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
approaches its August 2009 equinox.
This new moonlet, situated about 300 miles (480 kilometers), inward from
the outer edge of the B ring, was found by detection of its shadow which
stretches 25 miles, or 41 kilometers, across the rings. The shadow length
implies the moonlet is protruding about 660 feet, or 200 meters, 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 1,300 feet, or
400 meters, across.
This object is not attended by a propeller feature, unlike the band of
moonlets discovered in Saturn's A ring earlier by Cassini (see PIA07792 and
PIA06196). The A ring moonlets, which 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 a propeller
feature surrounding the new moonlet is likely 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. Also, 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 1
kilometer (4,680 feet) per pixel.
The Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. 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 and its two onboard cameras were designed, developed and
assembled at JPL. The imaging operations center is based at the Space
Science Institute in Boulder, Colo.
For more information about the Cassini-Huygens mission visit
http://saturn.jpl.nasa.gov/. The Cassini imaging team
homepage is at http://ciclops.org.