Smallest bodies seen around Saturn, 2 and 2.5 miles in
diameter
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.
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 system."
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."