Larger than the planet Mercury, Titan appeared to the Voyager probes in the 1980s as a mysterious yellow-orange globe, its surface hidden by its soupy methane atmosphere [see image, "Smoggy Sphere"]. Cassini is equipped to peer through those clouds with special camera filters and radar, but really getting up close and personal with this enigmatic world is the job of ESA's Huygens.

Launched from Cassini, Huygens will soon slam into Titan's atmosphere at 21 000 kilometers per hour and begin a one-way, two-and-a-half hour descent to the surface, slowed by parachutes. The lander is fitted with cameras pointing down and sideways, instruments designed to unlock the atmosphere's chemical secrets, and a microphone to pick up wind sounds. Investigators have speculated there might be seas of liquid methane and ethane on Titan, so Huygens has been designed to float. Although its batteries will be nearly exhausted by the time it finally reaches the surface, researchers hope it will be able to make a few measurements of the physical composition of the landing site [see illustrations, "Titan's Tiny Visitor"] and "Dicey Descent"].

Scientists believe the information gathered during the descent will open not only a window onto a mysterious world at the far end of the solar system but one onto the past as well, since Titan's atmosphere is believed to be similar to that of the primordial Earth.

Getting Huygens's once-in-a-lifetime readings and observations back to Earth is a two-stage process. Huygens is too small to be equipped with a radio transmitter powerful enough to reach Earth, so instead a receiver onboard Cassini will pick up Huygens's transmissions. With its powerful 4-meter main antenna, Cassini will then relay the data back to a small army of researchers, some of whom have been waiting decades for the insights they hope Huygens will provide.

When the Cassini-Huygens mission blasted off from Cape Canaveral in October 1997, no one suspected that a critical design flaw was lurking deep within the telemetry system onboard Cassini that was dedicated to harvesting Huygens's broadcast. Uncorrected, the flaw meant the data flowing from the hardy lander was in danger of being hopelessly scrambled, its seven-year odyssey across the solar system in vain.

"We have a technical term for what went wrong here," one of Huygens's principal investigators, John Zarnecki of Britain's Open University, would later explain to reporters: "It's called a cock-up."

But back in 1998, as Cassini was swinging past Venus and the Earth to build up speed for its run out to Saturn, Zarnecki and the other scientists and engineers at ESA and NASA were still blissfully unaware of any problem.

In fact, everything was working fine. The mission builders felt confident in their work: both the Cassini orbiter and the Huygens lander had been extensively tested on the ground, both separately and together. However, a proposal for a so-called full-up high-fidelity test of the radio link between the probes (where every system is subjected to a simulation of the exact signals and conditions it will experience during flight) had been rejected because it would have required disassembly of some of the communications components.