Saturn-Bound Craft to Pass Close to Earth
launched two years ago amid vigorous protests over its radioactive fuel supply--will fly within 800 miles of Earth at 8:28 tonight, taking pictures as it passes on its way to Saturn.
Critics had warned that the 72 pounds of radioactive plutonium on board Cassini posed an unacceptable threat to human life if the craft crashed, either on launch or during the fly-by. But the Oct. 15, 1997, launch was successful, and protests have mostly quieted since then.
Marvin Goldman, emeritus professor at UC Davis, contends that the Cassini fly-by presents essentially no risk. Goldman was involved in a seven-year, independent environmental analysis of Cassini undertaken before the launch.
“This is as close to zero [risk] as you can get,” Goldman said.
It is extremely unlikely that Cassini would reenter Earth’s orbit, he said, about 1 chance in 10 million. If reentry occurred, only some of the plutonium would be in a dangerous form. That fraction, he said, would be dispersed through Earth’s atmosphere, resulting in a tiny increase in annual radiation exposure.
Cassini is scheduled to reach Saturn on July 1, 2004. The data it collects on the way, including a variety of observations of the moon, will help scientists calibrate the many instruments on board, and will aid in the eventual interpretation of an unfamiliar target.
“The purpose [of the Cassini mission] is to make a detailed exploration of the entire Saturn system,” said Jet Propulsion Laboratory’s Bob Mitchell, Cassini program manager, “[including] Saturn’s atmosphere, rings, satellites and magnetic field.”
Of particular interest is Titan, one of Saturn’s moons. With a nitrogen-rich, smoggy atmosphere possibly hiding lakes of carbon compounds, Titan’s surface features have remained mostly mysterious. What scientists do know about Titan, however, paints a familiar picture.
Because Titan has not evolved--primarily because of its frigid surface temperature of -288 degrees Fahrenheit--some scientists believe that it might look more like prehistoric Earth than Earth does today.
“Titan today is very similar to what Earth looked like some 3 billion or 4 billion years ago,” Mitchell said.
“The hope is that by being able to study a body that looks like Earth long ago, we can understand how Earth formed, evolved and came to be what it is today,” Mitchell said.
Cassini’s overhead pass will not be visible from most places, although it could possibly be seen from Easter Island. The craft will pass 725 miles above the eastern South Pacific, off the west coast of Chile.
Included in Cassini’s interplanetary itinerary were four fly-bys to pick up additional energy; the gravitational pull of a planet propels the craft like a slingshot. Two passes by Venus have already occurred--in April 1998 and June 1999. Today’s cruise by Earth marks the third fly-by; the fourth is a scheduled encounter with Jupiter in December 2000.
Other methods of powering the spacecraft would not have worked, Goldman said. Batteries would not last the full 11 years of the mission--seven years to reach Saturn and four years for observations. Cassini’s energy needs would have necessitated solar panels the size of several tennis courts.
Previous space missions--including Pioneer, Voyager, Viking, Galileo and Ulysses--relied on “nuclear batteries” similar to those in use by Cassini.
