Click on the image for movie of
Soft Collision
The moon Prometheus slowly collides with the diffuse inner edge of
Saturn's F ring in this movie sequence of Cassini images. The oblong moon
pulls a streamer of material from the ring and leaves behind a dark
channel.
Once during its 14.7-hour orbit of Saturn, Prometheus (102 kilometers, or
63 miles across) reaches the point in its elliptical path, called apoapse,
where it is farthest away from Saturn and closest to the F ring. At this
point, Prometheus' gravity is just strong enough to draw a "streamer" of
material out of the core region of the F ring.
Initially the dust-sized material drawn away from the ring appears to form
a streamer pointing ahead of Prometheus in its orbit. (All orbital motion
is towards the right in the movie.) Over time, the streamer falls
increasingly farther behind Prometheus because material in the F ring is
orbiting slower than the moon. The streamer gets longer and a darker
"channel" starts to be seen (to the left of the streamer in the movie).
The creation of such streamers and channels occurs in a cycle that repeats
each Prometheus orbit: when Prometheus again reaches apoapse, it draws
another streamer of material from the F ring. But since Prometheus orbits
faster than the material in the ring, this new streamer is pulled from a
different location in the ring about 3.2 degrees (in longitude) ahead of
the previous one.
In this way, a whole series of streamer-channels is created along the F
ring. In some observations, 10 to 15 streamer-channels can easily be seen
in the F ring at one time (see PIA07712). Eventually, a streamer-channel
disappears as shearing forces (i.e., Keplerian shear) act to disperse the
constituent dust particles.
The movie shows just under half of a complete streamer-channel cycle. The
dark frames in the movie represent the period during which Prometheus and
the F ring pass through Saturn's shadow.
The images in the movie were acquired by the Cassini spacecraft
narrow-angle camera on Nov. 23 and 24, 2006. The movie sequence consists
of 72 clear spectral filter images taken every 10.5 minutes over a period
of about 12.5 hours.
The original images were cropped to show only the region around Prometheus
and the nearby portion of the F ring. The movie covers the region between
138,000 and 142,000 kilometers (86,000 and 88,000 miles) radially from
Saturn and 1 degree in longitude from Prometheus on each side. Each frame
was reprojected such that the vertical axis represents distance from
Saturn and the horizontal axis represents longitude around Saturn. Image
scale is 10 kilometers (6 miles) per pixel in the vertical direction; the
images cover 0.005 degrees of longitude in the horizontal direction.
Because of the reprojection, the F ring appears straight, rather than
slightly curved, as it otherwise would.
Since the F ring has an elliptical shape, its radial distance from Saturn
varies by about 1,000 kilometers (620 miles) around the ring. This
accounts for the apparent vertical movement of the ring over the course of
the movie. Only a very small part of the ring appears in each of the
reprojected frames, so the difference in the ring's radial distance from
left to right across any single frame is small enough as to be effectively
unnoticeable.
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/home/index.cfm. The Cassini imaging team
homepage is at http://ciclops.org.
