BioEd Online

Scientists have been celebrating throughout the weekend after the Huygens probe successfully landed on Titan, Saturn's largest moon, on 14 January.

After some 20 years of planning and a seven-year journey through the Solar System, the probe penetrated Titan's thick atmosphere, and landed safely on solid ground two hours later, at 12:34 GMT.

During its parachute flight, Huygens carried out several experiments to investigate Titan's atmosphere and surface. Photos have provided the first glimpse of the moon's surface, which is normally obscured by the atmosphere.

Signs of life

At the beginning of the day, scientists and officials involved in the mission looked tense as they waited in the control room of the European Space Operations Centre in Darmstadt, Germany, for signs of life from the probe.

Then at 10:40 GMT the news arrived. The Robert C. Byrd Green Bank Telescope of the US National Radio Astronomy Observatory in West Virginia had retrieved a carrier signal from Huygens at 10:25.

This meant that the most critical phase of diving into the atmosphere had been accomplished. In a brutal braking manoeuvre, Huygens had been slowed by Titan's atmosphere from more than 20,000 kilometres per hour to a tenth of that, a process that heated the front shield to 1,600 °C. The front and back shields had then detached, and the probe was transmitting a continuous signal.

The carrier signal identifies Huygens, but contains no data from the probe's instruments. So many questions remained. Would the scientific experiments be properly carried out during the two-hour parachute descent? And what would happen to Huygens after the impact on Titan?

Tension mounts

The nervousness increased when the moment for receiving data arrived. At 16:16 GMT, the signals bearing the scientific information should have finished their 1.2-billion-kilometre journey from Cassini to Earth. The eyes of the scientists and European Space Agency officials were glued to the computer screens in the control room. Two dozen television cameras and hundreds of journalists in the overcrowded press centre watched them, trying to judge from their reactions what might be happening.

The moment came and went. Then, at 16:19, the tension was broken. Scientists started jumping up and down, and Claudio Solazzo, Huygens' operations manager, fell into the arms of his colleagues.

Some minutes later, they arrived at the press centre to explain that Huygens' scientific instruments were working properly, and all the expected information was being retrieved.

"This means that all six experiments are getting good science," explained Jean-Pierre Lebreton, Huygens' mission manager.

"It is a historic event," added David Southwood, ESA's director of science, with a broken voice and tears in his eyes.

Alphonse Diaz, a NASA associate administrator of science later summarized his emotions, saying: "In the morning I was tense, at noon I was relieved, and in the evening I was ecstatic."

Several hours later, scientists rushed into the canteen to feed the crowd of journalists, who were finishing dinner, with more information. Marty Tomasco, from the Planetary Laboratory at the University of Arizona, Tucson, presented the first raw pictures of Titan's surface. In a view from about 16 kilometres up, drainage channels can been seen, which scientists say might have been shaped by liquid methane or ethane.

Another picture taken on the ground doesn't look particularly alien at all. It shows a distinctly Earth-like location, with flat ground strewn with icy blocks of about 20 centimetres across. "To see this on Titan gives you a feeling of connection," says Tomasco.

Cold facts

Scientists hope to learn about the early evolution of Earth from the experiments on Titan. The moon, which is the only one with an atmosphere in our Solar System, is about ten times farther from the Sun than Earth is, and the amount of energy that reaches it is about a hundred times less.

That means that physical and chemical processes have been happening much more slowly on Titan than on Earth, and scientists believe it may resemble a cold version of the Earth billions of years ago, before life began.

Detailed analysis of all the data retrieved by Huygens will occupy scientists involved in this mission for years to come, but John Zarnecki from the Open University in Milton Keynes, UK, and head of one of the probe's experiments, has already had the opportunity to present one unimpeachable success.

In a sweepstake of scientists involved in the mission, his prediction for the time of Huygens' impact on Titan was only seven seconds out. His prize, a bottle of scotch, was consumed by his team in the early hours of 15 January.