Click on the image for the movie
The Cassini spacecraft's narrow-angle camera captured Saturn's moon Rhea
as it gradually slipped into the planet's shadow—an event known as
"ingress" —on Aug. 19, 2008.
Seventeen consecutive clear-filter images are in this movie showing an
eclipse ingress across the Saturn-facing hemisphere of Rhea. Because
sunlight still penetrates the planet's upper atmosphere and the sun has a
finite size, the shadow of Saturn cast onto Rhea does not form a sharp
line.
North on Saturn and Rhea is up in these images. The shadow moves from the
upper left to lower right (northwest to southeast) because the moon's
orbital plane around Saturn is inclined relative to the path, called the
ecliptic, that the sun takes around the moon as viewed from its surface.
With Saturn approaching its August 2009 equinox in its orbit around the
sun, the motion of the planet's shadow during future eclipses will trend
more towards west-to-east (left-to-right) on ingress. After 2009, the
trend will reverse, and the motion of Saturn's shadow will become more
southwest-to-northeast, until the inclination of the orbital plane
(relative to the ecliptic) becomes so large that Saturn's shadow no longer
intersects with the orbits of moons such as Rhea and eclipse "season" ends.
The first frame of the movie displays Rhea's hemispherical albedo
(brightness) dichotomy. The left half of the moon, its leading hemisphere,
is remarkably brighter than the right half, its trailing hemisphere.
Saturn's E-ring particles, which have their origins in the famous jets of
the small moon Enceladus, preferentially impact Rhea's leading hemisphere,
churning up its surface and enhancing its . Saturn's extremely tenuous E
ring is so expansive that at least 11 moons orbit within it.
These images were acquired by NASA's Cassini spacecraft at a solar phase,
or sun-Rhea-spacecraft, angle of about 28 degrees at a distance of
approximately 450,000 kilometers (280,000 miles). Image scale is around
2.7 kilometers (1.7 miles) 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.
