Space Topics: Saturn
Prometheus (S/1980 S27)
Inner Shepherd of the F ring
Saturn's moon Prometheus
This image was taken by Cassini through a red filter on June 7, 2005. Source
Credit: NASA / JPL / Space Science Institute
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Size: Irregular, 145 x 85 x 62 kilometers - 11th largest
moon of Saturn
Orbital radius: 139,353 kilometers - 2.28 Saturn radii
- inner shepherd of the F ring
Orbital period: 0.613 days - about 1/26 of Titan’s
Discovery: 1980 by S. Collins et al.
Prometheus is a tiny moon that orbits just inside Saturn's F ring. Along
with Pandora, the F ring's outer shepherd, Prometheus is probably responsible
for maintaining the edges of the F ring and keeping the gaps on either side of
the F ring clear. But Prometheus, like the F ring, has an eccentric orbit. Over
a period of many years, the orbits of Prometheus and the F ring evolve so that
at some times, Prometheus rises into the F ring with each orbit of Saturn. As
Prometheus falls back out, it pulls out streamers of material, making a spiny-looking
edge to the F ring. Evidence from Cassini suggests that these streamers
eventually return to the F ring, so that in the end, Prometheus is not
stealing any material from it.
Credit: NASA / JPL / Space Science Institute |
This movie sequence from Cassini shows dark drapes in the
inner strands of Saturn's F ring caused by the gravitational influence of the
shepherd moon Prometheus. Prometheus appears first in the sequence, interior
to the F ring, and Pandora follows along outside of the ring. Prometheus orbits
closer to Saturn, and thus faster, than the icy particles that make up the
F ring. The moon passes closest to the ring at "apoapse", when it
is farthest from Saturn. It is during these apoapse passages that Prometheus
has its greatest influence on the fine ring material. With time, the ring material
previously affected falls behind so that on the next apoapse passage of Prometheus,
a new gore in the inner ring material is made. The material closer to Prometheus
orbits the planet faster than the material closer to the bright F ring core.
The gores, together with the sheared-out material, create the dark, diagonal
drapes. Visit the Planetary
Photojournal to download a high-resolution, 170-frame version of this
movie.
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