February 22, 2005
Titan Rising, Part II
Just over a month ago, the European Space Agency's Huygens probe descended through the atmosphere of Saturn's giant moon Titan. The probe sent back stunning close-up images of a world never before seen is such detail. In this, the second of a two-part series, science reporter Michael Benson shares his impressions of the event from his front-row seat at ESA's control center in Darmstadt, Germany.
Astrobiology Magazine -- Saturn, and therefore Titan, is more than nine times the distance from the Earth to the Sun. This is very far away. To give it some perspective, consider that the International Space Station orbits at an altitude of only 400 kilometers - about what the family car could do in a day's drive if it could go straight up (to paraphrase Timothy Ferris paraphrasing Isaac Asimov). By contrast, it would take about a hundred and seventy five million years to drive to Saturn, even at a brisk autobahn speed of 200 kph. It's not surprising, then, that it took Cassini-Huygens almost seven years to get there - at the speed of a rifle bullet.
Because it was supposed to swing and spin under its parachutes - at least, if everything worked right - Huygens' transmitter used an omni-directional antenna. And because the whole probe was powered by only five relatively small batteries, that transmitter was no stronger than a typical cell-phone. In fact Huygens wasn't designed to uplink to Earth directly, but exclusively via nearby Cassini, which was supposed to record the transmission and then send it to Earth later, after Huygens' short Titan mission was over. So how to receive that incredibly distant and weak signal on Earth?
To begin with, only the fact of a signal could ever be heard from Earth - not an actual data stream. The point of listening from as far away as Earth was simply to know that the probe had managed to deploy its parachutes and start its mission (rather than plunging, like ESA's recent ill-fated "Beagle" Mars probe, to its death). Although that transmission wouldn't be strong enough to decode its zeroes and ones from this great a distance, it would still be a binary signal. That's because there would either be a big fat zero (i.e., the mission had failed); or a signal (meaning everything was going well).
So far the sequence had gone like this: on Christmas day, 2004, Huygens sprang free, spinning away from Cassini. It then coasted silently on a collision course with Titan, conserving energy while four internal clocks counted down the hours and minutes. (Cassini, meanwhile, used its rockets to avoid collision with Titan and to pass near the moon instead, with its high-gain antenna focused on the area where Huygens was supposed to go in.) A few hours before entry, Huygens was programmed to wake up and start heating its electronics. Twenty minutes before the key hoped-for moment, Huygens should have plowed into Titan's atmosphere, creating a fire-ball that converted its speed into energy while the spacecraft slowed down. And at any minute, the probe was supposed to ditch its smoking heat shield, deploy the first of a series of three parachutes, pop the cap off its three vertically-stacked lenses, activate its six science sensors, and start communicating.
The finger-length ruby receiver prongs at the focal point of Greenbank's dish are cryogenically cooled to a temperature at the outskirts of absolute zero. This is done by placing a freezer tank within a freezer tank within a freezer tank, matrioshka doll style. That temperature helps the prongs differentiate between the spurious random noise of their surrounding electronics and faint, distant signals from space. And the almost infinitesimally faint vibrations focused by that dish onto oscillating ruby are further processed by extremely advanced electronics, further shifting the signal-to-noise ratio in favor of signal.
Shortly after 11:20 AM CET, Greenbank's ruby prongs started to vibrate. By 11:24, the receiver supplied by NASA's Jet Propulsion Laboratory managed to differentiate a signal from the surrounding hiss of space and extraneous electronic fields; and at 11:25, a technician in Greenbank turned to face the webcam and held up a hard-copy print of the crescent-shaped wave-form. "We have it?" blurted a Huygens engineer. "We have it!"
An eruption of cheering. Tears were visible in the eyes of grown men who had worked towards this day for two decades.
