Although the Cassini spacecraft is scheduled to officially arrive at the planet Saturn on June 30, scientists studying the planet's magnetosphere received an official welcome on June 27 when a burst of plasma wave noise indicated that Cassini had crossed the planet's bow shock -- the region where charged particles flowing outward from the sun collide with Saturn's magnetic field or magnetosphere.

University of Iowa Space Physicist Don Gurnett, head of the team that is analyzing radio and plasma wave emissions, says, "This is exciting. After nearly seven years, we finally got there! This marks the beginning of the scientific investigation for the people who will study the planet's magnetosphere."

Bill Kurth, Cassini team member and UI senior research scientist, compared the bow shock to a sonic boom.

"The bow shock is similar to a jet aircraft sonic boom that forms across the front of the plane. The charged particles flowing from the sun, called the solar wind, pass Saturn and the other planets at a speed of about one million miles an hour. We can compare the position of the bow shock with the pressure of the solar wind to learn something about the size of Saturn's magnetosphere and how much its size is controlled by the solar wind," he says.

The June 27 Cassini bow shock crossing occurred at a distance of 49.2 Saturn radii (2.97 million kilometers or 1.84 million miles) from Saturn and stands in contrast to first encounters by previous spacecraft, all of which took place much closer to the planet. The Pioneer spacecraft first crossed Saturn's bow shock at 23.7 Saturn radii, while Voyager 1 and Voyager 2 recorded crossings at 26.2 and 31.9 Saturn radii, respectively. Gurnett says the difference between Cassini and the other spacecraft is probably due to different flight trajectories.

"Cassini has encountered the bow shock quite a bit further out because the spacecraft is coming in from the side of the planet. So our approach angle is different from those of the other craft, primarily because Cassini is going to be placed into orbit about Saturn, while the other spacecraft made fly-bys," he says.

The radio sounds of Saturn and other sounds of space can be heard by visiting Gurnett's Web site at: http://www-pw.physics.uiowa.edu/space-audio

Cassini, carrying 12 scientific instruments, is on its way to the June 30, 2004 planetary rendezvous, when it will become the first spacecraft to orbit Saturn and begin a four-year study of the planet, its rings and its 31 known moons. The spacecraft is part of the Cassini-Huygens Mission that includes the Huygens probe, a six-instrument European Space Agency probe, scheduled to land on Titan, Saturn's largest moon, in January 2005.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology, Pasadena, Calif. manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter.