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European Huygens probe plunges towards Titan
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Arrow  On the morning of 25 December 2004, the European Space Agency’s Huygens probe was successfully released by NASA’s Cassini orbiter. It is now on a controlled collision course with Saturn’s largest and most intriguing moon, Titan.

Artist's impression of the Cassini-Huygens separation: Image © ESA
Artist's impression of the Cassini-Huygens separation: Image
“Today’s release is another successful milestone in the Cassini-Huygens odyssey,” said David Southwood, ESA’s Director of Science Programmes. “This was an amicable separation after seven years of living together. Our thanks to our partners at NASA for the lift. Each spacecraft will now continue on its own but we expect they’ll keep in touch to complete this amazing mission. Now all our hopes and expectations are focused on getting the first in situ data from a new world we’ve been dreaming of exploring for decades.”

Cassini relayed information about the separation to scientists on Earth, a signal that took one hour and eight minutes to cross 1.2 trillion kilometres.

Nearing the end

The Cassini-Huygens mission, jointly developed by NASA, ESA and ASI, the Italian Space Agency, began in October 1997, when the combined spacecraft lifted off from Cape Canaveral, Florida, atop a Titan 4B/Centaur rocket. Together, the two probes weighed 5548 kilograms at launch and represent the largest space mission ever sent to the outer planets.

To gain sufficient velocity to reach Saturn, Cassini-Huygens carried out four gravity-assist manoeuvres, flying by Venus twice, and by the Earth and Jupiter once each. On 1 July 2004, Cassini-Huygens became the first spacecraft to enter orbit around Saturn.

On 17 December, while on its third orbit around the ringed planet, the Cassini orbiter placed itself on a controlled collision course with Titan. Fine tuning of the trajectory took place on 22 December, as scheduled, placing Huygens on its nominal entry trajectory. Huygens will remain on this trajectory until it hits Titan’s atmosphere on 14 January 2005.

Fiery separation

The separation on 25 December was achieved with the help of some highly efficient pyrotechnic devices, along with special push-off springs, ramps and rollers. The playback signal from Cassini, containing telemetry data confirming the separation, was received by NASA’s ‘Deep Space Network’ stations in Madrid, Spain, and Goldstone, California.

The Huygens probe is now dormant and will remain so for 20 days as it coasts towards Titan. A triple-redundant timer will wake up the probe’s systems shortly before arrival on Titan.

Strange new world

Bigger than Mercury and slightly smaller than Mars, Titan is unique in that it has a thick hazy nitrogen-rich atmosphere, containing carbon-based compounds that could yield important clues to how the Earth came to be habitable. The chemical makeup of the atmosphere is thought to be very similar to that of Earth before life began, although much colder (-180°C) and therefore lacking liquid water.

Hence, in situ results from Huygens, combined with observations from repeated fly-bys of Titan by the Cassini orbiter, are expected to help us understand not only one of the most exotic members of our solar system but also the evolution of the early Earth's atmosphere and the mechanisms that led to life on our planet.

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