If we're to believe what we are told - which means, if we're to behave the way we were taught to behave - then we can easily understand the pragmatic reasons why the Huygens mission to Titan was important. It was important because Titan is a deep-frozen analogue to the early Earth, with all the requisite precursor chemistry for life. It was important because Titan's dense atmosphere is thought to produce a rainfall of "de-icer" fluids - methane and ethane - onto a surface with an average temperature of minus 180 Celsius (minus 292 Fahrenheit), which despite the frightening cold may possess petrochemical lakes and even oceans; Titan's weather could therefore teach us about our own world and its weather. It was important because it gave us a wider knowledge of the total picture of the solar system, and therefore helped us to understand how planets and moons formed around this particular star in the first place - a process eventually producing life, and us. It was important for all these and many other reasons. Still, somehow that missed it.
By Friday afternoon, all of the Huygens data had been received by Cassini and the NASA spacecraft had turned to face the Earth and transmit it. By Friday night the first pictures of Titan had been received. In blurry black and white, they revealed what appears to be a soggy, boggy landscape of drainage channels leading to the edge of a dark flat area that looked so much like a coastline that it was impossible to characterize it as anything else. Is that indistinct dark flatness a lake, or even an ocean? Or could it be the exposed bed of a lake or ocean? It was impossible to say; the data was so fresh that the scientists had barely had a chance to evaluate it (they later said it was most likely a still-moist lake-bed). Whatever its exact meaning, nothing quite like it had ever been seen before by a robot dispatched from our shores.
The motion of the probe under its parachute carried Huygens over the "coastline," and later pictures reveal the same blurry and vague landscape from an oblique angle, as though from an airliner. It looked oddly familiar - a picture, say from a cellphone camera, of a moonlit coast on Earth. In these images, some linear white features had formed at the edge of the dark flat area. Waves? Ground fog? Both? Despite the radical improvement in resolution made possible by actually penetrating through Titan's haze with a package of sensors, rather than peering from a distance during fly-bys of the moon from Saturn orbit (something that the Cassini Orbiter will do some 40 more times in the next 3 years), Huygens' pictures were frustratingly indistinct. Meanwhile other instruments had measured, in microseconds, the pace and texture of Huygens' abrupt deceleration when it finally impacted the ground, two and a half hours after it dropped its heat shield and deployed its first parachute. The surface wasn't exactly dry, but Huygens didn't land in a liquid either. According to John Zarnecki, the principle investigator for Huygens' Surface Science Package, the ground appeared to be similar to wet sand or clay, or even "crme brule" - a comment immediately wired triumphantly around the globe.
So why was the European mission to Titan important? In an engineering achievement unimaginable to the ancients, humanity glimpsed a world beyond the world. It is a place possessing an eerily Earth-like topography evidently carved by ethane rivers, lakes and even oceans. True, the landing may be only one stop on a road with no apparent end: this is science, where the path is the destination. By Saturday morning, the Huygens imaging team had unveiled some picture mosaics which improved the over-all resolution somewhat and revealed that same mysterious shore-line, now looming in a wider context of murky orange-brown land-forms.
The chirpy PR professional had thankfully long vanished, and ESA science director David Southwood stood in front of the auditorium, a gray-haired Brit with a shaggy moustache and an exultant glitter in his eye. He had been repeatedly asked, he said, to describe what it felt like to succeed in this decades-old mission. What was it like to see Titan for the first time? "I'm just a scientist-*****-apparatchik administrator," he said, to laughter. "When in doubt, turn to the poets." He quoted Keats' poem, "On First Looking into Chapman's Homer":
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez when with eagle eyes
He star'd at the Pacific--and all his men
Look'd at each other with a wild surmise"¦
Later, after comparing the fog and apparent rain of gloomy Titan to the city of Manchester, Southwood continued more seriously. "Science is more like a detective story than you would imagine," he said. "You're continually piecing together clues. I've always felt I was Hercule Poirot. Hercule Poirot never really finds the murderer. You know, you're continually looking at how things fit together, and thinking always of different scenarios. Very rarely do you get the eureka' moment, when you declare that the butler did it. You don't get an absolute truth, I genuinely believe this." And ESA's science director laughed in pure enjoyment.
Read...Titan Rising, Part I
Listen to sounds from the microphone onboard the Huygens during its descent (wav file format, approx. 600 kB each):
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