http://www.latimes.com/news/science/la-sci-cassini2jul02,1,6550325.story?coll=la-home-science
http://www.latimes.com/news/science/la-sci-cassini2jul02,1,6550325.story?coll=la-home-science
CELESTIAL BEAUTY & WONDER: A RINGSIDE SEAT REVEALS UNEXPECTED, PUZZLING
PHENOMENA BY CASSINI PROBE
By Thomas H. Maugh II, Times Staff Writer, Saturday, July 2, 2004
Safely in orbit around Saturn, NASA's Cassini spacecraft sent back its
first close-up images of the massive planet's rings Thursday, revealing
an unexpectedly varied terrain featuring surprisingly sharp edges,
braids and delicate ridges.
"We're seeing phenomena today that have never been imaged before," said
Ed Weiler, NASA's associate administrator. After seven years of
traveling to reach Saturn, "now the science is going to start."
Voyager flew part of the same route through the rings in 1981, but the
cameras on Cassini have five times the resolution of Voyager's, said
imaging team leader Carolyn Porco of the Space Science Institute in
Boulder, Colo.
"The beauty and clarity of these images are so shocking that I thought
at first that my team was playing tricks on me, showing me simulations"
rather than real pictures, she said.
The school bus-sized spacecraft flashed through the gap between two of
Saturn's rings, known as F and G, Wednesday evening, fired its rocket
motor for 95 minutes to slow the craft just enough for it to be captured
by Saturn's gravity, then flew once more through the gap in the rings,
taking black-and-white pictures all along the way.
Cassini will never be as close to the rings again, so the 61 images
beamed 900 million miles back to Earth by the craft after its crossing
offer a treasure trove of information about the origin, structure and
dynamics of the rings. "Scientists will have a field day with this,"
Porco said.
Astronomers are eager to study Saturn's ring system because it is a
nearly perfect model of the processes that produced our solar system and
can be seen producing similar systems around distant stars.
Cosmologists think that our sun was once surrounded by a gigantic ball
of dust that collapsed first into a series of rings and eventually into
planets. Understanding how Saturn's rings formed and are maintained will
thus provide insight into how the solar system developed. If we want to
understand our own celestial history, "this is the place to be," Weiler
said.
Cassini, which was launched in 1997, is scheduled to spend at least four
years in the Saturnian system studying the planet itself, its rings and
its 31 known moons, especially the largest, Titan. Researchers hope that
data from the mission will lend new insights into the formation of the
solar system and perhaps even the origins of life on Earth.
About half of the pictures were taken from the sunlit side of the rings
and half from the back side. The latter are like photographic negatives
of the former. In the images taken from the sunlit side, the rings
ice and rock ranging from house-sized boulders to specks of dust are
bright, looking much like they do when seen from Earth, albeit in much
greater detail.
But in the images taken from the opposite side of the ring plane, dark
bands in the images occur where the rings block sunlight and bright
bands occur where there are just enough particles to scatter light. The
images were more detailed than scientists expected. "They can be read
like a book telling us what kinds of properties the particles have and
how densely they are packed," Porco said.
The photos reveal two kinds of waves in the rings, both produced by
interaction with Saturn's tiny moons. Density waves, which are usually
concentric circles spreading farther apart and fading as they recede
from the moons, are produced by clumping of the particles of ice and
rock in small, thin bands, leaving minute gaps between them.
The so-called bending waves are spiral in structure and extend outward
perpendicular to the plane of the rings, giving the rings the appearance
of corrugated cardboard. Bending waves are similar to the arms of spiral
galaxies, Porco said.
Other pictures showed delicately braided rings and scalloped edges
caused by a tiny moon orbiting in the ring gap. Repeated passage of the
moon "forces eccentricities in the orbits of particles on the edge of
the ring building up beautiful, classic sinusoidal patterns," Porco
said. "This is textbook ring physics . I can't describe how exciting
this is for us."
Some images left astronomers scratching their heads. One photo of the
thin band known as the A ring shows an unexpected structure that Porco
said looked a bit like straw. "We're seeing something here and we
literally don't have a clue about what it is," she said. "Maybe it is
something no one has ever predicted before."
Researchers hope that close examination will reveal how old Saturn's
rings are and how long they will persist. Most research suggests that
they are no older than a few hundred million years, Porco said. Cassini
itself behaved perfectly during the orbital insertion, said project
manager Robert Mitchell. "There is not a single red alarm, not a single
indication of any faults," he said.
The craft entered a broad, looping orbit that will take 116 days to
complete, almost exactly what the team had planned, said navigation team
chief Jeremy Jones. On its second orbit, the craft will fly by Titan
and is expected to get a gravitational tug that will tighten the orbit
and reduce its length to 60 days. A second passage will reduce the
orbital period to 32 days, eventually putting the craft into an orbit in
which it will repeatedly fly by Titan. That orbit is required for
Cassini to release the European-built Huygens probe on Christmas Eve.
Three weeks later, Huygens will plummet to the surface of Titan, thought
to be the only moon in the solar system with a dense atmosphere. Titan
is rich in organic compounds, and researchers think its conditions are
similar to Earth's before life evolved. Over the next four years,
Cassini will take an estimated 300,000 photos of the Saturnian system.
With its nuclear power plant, the craft could continue its mission for
as long as 15 years.
The $3.3-billion mission is a joint project of NASA, the European Space
Agency and the Italian Space Agency.