Cassini spacecraft

Resources

On October 15,1997, the Cassini spacecraft began a seven-year, 2.175 billion mi (3.5 billion km) journey to explore from orbit the planet Saturn and its system of rings and moons. It left that day from Cape Canaveral, Florida, aboard a Titan-IVB/Centaur rocket. The robotic spacecraft, constructed by the European Space Agency (ESA), is 22.3 ft (6.8 m) long and 13.1 ft (4 m) wide. It arrived at Saturn on June 30, 2004. Cassini fired its main engine one day later in order to become captured by Saturn’s gravity. Upon taking its place within the orbit of Saturn, Cassini began sending back images and data to Earth. 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 released its Huygens Probe on December 25,2004. It coasted for 21 days and, then, descended by parachute through the thick atmosphere of Saturn’s moon Titan. Huygens carried six science instruments onboard in order to study the dynamics of Titan’s atmosphere and information on its surface. During its descent, its camera took more than 750 images.

Huygens’ five other instruments took samples of the atmosphere to determine its composition and structure. The probe landed on Titan on January 14, 2005.

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 planet Earth.

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 there 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 French-Italian astronomer Jean-Dominique Cassini (1625–1712). Cassini discovered the prominent gap in Saturn’s main rings (what is called the Cassini Division), as well as the icy moons Iapetus, Rhea, Dione, and Tethys. Dutch scientist Christiann Huygens (1629–1695) discovered Saturn’s rings and, in 1655, its largest moon Titan.

Cassini ’s launch was accompanied by political controversy and protest, as the probe carried 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 (32.7 kg), 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 its flyby.

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

During its tour of Saturn, Cassini will fly 74 orbits about the planet, 44 flybys of its moon Titan, and many flybys of the other icy moons of Saturn. The year 2006 has been a busy year for Cassini. As of October 2006, Cassini did a flyby of Titan on January 15, allowing scientists to see the Sun shining through the atmosphere of Titan. Two days later, it did a flyby of Rhea, which was followed up with another flyby on March 21 to observe its craters and topography. On February 27, the craft performed another Titan flyby, this time when Titan was near apoapsis (its farthest point from Saturn). On March 19, Cassini did another Titan flyby in which it bounced a signal off the surface and returned it to Earth and, then, sent a radio signal through the atmosphere of Titan to the Earth. In all, Cassini has performed numerous flybys of Titan and Rhea. It also performed flybys of Enceladus (one of the innermost moons of Saturn) and Methone (a tiny moon between Mimas and Enceladus).

The sophisticated instruments onboard Cassini and Huygens are continuing to provide important data and images of the Saturnian system. 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.

Resources

OTHER

Cowing, Keith. SpaceRef.com. Keck Observatory Provides Clearest Peek Yet of Titan’s Surface. <http://www.spaceref.com/news/viewnews.html?id=65> (accessed October 2, 2006).

CNN Interactive. Much Ado About Cassini’s Plutonium. <http://www.cnn.com/TECH/9710/10/cassini.advancer/> (accessed October 2, 2006).

European Space Agency. “Cassini-Huygens.” <http://www.esa.int/SPECIALS/Cassini-Huygens/> (accessed October 2, 2006).

Jet Propulsion Laboratory, California Institute of Technology. Cassini-Huygens Mission to Saturn and Titan. <http://saturn.jpl.nasa.gov/index.cfm> (accessed October 2, 2006).

Larry Gilman