Huygens 'sees' Titan's pebbles
The European Space Agency (ESA) probe Huygens, which landed on the surface of Saturn's moon Titan in January 2005, has given an unexpected bonus to researchers on Earth - an appraisal of the size of stones and pebbles on the planet's surface. When Huygens was designed, it was not expected to survive the landing impact. However, the probe remained alert and transmitted data for a further 71 minutes after landing, until Cassini moved beyond Titan's horizon, cutting off the communication link. The new information is essentially an accident - but a very useful one. 'Huygens had not been designed to necessarily survive impact so we had never thought about what the signal would look like from the surface,' said Miguel Pérez-Ayúcar, a member of the Huygens Team at ESA's European Space Research and Technology Centre (ESTEC) in the Netherlands. The signal received by Cassini oscillated in an unexpected manner, fuelling speculation about little green men on this distant, freezing planet. Unfortunately, the real answer is more mundane, but no less interesting. The main signal sent by Huygens to Cassini was subject to interference from Huygens itself. The probe interfered with its own signal, as radio waves bounced between the probe's exterior and Titan's surface, mingling with the probe's direct signal to Cassini. As the angle between Huygens and Cassini narrowed, as Cassini orbited Titan, so the information changed. The interference signal gave high quality information about the terrain directly in front of Huygens, stretching from one metre when it landed, to two kilometres just prior to cut-off, 71 minutes later. To decode the data, the researchers first had to find out what the interference was. The team considered that perhaps Huygens was indeed interfering with its own signal, and ran a computer model to eke out the information. The resulting information was then compared to the famous 'orange desert' picture taken by Huygens when it landed, which pointed in the same direction as the beam, allowing a direct comparison. This piece of luck gave the researchers the chance to confirm their suspicions. They found that the surface of Titan is mostly flat, and covered with pebbles, between five and 10 centimetres in diameter. 'This is a real bonus to the mission. It requires no special equipment, just the usual communications subsystem,' explained Dr Pérez.' This accidental find will now almost certainly be incorporated into new missions. 'This experience can be inherited by any future lander,' says Dr Pérez, 'All that will be needed is a few refinements and it will become a powerful technique,' he says. But rather than rely on an accidental signal, the system can be refined, and could help deduce the chemical composition of a planet's surface.
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Netherlands, United States