NASA/Spacelink FACT SHEET: THE CASSINI MISSION SPACELINK NOTE: The February 1991 launch manifest shows December, 1995 as the launch date for Cassini. The ringed planet Saturn, its major moon Titan and complex system of at least 16 other satellites will be the destination for NASA's and the European Space Agency's Cassini Mission. Named for the Italian-French astronomer who discovered several of Saturn's moons, Cassini is under study at the Jet Propulsion Laboratory as one of two initial projects in the Mariner Mark II series of missions to the outer solar system. Early during the spacecraft's four-year tour orbiting Saturn, it will launch a parachuted probe descending through Titan's dense atmosphere to the surface of the satellite -- which boasts unique organic-like chemistry that could provide clues to the origin of life on Earth. On its way to Saturn, Cassini will also execute an encounter with an asteroid and will fly down the magnetotail of Jupiter, performing studies complementing NASA's Comet Rendezvous Asteroid Flyby (CRAF) mission and the Galileo mission to Jupiter. Recommended by NASA's Solar System Exploration Committee in its report to the space agency in the early 1980s, the Mariner Mark II series will use shared design concepts and advanced manufacturing techniques to build a number of unmanned spacecraft to explore the solar system beyond the orbit of Mars. CRAF and Cassini are the first two missions proposed in the series. MISSION OVERVIEW As currently planned, Cassini would be launched in April 1996 on an expendable rocket. NASA would provide the Titan-Centaur launch vehicle and the Cassini orbiter spacecraft; the European Space Agency (ESA) would contribute the Titan probe. NASA and ESA member countries would provide science instruments for both the orbiter and probe. In order to reach Saturn, Cassini will first execute flybys of the Earth and of Jupiter in order to gain "gravity assist" boosts in velocity to send it on its way. The first flyby of Earth will take place 26 months after launch, followed by the Jupiter flyby some 19 months later. Cassini would then arrive at Saturn in October 2002. During the first leg of its trip -- after launch and before its first Earth flyby -- Cassini will navigate through part of the asteroid belt and could perform an encounter with the asteroid Maja in March 1997. After flying by the Earth at a distance of 300 kilometers (about 190 miles), the spacecraft will be flung out through the asteroid belt where it may be possible to encounter another asteroid. Maja is a carbonaceous, or "C" type, asteroid 78 kilometers (about 50 miles) in diameter. Two small asteroids are being considered as possible additional targets (only one of the two asteroids could be visited). Cassini's final encounter before proceeding to Saturn will be with Jupiter, which it will pass at a distance of about 3.6 million kilometers (about 2.2 million miles), some 50 times the radius of Jupiter itself. Cassini's flight path will take it for 130 days down through a region that no spacecraft has explored more than briefly -- the giant planet's magnetotail, a long tube of Jupiter's energy field trailing many millions of miles away from the Sun. Upon reaching Saturn the spacecraft will swing within 1.8 Saturn radii of the planet to begin the first of some three dozen highly elliptical orbits during the remainder of its mission. Eighty-five days after reaching Saturn -- traveling back in toward the planet as it completes its first orbit -- Cassini will release its instrumented probe to begin descent to Titan. Eleven days later the probe will enter Titan's dense atmosphere, buffered at first by a heat shield. Throwing off that protection, the probe will then deploy its parachute for final descent to the surface. After relaying to Earth data uplinked from the Titan probe, Cassini will continue with orbits of Saturn and flybys of most of the planet's 16 or more other moons. Plans call for 40 targeted flybys including 36 close encounters of Titan, two of the major moon Iapetus, and one each of Enceladus and Dione. Cassini will make 26 non-targeted flybys of the Saturnian moons. In addition, the spacecraft's orbits will allow it to study Saturn's polar regions after examining the planet's equatorial zone. SATURN AND ITS MOONS: UNRESOLVED QUESTIONS Much has been learned about the ringed planet since Renaissance astronomers first turned their telescopes on it in the 17th century. Many questions remain, however, which could provide clues to how the solar system evolved and how life began on Earth. One of the bright planets known to the ancients, Saturn was first studied via telescope by Galileo Galilei (1564-1642) and, in the generation following him, by such European astronomers as the mission's namesake, Cassini. Glimpsing the planet's famous rings through an early telescope, Galileo first thought them to be two smaller planet-like bodies flanking the main planet Saturn; only observations by others with later, better telescopes revealed their true nature. The greatest wealth of information on the planet has come from unmanned spacecraft. Initially reconnoitered by NASA's Pioneer 11, the planet was encountered by Voyager 1 and Voyager 2 during flybys in 1980 and 1981, respectively. Those missions showed Saturn to be a giant gaseous globe of hydrogen and helium 95 times more massive than the Earth, buffeted by winds of up to 500 meters per second (1,100 miles per hour). The ring system proved to be much more complex than previously realized, with intricate "braiding" in some parts of the system. Small moons were also discovered by the spacecraft, bringing the total of Saturnian moons to at least 17. Because of the brief nature of those flybys, many questions remain for Cassini to explore. In studying Saturn's