Successful Flyby Exposes Titan

PLANETARY EXPLORATION

The Saturn-orbiting Cassini spacecraft made its first close-up flyby of the giant moon Titan on Oct. 26. Finally, scientists should be able to answer a long-standing question: What covers Titan's surface? Lakes of methane and ethane? Or mixtures of ice and liquid?

Titan is the largest of Saturn's 31 moons and second in size only to Jupiter's moon Ganymede. Titan is bigger than the planets Mercury and Pluto and is the only moon with an atmosphere. That atmosphere, composed of nitrogen and a telescopically impenetrable methane haze, has frustrated astronomers, who hadn't been able to peer through it to glimpse Titan's surface. But recent images from more powerful ground telescopes and from Cassini--including one taken by the craft on Oct. 24--show patches of bright and dark material, which could be ice and liquid.

During the flyby, Cassini skimmed the tops of Titan's clouds, only 745 miles above the surface at its closest approach. Eleven of its 12 instruments were trained toward Titan, including an ion and neutral mass spectrometer designed to take the first-ever sample of Titan's atmosphere. A radar-imaging instrument pierced the atmosphere, and NASA hopes that it will reveal the surface topology.

Atmospheric information from the flyby will also help scientists ready Cassini for releasing the Huygens probe to the surface of Titan on Dec. 25. Huygens, which is managed by the European Space Agency, will plunge through the atmosphere and, it is hoped, land successfully.

Conditions on the surface of Titan are believed to be similar to those faced by early life on Earth. Whether Titan's environment could produce molecules that are precursors to life has been a mystery. Instruments on board Huygens will help clarify that picture. For example, a gas chromatograph and mass spectrometer will be able to identify organic molecules of low- to midrange mass.

During its four-year mission in orbit around Saturn, Cassini will perform 45 Titan flybys, some of them even closer than this first.

With a price tag of $3 billion, Cassini is the most elaborate and most expensive robotic interplanetary mission NASA has ever undertaken. According to the Jet Propulsion Laboratory in Pasadena, Calif., which manages the mission, the craft has been performing flawlessly since its June 30, hair-raising "orbit insertion," in which the craft slid through gaps in Saturn's rings to orient itself around the planet.