisional designation of S/2009 S1 by the IAU. However, if there is indeed a 300-m diameter body orbiting in the middle of the B-ring, then why does the body not seem to be perturbing the ring at all? Why was no propeller-shaped structure (e.g., Tiscareno et al. 2006, Nature) seen surrounding the object, as the simulations of Michikoshi and Kokubo (2011, ApJL) indicated it should? The answer may be that the ring is indeed perturbed and the bright feature at the base of the shadow is in fact the propeller, with the body itself not resolved. That hypothesis is further supported by an analysis of the shadow that implies that the object casting the shadow is roughly spherical (and therefore 300 m wide), much smaller than the 2-km-wide projected point-spread, while the bright feature at the base of the shadow is larger than the point-spread, and is at least 10 times wider than the height of the shadow caster. Moreover, the bright feature is slightly canted in a direction consistent with Keplerian shear. Based on the assumption that the bright feature is the only propeller to have been observed in the B-ring to date, we will discuss what can be learned by comparing B-ring and A-ring propellers, as well as the origin, evolution and fate of the observed body.

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