magnetosphere -- the pocket of energy enveloping the planet, much like Earth's radiation belts -- scientists hope to understand the configuration and dynamics of the magnetosphere; the nature, source and fate of its energy particles; how it interacts with the solar wind, satellites and rings; and how Titan interacts with the solar wind and Saturnian magnetosphere. At the planet itself, topics of study include cloud properties and composition of the atmosphere; winds and temperatures; Saturn's internal structure and rotation; the planet's ionosphere; and Saturn's origin and evolution. Science objectives in respect to Saturn's rings include studies of their structure and composition, dynamic processes, interrelations of the rings and Saturnian satellites, and the dust/micrometeoroid environment at the rings. Because of the dense atmosphere shrouding the moon Titan, little is known of its surface -- whether it is solid or covered with liquid oceans. At Titan, scientists hope to gain a better understanding of abundances of elements and compounds in its atmosphere; distribution of trace gases and aerosols; winds and temperatures; surface state and composition; and the satellite's upper atmosphere. Saturn's other major moons are ice-covered bodies. Scientists wish to use Cassini to study their characteristics and geological histories; how their surfaces are modified over time; composition and distribution of materials on their surfaces; overall composition and internal structure of the satellites; and how they interact with Saturn's magnetosphere. To carry out those studies, the Cassini orbiter will carry 15 science experiments, with nine more on the Titan probe. The orbiter will house several instruments on its directable, high-precision scan platform: a solid-state imaging system; ultraviolet spectrometer; near-infrared spectrometer; mid-far-infrared spectrometer; a microwave radiometer/spectrometer; and a high-speed photometer. Instruments on the orbiter's turntable platform are: a dust analyzer; energetic gas/hot plasma detector; a plasma/radio-wave spectrometer; and a plasma spectrometer. Located on a separate orbiter aeronomy platform will be a neutral/ion mass spectrometer and an ion analyzer/Langmuir probe. A magnetometer will be mounted on a separate boom. The orbiter's high-gain antenna will also be used for radar to map Titan through the satellite's atmospheric veil. The spacecraft's radio system will also be used for radio science experiments. The Titan probe's experiments include an imager/ spectrometer to relay photos as the probe descends; a radar altimeter; doppler tracking; a lightning/radio detector; a laser spectrometer/particle size counter; a gas chromato- graph/mass spectrometer; an aerosol collector/pyrolyzer; an atmospheric structure instrument; and a surface science package. THE ASTRONOMER IN HISTORY Gian Domenico Cassini -- or, in his adopted country, Jean-Dominique Cassini -- was born in Perinaldo, Italy, in 1625 and later settled in Paris. Before his death in 1712, Cassini became renowned for diverse work in astronomy, including the discovery of four of Saturn's major moons and a dark, narrow gap ("Cassini's Division") splitting the planet's rings. Interested in mathematics and astronomy as a youth, Cassini in his early 20s was invited by a rich marquis and amateur astronomer to work at the nobleman's observatory near Bologna, Italy. At the age of 25 he was named to the principal chair of astronomy at the University of Bologna. Alternating astronomy with hydrology studies solving problems of river flooding for the pope, Cassini began developing theories of planetary motion, observing comets and mapping solar eclipses. Through friendships with lensmakers, Cassini was able to obtain very powerful telescopes; by the time he was 40 he began observations of planets which cemented his fame. He studied the rotation rates of Jupiter and Mars, coming up with periods very close to those currently accepted. When he was 43, Cassini published a table of movements of the moons of Jupiter, which was used by astronomers and navigators for many years on expeditions around the world. By observing eclipses of Jupiter's moons from two distant sites, explorers could develop accurate longitudes of various points around the Earth. In 1669 Cassini accepted a generous offer to leave Italy and join the new Academie Royale des Sciences in Paris; four years later he became a French citizen, and when he was 49 he married the daughter of a French lieutenant general. Using new telescopes, Cassini made his most celebrated discoveries related to Saturn from 1671 to 1675. The planet's largest moon, Titan, had been discovered some 20 years previously by the Dutch scientist Christiaan Huygens. Cassini discovered Saturn's four other large moons, Iapetus, Rhea, Tethys and Dione. He also noticed changes in the brightness of Iapetus; to explain it, Cassini developed a theory that the satellite always turned the same face toward its parent planet. In 1675 Cassini discovered that Saturn's rings are split into two parts by a narrow division, which became named for him in the language of astronomy used today. He also put forth the notion, since proven, that the rings are not solid but rather are made up of huge numbers of small particles. Cassini's heirs were also influential in French astronomy. They included his son, Cassini II (Jacques Cassini, 1677-1756); grandson Cassini III (Cesar-Francois Cassini de Thury, 1714-1784); and great-grandson Cassini IV (Jean-Dominique Cassini, 1748-1845). ___________ At JPL, Ronald F. Draper is Mariner Mark II project manager. Charles Kohlhase is Cassini science and mission design manager. Linda Horn is Cassini deputy study scientist. Cassini is managed by JPL for NASA's Office of Space Science and Applications.