Cassini Spacecraft

In the fall of 1997, the Cassini spacecraft began a seven-year, 2.175 billion mi (3.5 billion km) journey to Saturn. The 22.3 ft (6.8 m) robotic spacecraft is still functioning perfectly. When it arrives on July 1, 2004, it will spend four years probing the Saturnian system. Cassini, the first spacecraft to visit Saturn since Voyager 2 swung past it in 1980 and the first spacecraft ever to take up orbit around Saturn, will observe Saturn's atmosphere, magnetic field, rings, and moons. Cassini will also release a small probe that will descend by parachute through the atmosphere of Saturn's moon Titan and impact on its surface.

Cassini bears a number of specialized cameras and other instruments. The Magnetosphere Imaging Instrument (MIMI), for example, designed by the Applied Physics Laboratory at Johns Hopkins University, will allow the first-ever imaging of a planet's magnetic field. MIMI will obtain images of the plasma and radiation surrounding Saturn and enveloping its moons and will observe the glow of Titan's exosphere (highest layer of atmosphere) caused by bombardment of high-speed protons trapped in Saturn's magnetic field.

Saturn's sixth moon, Titan—the second-largest moon in the solar system (larger than the planet Mercury) and one of only two moons in the solar system to have an atmosphere (the other is Neptune's moon Triton)—is of particular interest. Titan's atmosphere is so smoggy that observations of its surface have been limited, although images acquired by the Keck Observatory in Hawaii in 2001 at infrared wavelengths show that Titan has a blotchy surface. The dark parts, scientists theorize, may be pitch-black oceans of liquid hydrocarbons that are fed by methane rain falling on Titan's surface. Titan's surface chemistry may resemble that of a frigid, primordial Earth; if so, an understanding of Titan's atmosphere may help to understand the evolutionary origin of life on our planet.

A detailed study of Saturn's rings is another of the Cassini spacecraft's goals. Researchers hope that long-term, close-up observations of the planet's rings will help resolve whether they are material left over from Saturn's original formation or remnants of one or more moons shattered by comet or meteor strikes. This data should also help prove or disprove theories about the origin and evolution of the dust and gas from which the planets first formed. Researchers timed Cassini's arrival at Saturn so that the rings will be illuminated by sunlight. The tilt of the ring plane and resulting illumination angle will offer Cassini's instruments an excellent view of the ring disk.

The Cassini-Huygens mission, the result of an international collaboration between the U.S. National Aeronautics and Space Administration (NASA), the European Space Agency, the Italian Space Agency, and several other European academic and industrial partners, was named in honor of the seventeenth-century, French-Italian astronomer Jean Dominique Cassini. Cassini discovered the prominent gap in Saturn's main rings as well as the icy moons Iapetus, Rhea, Dione, and Tethys.

Cassini's launch was accompanied by political controversy and protest, as the probe carries 72 lb (34 kg) of plutonium with which to generate electric power during its seven years in space. All probes to the outer solar system have been powered by similar systems, as the dimness of the Sun's light at such distances makes the use of solar power difficult. Cassini caused unique worry because unlike earlier plutonium-carrying spacecraft, it was designed to follow a complex, looping path from the earth to Venus, the Sun, and past Earth again on its way to Saturn. There was concern that if the Cassini struck Earth by accident (as the Mars Climate Orbiter spacecraft struck Mars in 1999), its plutonium load might be vaporized in the atmosphere and constitute a global health hazard. (Plutonium has a half-life of 24,000 years and is highly carcinogenic; 72 lb [34kg], if divided into small particles and inhaled, would be sufficient to cause cancer in billions of people.) NASA disputed the protestors' claims. Whatever the merits of this controversy, Cassini did not crash into Earth during flyby and is now safely on its way to Saturn.

Cassini acquired valuable science data during its flyby of Jupiter on December 30, 2000, taking many photographs, measuring magnetic fields in collaboration with the Galileo spacecraft orbiting Jupiter, and taking advantage of Jupiter's gravity to boost itself toward Saturn at increased speed.

After the primary mission to study Saturn and its moons is finished in 2008, Cassini may fly closer to Saturn and pass inside the G ring while evading the regions known to contain a high density of potentially damaging ring particles. The spacecraft may also be sent into orbit around Titan to make a closer study.