Cassini Spacecraft Nears Saturn, Photographs Moon

John Roach
for National Geographic News
June 14, 2004
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.

"There's no first-time event, and that's what always gets you—when you have first-time events, something comes up you didn't count on. We've scrubbed all those out, and that's why we're confident," Matson said.

Why Saturn?

Scientists are eager to study the Saturnian system in detail, because they believe it will yield answers to fundamental questions about chemistry and physics, planetary formation, and the conditions that give rise to life.

Four years of detailed investigation of the system will allow scientists for the first time to understand how the various components interact with each other—the difference between knowing the individual parts and the essence of the whole, Matson said.

He uses an analogy of a mechanical clock, where one can measure the wheels and various gears but totally miss out on the essence of what they do. "The essence of it is totally bound up in the interaction between the parts," he said.

Saturn and Jupiter's systems of planet, rings, and moons have long been considered physical models of the process of planetary formation and are referred to as miniature solar systems.

"[Studying] these different experiments in planetary formation will help us understand how the process works," said Ralph Lorenz, a planetary scientist at the University of Arizona in Tucson. Lorenz is an expert on Saturn's moon Titan.

Saturn's largest moon, Titan is a major focus of the Cassini-Huygens mission. Its surface, covered in a thick, smoggy haze, has never been studied in detail, but scientists believe that Titan's atmosphere is rich in organic material. This suggests that Titan may be similar to what Earth was like before life evolved.

"The pre-biotic Earth was probably not as reducing and not as cold, but many of the same chemical steps are there—long overprinted and destroyed on Earth but frozen in place for us to study on Titan," Lorenz said.

On December 24 Cassini will release the European Space Agency's Huygens probe. The probe will enter Titan's thick, nitrogen-rich atmosphere on January 14, 2005, deploy a set of parachutes and begin 2.5 hours of intensive scientific observations.

Images and data will be relayed via Cassini to scientists on Earth, providing details of the unknown landscape. What the probe will land on or in is unknown, but Lorenz has his fingers crossed it will splash into a lake of methane or ethane.

According to Matson, "Titan won't have any secrets left when we get through with it."

Mission Controversy

Anti nuclear activists staged several protests against the Cassini-Huygens mission, because at launch it carried about 70 pounds (30 kilograms) of plutonium dioxide to power the spacecraft's electrical system via radioisotope thermoelectric generators, or RTGs.

RTGs rely on heat, which is produced as the plutonium decays.

Activists were concerned that Cassini-Huygens posed an unacceptable risk of exposing billions of people to radioactive material if the spacecraft failed and blew up at launch or burned up in Earth's atmosphere.

Both of the events passed safely, but activists are still concerned about NASA's use of RTGs as well as RHUs, or radioactive heater units.

"I certainly do still feel that launching Cassini was a big mistake," said Russell Hoffman, who maintains the Stop Cassini Web site from Carlsbad, California. "Just because it now appears that the mission might succeed is no reason to change my mind. The statistics have not changed significantly—the failure rates for launches, for example. Indeed, we've suffered a number of spectacular space failures since Cassini was launched."

According to a NASA document on Cassini's power source, extensive testing shows that the use of RTGs is "very safe" even in the event of an accident. Although activists disagree with NASA's assessment, the space agency plans to continue their use.

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