PASADENA, Calif. - NASA's Cassini spacecraft streaked by Saturn's smoggy moon Titan today, targeted to pass within just 750 miles of the planet-sized satellite to give scientists their first detailed glimpse of a world that, until now, has been shrouded in mystery.

Moving through space at some 14,000 mph, Cassini made its closest approach to Titan at 12:44 p.m. EDT, using the moon's gravity to change its trajectory slightly for another Titan flyby Dec. 13.

Today's encounter, the first of 45 Titan flybys planned over the course of Cassini's four-year primary mission, occurred while the $3 billion spacecraft was out of contact with flight controllers at the Jet Propulsion Laboratory in Pasadena, Calif.

The high-gain dish antenna normally used to communicate with Earth was aimed instead at Titan for cloud-piercing synthetic aperture radar observations. Those images, along with other radar data and high-resolution visible, infrared and ultraviolet observations, should resolve long standing questions about Titan, including whether the moon harbors standing lakes or pools of liquid ethane and hydrocarbon sludge.

But Cassini will not turn its high-gain antenna back toward Earth and begin playing back recorded data until late this evening. It will take those radio signals, traveling 186,000 miles per second, one hour and 14 minutes to reach NASA's Deep Space Network antennas some 826 million miles away.

The first low-resolution pictures are expected to begin showing up around 9:40 p.m. High-resolution imagery will reach Earth starting around 12:51 a.m. Wednesday, with narrow-angle resolutions of a tenth of a mile per pixel. That will improve to 17 feet per pixel when the highest resolution pictures flow in around 2:40 a.m.

Data playback will end at 5:22 a.m. and Cassini will make this orbit's closest approach to Saturn at 7:33 a.m.

A timeline of major events that includes the number of images expected from the narrow- and wide-angle cameras (in EDT; resolution in statute miles) is available here.

Along with collecting priceless imagery and data about Titan, today's encounter, known as Titan A or TA for short, also collected critical atmospheric data that will be used to determine just how close Cassini can safely pass during upcoming flybys.

That same data also will shed light on what Cassini's Huygens probe can expect when it slams into the atmosphere of Titan Jan. 14.

Built by the European Space Agency, Huygens will descend by parachute all the way to the moon's surface, using a suite of instruments to probe its environment. Data will be relayed back to Earth by Cassini, which will be flying past at the same time.

The density of Titan's atmosphere, however, is a critical factor in the Huygens' descent. The probe is scheduled to be released from Cassini on Christmas Eve and depending on what today's TA flyby data show, engineers could elect to make slight changes to its trajectory.

The data were considered so vital that engineers programmed playback through two DSN ground stations to ensure successful capture.

Along with characterizing the moon's atmosphere, Cassini also was programmed to photograph the Huygens landing site at a resolution of .62 miles per pixel, hopefully providing insights into what the probe can expect when it reaches the surface in January.

Cassini braked into orbit around Saturn the night of June 30, firing its main engine for a nerve-wracking 96.4 minutes. Another long rocket firing in late August raised the low point of Cassini's orbit and set the stage for an extended voyage of discovery.

Equipped with state-of-the-art telescopes, an imaging radar system and a battery of other powerful instruments, Cassini will spend at least four years orbiting the sixth planet from the sun, studying its rings in unprecedented detail, making high-resolution movies of its windy atmosphere, charting its magnetic field and mapping a host of icy moons.

Titan will get special treatment. Bigger than Mercury and Pluto, Titan is the only moon in the solar system with a thick atmosphere, one in which hydrocarbons fall as rain and liquid ethane pools on its ultra-cold surface. Or so astronomers believe.

TITAN FACTS AND FIGURES
   
Discovered by...........Christiaan Huygens, 1655
Mass (Earth=1)..........0.02259
Radius..................1,600 miles
Diameter................3,200 miles
Distance from Saturn....745,000 miles
Rotation period.........15.94 days
Orbital period..........15.94 days
Orbital inclination.....0.33 degrees
Atmospheric pressure....1.6 times Earth's
Temperature.............-290 Fahrenheit
Daylight at surface.....1/1000 the intensity
						  of sunlight on Earth

In a pre-launch news conference seven years ago, Jonathan Lunine, a University of Arizona physicist and a member of the Cassini science team, provided an educated guess about what today's flyby and the Huygens probe might reveal.

"Imagine a world that's smaller than Mars and bigger than the planet Mercury, where the air is four times denser at its surface than the air in this room and the surface pressure is about the same as you'd experience at the bottom of a neighborhood swimming pool," he said. "On that world, the distant sun is never seen and at high noon, things are no brighter than a partly moonlit night on the Earth.

"Because of its great distance, the cold is so enormous that water is always frozen out of the atmosphere. Nitrogen is nearly so, but not quite. And the simplest organic molecule, methane, is there to take the place of water as a cloud former, possibly a rain maker and maybe even the stuff of lakes or seas of hydrocarbons.

"The methane is lofted hundreds of miles above the surface of this world," Lunine said before Cassini's launch in 1997. "It's cracked open by sunlight and cosmic rays and a menagerie of more complicated organics is produced from the methane and these then float down to the surface to accumulate over time, perhaps to depths of hundreds of meters or more. Volcanism and impacts shape the surface and provide energy to make ever more complex organic molecules in a planet-wide tapestry that is an organic chemist's dream.

"What I have described to you is Titan, the second largest moon in the solar system, nearly the largest. It was partly revealed to us by Voyager 1 in 1980. Through its many instruments, Voyager discovered and characterized a dense atmosphere around this cold world. Yet ... Voyager's cameras could not penetrate the organic haze and so we still do not know what awaits Cassini-Huygens at the end of its journey."

But in the years since Cassini's launch, optical and radar observations from Earth have given scientists at least a hint of what the spacecraft might find. Scientists are convinced lakes or small oceans of liquid hydrocarbons exist on Titan, but not a globe-spanning sea. One way or the other, Cassini and Huygens should resolve the matter.

"Titan is almost certainly not the home of life today," Lunine said. "But the organic chemical cycles that go on may constitute a chemical laboratory for replaying some of the steps that led to life on Earth. Titan is in some ways the closest analogue we have to the Earth's environment before life began and this makes Titan very important."