Donald Savage Headquarters, Washington, D.C. (Phone: 202/358-1547) June 27, 1994 Diane Ainsworth Jet Propulsion Laboratory, Pasadena, Calif. (Phone: 818/354-5011) RELEASE: 94-104 ULYSSES STARTS PRIMARY MISSION AT SUN The Ulysses spacecraft became the first robotic exploration vehicle in history to reach a polar region of the Sun when it passed over the Sun's southern polar area June 26 after a journey of almost four years from Earth. The spacecraft, built by the European Space Agency (ESA), will now climb to a position 70 degrees south of the Sun's equator to begin a four-month study of the complex force at work in the polar region. Ulysses was deployed from the Space Shuttle Discovery in October 1990. In February 1992, Ulysses spent nearly 11 days exploring unknown regions of Jupiter before gaining enough momentum to loop out of the ecliptic and on an orbit that passes over the poles of the Sun. Scientists are elated to be able, finally, to carry out observations in the Sun's polar regions, said U.S. Project Scientist Dr. Edward J. Smith of NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. "We have been like explorers confined to travel near the Earth's equator without being able to journey to the arctic regions." "For a long time, we have suspected that much of the solar phenomena studied at, and near, the Earth was controlled by conditions in the Sun's polar regions," added Dr. Richard G. Marsden, ESA project scientist. "But never before have we been able to observe those processes." The lack of knowledge about these vital regions of the Sun has resulted in large part from the limited view that Earth-based instruments or spacecraft orbiting the Earth have had of the Sun. In addition, many complex solar phenomena cannot be observed remotely, but require direct measurements. The advance of space technology has made it possible in the last few decades to send - more - - 2 - proper instrumented robotic spacecraft such as Ulysses to previously inaccessible regions to make those direct measurements. The polar regions of the Sun -- although mysterious in many ways -- have long captivated scientists' interest. "The existence of a global magnetic field means that the Sun has magnetic poles much like the Earth," Smith said. "However, the properties of the polar magnetic fields, which switch polarity every 11 years in conjunction with the sunspot cycle, are poorly understood. Nevertheless, their existence introduces a basic north-south asymmetry into the solar atmosphere and space surrounding the Sun." A better understanding of the Sun's magnetic field will be important, the scientists contended, because magnetic fields play a key role in the physics of the Sun's outer atmosphere -- called the corona -- and its extension outward into space as the so-called solar wind. "The characteristic structure of the corona is imposed by the Sun's magnetic field," Smith said. "Furthermore, the source of the heat which creates the corona is unknown, but it is generally believed to be energy originally stored in the Sun's twisted and irregular magnetic fields." Whatever the heat source may be, scientists think the corona is generally too hot to be restrained by even the massive gravity field of the Sun. Unless the magnetic field can hold back the coronal gas, that gas flows outward into space as the solar wind picks up speed. The solar wind is known to reach velocities of about a million miles per hour. On the other hand, Smith added, if magnetic fields are directed outward from the Sun, they can channel the flow and assist in the escape and acceleration of the coronal gas. Coronal holes, regions of the corona which appear to be dark compared to the rest of the corona, are known sources of the solar wind. Although these general observations are clear, many details remain obscure and will become the focus of Ulysses measurements, Smith added. For instance, in the Sun's polar caps, the magnetic field extends outward through semi-permanent, very large coronal holes. By virtue of being directly above these sources and in the absence of complications introduced by the Sun's rotation, Ulysses is expected to contribute significant new knowledge about the escape and acceleration of the solar wind and, possibly, about the heating of the corona itself. The magnetic field also exerts a crucial influence on matter arriving in the vicinity of the Sun from the Milky Way galaxy and, in particular, from the nearby interstellar medium. - more - - 3 - Incoming cosmic rays, the nuclei of atoms traveling at nearly the speed of light, are subject to forces exerted by the Sun's magnetic field and its superimposed irregularities. The structure of the Sun's magnetic field is thought to favor entry of the cosmic rays by way of the polar regions. Two questions of scientific importance, Smith noted, are the extent to which the galactic cosmic rays observed at Earth use this route, and the ways in which their properties are modified as a consequence. "We hope to gain more knowledge of the intensity and properties of the cosmic rays far from the Sun, something that is presently unknown," Smith said. "The Ulysses mission will be able to shed new light on these long-standing riddles, as the instruments on board simultaneously measure the magnetic field and the properties of the solar wind and the cosmic rays." The Ulysses mission is managed jointly by the European Space Agency and NASA to study the regions over the Sun's poles. JPL oversees the U.S. portion of the mission for NASA's Office of Space Science, Washington, D.C. - end - NOTE TO EDITORS: A videotape illustrating this news release is available to news media representatives by faxing the Broadcast and Imaging Branch on 202/358-4333.