Engineers at Caltech even had a betting pool going for what Huygens would find on touchdown, with options for “ice,” “tar,” “liquid,” “undeterminable,” “DOA,” and even a facetious vote for “eaten” — as in by sea monsters.
To conduct its science, Huygens was equipped with six main instruments, aimed to answer a slew of questions: What gases make up Titan’s atmosphere, and what kinds of particles, hazes, or clouds float there? What chemicals churn through the skies? How warm or cool is the atmosphere, and how do global wind patterns flow? What does the top of the atmosphere look like and how does it behave, including the ionosphere? And what are conditions like at the bottom of the atmosphere, just above or — fingers crossed — at the surface?
Huygens carried an aerosol collector to sample tiny particles floating through Titan’s skies and a spectrometer to analyze the gases making up the air it sailed through. Another instrument measured the physical properties of its surroundings: temperature, pressure, the probe’s speed, and how hard it might impact the ground or the rocking motion of Titan’s waves if it hit liquid. It could even measure how the winds pushed it across Titan’s skies. It carried a whole package of sensors to analyze whatever it landed on, be it liquid sea or solid ground.
And, of course, Huygens included a camera, to reveal what is still the most distant world on which humans have landed a spacecraft.
Can you hear me?
While the Huygens mission was a rousing success, it had two notable hiccups, both related to communications issues between the probe and Cassini, its relay to Earth.
In 2000, with Cassini well into its journey toward Saturn, an engineer took it upon himself to test the communications on the spacecraft. He pinged Cassini with a simulated message from the Huygens’ engineering model on Earth, hoping to receive it back. He did receive a response, but it was gibberish.
The flaw turned out to be in the way the receiver on Cassini handled Doppler shifting of signals it received. As the craft moved, any signals approaching it would shift in frequency, the same way a siren rises and falls in pitch as it speeds toward and away from a listener. Cassini’s receiver for communication with Huygens could not adjust for these changes. Worse, the receiver’s abilities were locked in, and Cassini was already hundreds of millions of miles away.
Luckily, the team had four years to work on a solution. If the receiver couldn’t handle Doppler shifting, then the team would avoid motions that caused such shifts. But this meant changing how the orbiter and probe would maneuver through the Saturn system. Instead of releasing Huygens on its first Titan pass, Cassini would now cart Huygens along for a few Titan flybys, slowing down with each pass until it could release the probe more gently, on a path that minimized the Doppler shifting of the probe’s transmissions. Another solution called for the probe to “wake up” earlier than initially planned after separating from Cassini, since the temperature of the instrument also influenced the signal.
Although these changes used up precious fuel (for Cassini) and battery life (for Huygens), the spacecraft had reserves of both. And they were well worth it to avoid Cassini relaying nothing but nonsense from Huygens’ precious stream of data. By the time Cassini arrived at Saturn, the new plan was well in place.
But this wasn’t the end of the mission’s communications problems. Only after Huygens was well into its descent at Titan did operators notice that only one of Cassini’s two channels was relaying information from the probe. Huygens was meant to send information over both of Cassini’s channels, Channel-A and Channel-B. But Cassini’s programming was missing a crucial command to turn on the Channel-A receiver.
While critical data was duplicated on both channels, and some other transmissions were eventually recovered directly by Earth-bound receivers, much other information, including half of Huygens’ images, was lost forever. Even so, the information that Huygens sent back was enough to take Titan from fuzzy orange ball to a fully realized world, in the span of only a few hours.
