for National Geographic News
After a seven-year, roundabout planetary voyage, the international Cassini-Huygens spacecraft is poised to begin a four-year tour of Saturn, its rings, and its 31 known moons, including Earthlike Titan. Already the craft has captured unprecedented views of the moon Phoebe (see image at right).
"In some sense, we'll write the book on Saturn. I know that's sort of a glib phrase, but that's what we'll do," said Dennis Matson, project scientist for the Cassini-Huygens mission at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.
Located between Jupiter and Uranus, Saturn is 890 million miles (1.43 billion kilometers) from the sun, or ten times the distance from Earth to the sun. Smaller only than Jupiter, Saturn is composed mostly of hydrogen and helium gas, making it the least dense planet in the solar system.
Three NASA spacecraft, Pioneer 11 and Voyager 1 and 2, flew by the ringed planet in 1979, 1980, and 1981. But Cassini-Huygens is the first mission dedicated to the study of the Saturnian system.
What is known about the planet suggests a cold and windy place. The temperature at Saturn's cloud tops is -218 degrees Fahrenheit (-139 degrees Celsius), and winds rip across its equator at 1,100 miles an hour (500 meters a second).
Voyage to Saturn
The U.S. $3.4 billion Cassini spacecraft (named after the 17th-century Italian-French astronomer Jean-Dominique Cassini, who made several key discoveries about Saturn) and the piggybacking Huygens probe named after the Dutch scientist Christian Huygens, who discovered the moon Titan) launched October 15, 1997, from Cape Canaveral, Florida.
The package has been on a 2.2-billion-mile (3.5-billion-kilometer) journey to Saturn, swinging by Venus, Earth, and Jupiter along the way for "gravity-assists," or extra propulsion, and is scheduled to enter orbit around Saturn on June 30.
On arrival, Cassini will thread a 625-mile-wide (1,006-kilometer-wide) gap between two of the planet's ice- and rock-strewn outer rings. It will fire one of its main engines to slow it down as it studies the rings, then re-thread the rings and jockey into position for the first of 76 planned orbits.
Project scientists hope a shield-like antenna will block any dust grains from damaging the spacecraft as it threads and studies the rings. Once the spacecraft is safely in orbit, it will orient the antenna for relaying data to Earth.
JPL's Matson said the international project team is ready for orbit insertion, having tested and retested all systems required for the tricky task. Pioneer 11 successfully threaded the gap between the F and G rings, proving it can be done.
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