S/2009 S 1

Lore & Background

S/2009 S 1 was first identified by the Cassini Imaging Team led by Carolyn Porco, in a single image taken by the Cassini spacecraft approximately 296,000 km from Saturn on 26 July 2009 at 11:30 UTC. The moonlet was discovered during Saturn's 2009 equinox, when it cast an approximately 36 km-long shadow on the planet's B ring. Based on the shadow's width, the Cassini Imaging Team inferred a diameter of 300 m for S/2009 S 1. The presence of a shadow suggests that it is most likely a solid body large enough to have existed since the formation of the B ring. The moonlet protrudes 150 m above the B ring plane, which has a vertical thickness of 5 m for comparison. Although it is embedded in the B ring, S/2009 S 1 does not appear to produce extensive, propeller-shaped disturbance features unlike the propeller moonlets in Saturn's A ring. This may be because the B ring is very dense at the moonlet's location, which would hinder the formation of visible propeller gaps around the moonlet. Some have proposed that it does not actually exist and the bright spot is just the effect of a coincidental meteoroid impact, though it was pointed out other propeller moonlets similarly appeared as a dusty blob around the time of equinox. Alternatively, the bright spot could be from the entire propeller structure rather than the moonlet itself.

Reader's Guide

S/2009 S 1 holds significance as the smallest known natural satellite in the Solar System and the innermost known moon of Saturn. Its discovery during the Saturnian equinox event of 2009, when the Cassini spacecraft imaged its shadow on the B Ring, provided direct evidence of a solid body embedded within the ring. The moonlet's size and location suggest it is a long-lived remnant from the formation of the B ring. However, the article notes uncertainty about its existence: some propose the bright spot could be from a coincidental meteoroid impact or from the entire propeller structure rather than the moonlet itself. This ambiguity highlights the challenges of observing small objects within dense rings. Its legacy lies in demonstrating that even tiny moonlets can be detected through their shadows during equinox, and in raising questions about the nature of such features in Saturn's rings.

Did You Know?

• S/2009 S 1 was discovered during Saturn's 2009 equinox event.
• It casts a 36 km-long shadow onto the B Ring.
• Some scientists propose the bright spot may be from a meteoroid impact rather than a moonlet.