Shortly before daybreak on the East Coast this morning, a pie-shaped robotic probe called Huygens was scheduled to dive into the smog-choked atmosphere of Saturn's moon, Titan.
If it survives, Huygens will become the first mechanical craft to touch down in the outer solar system. More importantly, it may answer a question that has tantalized astronomers for nearly 400 years: What lies beneath Titan's near-impenetrable haze?
Titan has long been a fascinating scientific target. More planet than moon, it's larger than Mercury and Pluto and one of only four solid bodies in the solar system with a substantial atmosphere (the others are Venus, Mars and Earth).
And it's not just any atmosphere. Composed mostly of nitrogen, spiked with methane and other hydrocarbons, Titan's skies are thought to resemble those of Earth before life took hold. Huygens, scientists say, could offer clues to how biology began on our own planet. But this thick chemical soup has also created an orange smog so thick it has stymied nearly all attempts to see what lies beneath.
Until Huygens. Built by the European and Italian space agencies and named for the Dutch astronomer who discovered Titan in 1655, Huygens could beam back the first images of the surface as early as this afternoon.
The 703-pound, 9-foot diameter probe has been drifting toward Titan since Christmas Eve, when it was sprung loose from NASA's Cassini spacecraft, now in orbit around Saturn after a seven-year journey to the ringed planet.
When it reached the moon's upper atmosphere early this morning, the heat-shielded probe was traveling at 13,500 mph. The journey to the frozen surface was expected to take nearly 2 1/2 hours and require the probe to deploy three braking parachutes.
Huygens is fitted with six sophisticated instruments, including sensors to measure the temperature and composition of the atmosphere and nature of the moon's winds.
But the instrument everybody will be watching is the Descent Imager and Spectral Radiometer, which consists of three digital cameras aimed up, down and sideways.
The cameras will start snapping pictures roughly 90 miles up and are designed to take 750 photos, many of which will be stitched together by scientists into sweeping panoramas of the ground and horizon, says the University of Arizona's Martin Tomasko, who is overseeing the cameras.
Even without having seen it up close, the one thing scientists do know is that Titan is a frozen world whose surface temperature hovers at a bitter minus 290 degrees Fahrenheit.
Over the years, scientists have theorized that the surface could be dotted with everything from tarry hydrocarbon sludge bogs and methane lakes to exotic "cryovolcanoes" that spew frozen slush rather than lava. On the way down, scientists also hope to see more familiar phenomena: clouds, snow or even lightning.
Whatever they find, it's likely to resemble something "out of a Lewis Carroll story," says planetary scientist Carolyn Porco.
Titan, she says, "gets curiouser and curiouser the more we look at it and the closer we get."
Although the probe will be arriving just after dawn on the moon, Tomasko says, it will look like dusk on Titan, where daytime light levels are just one-thousandth the brightness of Earth - thanks to the moon's distance from the sun and its gauzy atmosphere.
A big question is whether Huygens will land with a thud on granite-hard ice or splash down in one of the lakes of liquid methane that could theoretically exist on the moon's surface.
Although a liquid landing could speed Huygens' demise, most scientists are secretly hoping for a splashdown: Not only would that ensure Huygens' antennas are pointed in the right direction, it would also make scientific history by allowing scientists to conduct the first off-world oceanography.
The probe is designed to briefly float, measuring wave motion and liquid density as it bobs.
The mission will end either when the probe's batteries give out or when the Cassini spacecraft, which is relaying data from the orbiter, disappears over the horizon.
Even if Huygens fails, NASA still might be able to resolve some of Titan's mysteries. The moon is so scientifically important that Cassini is scheduled to swing by 45 times during the next four years, visiting it far more than any of Saturn's 30 other known satellites.