Out From the Shadows: Two New Saturnian Moons
With eyes sharper than any that have peered at Saturn before,
the Cassini spacecraft has uncovered two moons, which may be
the smallest bodies so far seen around the ringed planet.
Image above: Cassini recently detected two new moons orbiting between Mimas and Enceladus. A white box frames one of the moons, temporarily dubbed S/2004 S1. + Click for full animation and caption.
+ Click for full animation and caption of second moon.
Image credit: NASA/JPL/Space Science Institute.
The moons are approximately 3 kilometers (2 miles) and 4
kilometers (2.5 miles) across -- smaller than the city of
Boulder, Colorado. The moons, located 194,000 kilometers
(120,000 miles) and 211,000 kilometers (131,000 miles) from the
planet's center, are between the orbits of two other saturnian
moons, Mimas and Enceladus. They are provisionally named
S/2004 S1 and S/2004 S2. One of them, S/2004 S1, may be an
object spotted in a single image taken by NASA's Voyager
spacecraft 23 years ago, called at that time S/1981 S14.
"One of our major objectives in returning to Saturn was to
survey the entire system for new bodies," said Dr. Carolyn
Porco, imaging team leader, Space Science Institute, Boulder,
Colo. Porco planned the imaging sequences. "So, it's really
gratifying to know that among all the other fantastic
discoveries we will make over the next four years, we can now
add the confirmation of two new moons, skipping unnoticed
around Saturn for billions of years until just now.”
The moons were first seen by Dr. Sebastien Charnoz, a planetary
dynamicist working with Dr. Andre Brahic, imaging team member
at the University of Paris. "Discovering these faint
satellites was an exciting experience, especially the feeling
of being the first person to see a new body of our solar
system," said Charnoz. "I had looked for such objects for
weeks while at my office in Paris, but it was only once on
holiday, using my laptop, that my code eventually detected
them. This tells me I should take more holidays."
The smallest previously known moons around Saturn are about 20
kilometers (12 miles) across. Scientists expected that moons
as small as S/2004 S1 and S/2004 S2 might be found within gaps
in the rings and perhaps near the F ring, so they were
surprised these small bodies are between two major moons. Small
comets careening around the outer solar system would be
expected to collide with small moons and break them to bits.
The fact that these moons exist where they do might provide
limits on the number of small comets in the outer solar system,
a quantity essential for understanding the Kuiper Belt of
comets beyond Neptune, and the cratering histories of the moons
of the giant planets.
"A comet striking an inner moon of Saturn moves many times
faster than a speeding bullet," said Dr. Luke Dones, an imaging
team member from the Southwest Research Institute in Boulder,
Colo. "If small, house-sized comets are common, these moons
should have been blown apart many times by cometary impacts
during the history of the solar system. The disrupted moon
would form a ring, and then most of the material would
eventually gather back together into a moon. However, if small
comets are rare, as they seem to be in the Jupiter system, the
new moons might have survived since the early days of the solar
Moons surrounding the giant planets generally are not found
where they originally formed because tidal forces from the
planet can cause them to drift from their original locations.
In drifting, they may sweep through locations where other moons
disturb them, making their orbits eccentric or inclined
relative to the planet's equator. One of the new moons might
have undergone such an evolution.
Upcoming imaging sequences will scour the gaps in Saturn's
rings in search of moons believed to be there. Meanwhile,
Cassini scientists are eager to get a closer look, if at all
possible, at their new finds. Porco said, "We are at this very
moment looking to see what the best times are for retargeting.
Hopefully, we haven't seen the last of them."
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 Cassini-Huygens 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 team is based at
the Space Science Institute, Boulder, Colo.
For images and information about the Cassini-Huygens mission,
. Images are also available at the
Cassini imaging team home page, http://ciclops.org
Carolina Martinez (818) 354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
Donald Savage (202) 358-1727
NASA Headquarters, Washington
Heidi Finn (720) 974-5859
Cassini Imaging Central Laboratory for Operations
Space Science Institute, Boulder, Colo.