Cassini Probe to Fly by Saturn's Moon Titan Tuesday

John Roach
for National Geographic News
October 22, 2004
The Cassini spacecraft is set to buzz through the upper atmosphere of Saturn's moon Titan on Tuesday. If all goes according to plan, the probe will use high-tech cameras during the flyby to peer through Titan's hazy orange atmosphere and peek at the moon's mysterious surface.

"This is the first good look at Titan," said Dennis Matson, project scientist for the Cassini-Huygens Mission to Saturn and Titan at NASA's Jet Propulsion Laboratory in Pasadena, California.

Titan has long intrigued scientists. Its surface and atmosphere are thought to resemble that of Earth several billion years before life as we know it began pumping oxygen into the atmosphere.

Cassini came within 210,600 miles (339,000 kilometers) of Titan on July 2, days after it entered orbit around Saturn. The spacecraft will fly by Titan at an altitude of 746 miles (1,200 kilometers) on Tuesday, the first of more than 40 planned close encounters.

Scientists say that at such an altitude, the moon would appear as a featureless, fuzzy, beige ball to the naked eye, given the celestial body's hazy upper atmosphere and uniform cloud cover.

"However, Cassini is equipped with a suite of instruments that will allow us to peer through the atmosphere and reveal the surface," said Carl Murray, a member of the Cassini imaging team and professor of mathematics and astronomy at Queen Mary University of London in England.

The Cassini-Huygens mission is a joint project of NASA and the European Space Agency.

Images taken during the spacecraft's flyby will enable scientists to begin answering questions about Titan's surface, atmosphere, and chemical composition.

Does the moon have oceans and lakes of liquid methane? Is Titan covered with mountains of ice? Is the moon pockmarked with impact craters? Cassini-Huygens's July 2004 flyby only hinted at answers to such questions.

"At this point, it is way too early to tell what features we are seeing on the surface. But it doesn't look simply like a cratered surface," said Elizabeth Turtle, a planetary scientist at the University of Arizona in Tucson.

Revealing Images

Mission planners refer to the spacecraft's distant flyby of Titan last July as T0 (T-Zero). During that pass, special filters on Cassini's Imaging Science Subsystem (ISS) cameras revealed cloud cover around the moon's south pole. They also showed unexplained light and dark features on Titan's surface.

"From T-Zero we learned, if anything, Titan is mysterious and exotic," Matson said. "We see blurry boundaries between surface units of different reflection and color. We'd like to know what those units are made of and why the boundaries are blurry."

During Tuesday's flyby, Cassini will use a suite of remote-sensing instruments to peer through Titan's atmospheric haze and image its surface. Specialized instruments will gather data about the moon's surface composition, and radar will be used to begin mapping Titan's surface.

"This will certainly be the best resolution any instruments have had," Turtle said. "It will be a good test of how these instruments work close to Titan, how the atmosphere affects the instruments—and provide the first good look at the surface."

Turtle is particularly interested in any conclusive images of impact craters, which could reveal details of the moon's material properties and structure.

On January 15, 2005, the Huygens probe, which is currently piggybacking on Cassini, will detach itself and plunge through Titan's hazy atmosphere and land on the moon's surface. The proposed landing site will be imaged for the first time during Tuesday's flyby.

The information gleaned during the flyby will give scientists a sense of the images Huygens might be expected to capture later, according to Murray, the London mathematician and astronomer. He noted that the flyby will also provide a sketch of the terrain—an ocean, a crater, or mountains—the probe is likely to land in.

"This is really an alienlike world we are seeing for the first time," Murray said. "Uncovering Titan's mysteries will be an amazing accomplishment for Cassini and pave the way for the Huygens landing in January."

Atmospheric Drag

On Tuesday, Cassini will pass through the upper reaches of Titan's atmosphere. This will allow scientists to test, for the first time, their models of the moon's atmospheric density and structure, Turtle said.

Turtle said scientists expect the spacecraft to experience atmospheric drag similar to that felt by the International Space Station as it orbits Earth. During the Titan flyby, Cassini's thrusters will be turned on to react to any atmospheric effects.

According to Matson, a Cassini-Huygens project scientist, instruments on the spacecraft will collect data on the chemical composition and density of Titan's atmosphere. The composition is of interest to scientists who think it may be similar to that of Earth's before life evolved.

"From an engineering perspective, [the Tuesday flyby] gives us some direct measure from which to calibrate our atmospheric models that will tell us exactly how low it is to safely fly the spacecraft," Matson said.

On future flybys of Titan, the team hopes Cassini will come as close as 560 miles (900 kilometers) above the moon's surface.

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