In the most ambitious and expensive interplanetary space mission of all time, the Cassini-Huygens spacecraft made a seven-year trek across the Solar System to attempt first contact with the Earth-like moon of Titan by landing a probe on its unseen surface.
The first close up images of Saturn and its many moons were taken in the early 1980s by the Voyager One Deep Space Probe. One moon stood out from all the rest, the mysterious moon of Titan. Unlike any moon that had ever been seen, it had a thick almost Earth-like atmosphere. It was also shrouded in a thick orange haze which prevented Voyager from seeing down to the moon's surface. Scientists knew they had to go back.
Launched in 1997, the Cassini-Huygens spacecraft was the result of a unique transatlantic $3.2 billion collaboration between NASA and the European space agencies. Steered from NASA's JPL mission control in Pasadena California, the craft took seven years to reach Saturn. It took a long slingshot route via Venus twice, the Earth and Jupiter to pick up enough speed to reach its final destination.
When it finally arrived in July 2004, the spacecraft had to carry out a very dangerous manoeuvre and pass between Saturn's rings in order to get into orbit around the giant planet. Even the tiniest grain of dust could have ripped through the spacecraft and destroyed the mission.
On Christmas Day 2004, the European-built Huygens probe was finally released from the Cassini mothership, ready to descend to Titan. The probe's trajectory had to be absolutely spot on, as without any engines even a slight misjudgement could not be corrected later and would mean Huygens missing its target altogether.
January 14 2005. The Huygens probe finally reached Titan's upper atmosphere. Mission control had now transferred to ESA in Darmstardt, Germany, but all the scientists could do was sit and wait, as the probe was running on automatic. For any chance of success, the probe's heat shield had to protect the craft from the fierce temperatures of re-entry, and its three parachutes had to deploy correctly in sequence to slow its descent.
Amazingly, long before they expected to hear from Huygens, the probe's faint carrier signal was picked up on Earth by the massive Robert C Byrd radio telescope at Greenbank in West Virginia. Not much stronger than a mobile phone, and travelling over a billion kilometres through space, the signal was too weak to carry any real data, but at least they knew the probe had survived entry and was now under parachute.
Some hours later, the scientific data finally started coming through, relayed via the orbiting Cassini. To their horror, one of the vital data-streams had not been switched on. Fortunately most of the data was coming through on the single channel, but crucially half the images were lost.
After years of waiting, Titan was finally revealed. With Huygens built to sniff and taste the atmosphere on its way down, it discovered it was similar in many ways to that of the Earth in its infancy, four billion years ago. Titan's chemistry is still a long way from what we see as 'living', yet it was found to contain a rich cocktail of organic carbon-based chemicals, thought to be important as the precursors to life.
Now visible beneath the impenetrable orange haze, Titan appears to look a lot like Earth. The images beamed back from over a billion kilometres away show lake beds, river channels, gulleys and canyons. But these river channels are gouged not by water, but by a rain of liquid methane. The surface itself is not made of rock, but of solid ice, and Huygens' landing site was strewn with small round ice pebbles, lying in a bed of icy sand grains. Although home to a somewhat cold alien chemistry, in many respects Titan is driven by exactly the same geological and meteorological processes that shape and contour our own planet. Titan is certainly a place like home.
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