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The Cassini spacecraft captured this image of a small object in the outer portion of Saturn's B ring casting a shadow on the rings as Saturn approached its August 2009 equinox.
The new object, situated about 650 kilometers (400 miles) inward from the outer edge of the B ring, was found by the presence of a localized bright feature and the detection of its shadow, which stretches 36 kilometers (22 miles) across the rings. Cassini scientists suspect the object is a solid moonlet, as opposed to a diffuse debris cloud, as might result from an impact, because no shadows have been observed accompanying any known impact plumes observed by Cassini. The shadow's length implies that the moonlet is protruding about 150 meters (500 feet) above the ring plane. If the moonlet is orbiting in the same plane as the ring material surrounding it, which is likely, it must be about 300 meters (1,000 feet) from bottom to top. However, the width of the bright feature implies that the moonlet must be several times wider than that height.
Unlike the band of moonlets discovered in Saturn's A ring earlier by Cassini (see PIA07792 and PIA07790), this object is not attended by a long propeller feature. However, the relatively large width of the bright features relative to the height implied by the long shadow may indicate that a fairly localized, perhaps truncated propeller feature surrounds a roughly spherical body that is not directly imaged. The A ring moonlets, which also have not been directly imaged, were found because of the propeller-like narrow gaps on either side of them that they create as they orbit within the rings. The lack of an extended propeller feature surrounding the new moonlet may not be not surprising because the B ring is dense, and the ring material in a dense ring would be expected to fill in any gaps around the moonlet more quickly than in a less dense region like the mid-A ring. Alternatively, it may simply be harder in the first place for a moonlet to create propeller-like gaps in a dense ring.
Straw-like patterns of clumping ring material are also visible along the edge of the outer B ring near the right of this image. See PIA09855 to learn more about these features.
This image and others like it (see PIA11656 and PIA11659) are only possible around the time of Saturn's equinox which occurs every half-Saturn-year (equivalent to about 15 Earth years). The illumination geometry that accompanies equinox lowers the sun's angle to the ring plane and causes out-of-plane structures to cast long shadows across the rings.
This view looks toward the sunlit side of the rings from about 42 degrees below the ring plane. Background stars are visible on the right of the image. They appear elongated by the camera's exposure time.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 26, 2009. The view was obtained at a distance of approximately 296,000 kilometers (184,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 120 degrees. Image scale is about 1 kilometer (4,680 feet) per pixel.
[Caption updated Nov. 1, 2010.]
The Cassini Equinox Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.