|Volume: 02, Issue: 14||09/08/2004|
Cassini Unlocking Saturnís SecretsIn June 2004, the international Cassini-Huygens mission kicked into high gear when the spacecraft entered Saturnís orbit after a nearly seven-year journey. Since then, the orbiter has recorded unparalleled information about Saturnís atmosphere, rings, and moons.
"The Saturn system represents an unsurpassed laboratory, where we can look for answers to many fundamental questions about the physics, chemistry, evolution of the planets, and the conditions that give rise to life," said Dr. Ed Weiler, NASA's Associate Administrator for Space Science in Washington.
Cassini studied Saturnís rings as the spacecraft entered the planetís orbit. The infrared spectrometer instrument recorded the most detailed temperature measurements of the rings to date, confirming that the rings have both cool and relatively warm regions. The warm regions tend to appear more transparent, while the cooler regions are more opaque.
The spacecraft has also begun to make exciting new discoveries regarding Saturnís moons. Its June 11, 2004 flyby of Saturnís moon Phoebe revealed that Phoebe may be a frozen artifact dating back some four billion years. Phoebe is likely a primordial mixture of ice, rock and carbon-containing compounds similar in many ways to material seen in Pluto and Neptune's moon Triton.
Perhaps Cassiniís most thrilling breakthrough to date, however, is the discovery of two new moons. With eyes sharper than any before that have peered at Saturn, Cassini uncovered the two moons, which may be the smallest bodies thus far seen around the ringed planet.
The moons are approximately 3 kilometers (2 miles) and 4 kilometers (2.5 miles) across -- smaller than Boulder, Colo. 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 Mimas and Enceladus. They are provisionally named S/2004 S1 and S/2004 S2. One of them, S/2004 S1, may be an object that had been spied in a single image taken by NASA's Voyager spacecraft 23 years ago, S/1981 S14, but was unconfirmed until now.
"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. "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, unnoticed around Saturn for billions of years until 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," Charnoz said.
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, but they are 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 that are believed to be there. Meanwhile, Cassini scientists are eager to get a closer look, if at all possible, at their new finds.
"We are at this very moment looking to see what the best times are for retargeting,Ē Porco said. ďHopefully, we haven't seen the last of them."
Cassini was launched Oct. 15, 1997 on a journey covering 3.5 billion kilometers (2.2 billion miles). It is the most instrumented and scientifically capable planetary spacecraft ever flown, with 12 instruments on the Cassini orbiter and six on the Huygens probe. The mission represents the best technical efforts of 260 scientists from the United States and 17 European nations. The $3 billion, four-year study of Saturn will examine the planetís rings, icy satellites, magnetosphere, and Titan, the planet's largest moon.
The study of Titan is one of the major goals of the mission. Titan may preserve, in deep-freeze, many of the chemical compounds that preceded life on Earth. Cassini will execute 45 flybys of Titan within approximately 950 kilometers (590 miles) of the surface. This will permit high-resolution mapping of the moon with the Titan radar-imaging instrument. The radar can see through the opaque haze of Titan's upper atmosphere.
"Titan is like a time machine taking us to the past to see what Earth might have been like," said Dr. Dennis Matson, Cassini project scientist at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. "The hazy moon may hold clues to how the primitive Earth evolved into a life-bearing planet," he said.
On Dec. 24, 2004, Cassini will release the wok-shaped Huygens probe for its journey to Titan. Huygens is the first probe designed to descend to the surface of the moon of another planet, and the most distant descent of a robotic probe attempted on another object in the solar system. On Jan. 14, 2005, after a three-week ballistic freefall, Huygens will enter Titan's atmosphere. It will deploy parachutes and begin 2.5 hours of intensive scientific observations. The Huygens probe will transmit data to the Cassini spacecraft, which will relay the information back to Earth.
JPL designed, developed and assembled the Cassini orbiter. A team at the European Space Technology and Research Center in Noordwijk, the Netherlands, managed the development of the Huygens probe. The Italian Space Agency provided the high-gain antenna, much of the radio system and elements of several of Cassini's science instruments. JPL manages the overall program for NASA's Office of Space Science.
Images are also available at the Cassini imaging team Internet site:
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