GERMANY: Imagine a universe in which there is only one horse.We would never know how good it was without another horse to compare it with.Then a second horse appears - now we have a race.
For generations, modern science has faced such a dilemma in trying to understand planet Earth. The Blue Planet has no family bar two dysfunctional cousins, Mars and Venus. Mars gasps under a puny atmosphere while Venus is crushed by a gas envelope. Enter Titan.
When the Dutch/American astronomer Gerald Kuiper discovered an atmosphere on Titan, the focus for many climate researchers switched unexpectedly to an icy moon of Saturn 2 billion km from Earth. Titan's atmosphere is mostly nitrogen, like Earth's, and its surface density is half as much again as the atmospheric pressure here. It could be Earth's sibling, except that Titan is very cold and the chemistry that takes place in its atmosphere is strange and exotic. Thus the moon has become an unlikely Holy Grail for modern planetary science.
"I believe tomorrow's arrival at Titan could be more significant than the first Moon landing," said said Prof Richard Butler, head of the chemistry department at NUI Galway. "If we are to better understand our own planet's atmosphere, we need to understand a place like Titan."
What follows here is a timetable for the descent of Europe's Huygens probe through Titan's atmosphere. The times are so-called "Earth received time" (in Irish time), but spacecraft events occur 67 minutes earlier, as this is the time it takes for signals to reach Earth, travelling at the speed of light.
09.51 a.m - Huygens turns transmitters on. Huygens separated from the US Cassini spacecraft on Christmas Day and has been coasting towards Titan since. It is ready to relay data to Mission Control in Darmstadt, Germany, via Cassini which is still in orbit around Saturn.
10.13 a.m. - Huygens enters Titan's atmosphere. 1,270 km above the moon's surface, Huygens begins its plunge. Mission scientists were surprised recently to discover large amounts of complex petrochemicals at this high altitude, including benzene and acetylene - used on Earth in the manufacture of plastics. These complex substances can only be formed by the destruction of methane gas in a process that may closely mimic the destruction of ozone in the Earth's atmosphere.
10.14 a.m. - Peak heating. Huygens now glows hotter than the Sun's surface as it passes through Titan's unique haze layer. Recent pictures from Cassini showed a succession of thick cloud decks detached from the main atmosphere below. Speed fluctuations at this altitude will tell much about the composition and density of this mysterious haze layer.
10.16 a.m. - Parachute deploys. The main parachute, 8.3 m in diameter, deploys when Huygens detects it has slowed to 400 m per second, at about 180 km above Titan's surface. 42 seconds later, a battery of sensors begins an almost superhuman exploration of Titan in all five senses: they will touch, see, smell, taste and even hear the moon's atmosphere and petrochemical smog.
10.32 a.m. - Drogue parachute deploys. This is 3 m in diameter. At this level - 125 m in altitude - the main parachute would slow Huygens down so much that the batteries would not last for the descent to the surface. The onboard laboratory is now analysing some of the most enigmatic chemistry known to science. In the 1950s, researchers raised the possibility that Titan's atmosphere might closely resemble the "primordial soup" from which life emerged on Earth. But we now know Saturn's moon lacks a plentiful supply of life's vital ingredient: oxygen.
10.49 a.m. - Surface proximity sensor activated. At a height of 60 km, Huygens will detect its own altitude using radar. By now the cameras should see the surface, which remains an almost complete mystery.
10.56 a.m. - Possible icing of probe. The probe has been designed to withstand icing as it descends to 50 km above the surface, through the coldest part of the atmosphere. Researchers will scour the high-resolution panoramas for signs of meteorite craters.
12.30 p.m. - Descent imager/ spectral radiometer lamp turned on. Close to the surface, Huygens's camera instrument will turn on a light. The descent is expected to take 137 minutes, plus or minus 15 minutes. Throughout its descent, the spacecraft will continue to spin at a rate of 1 to 20 rotations per minute, allowing the camera and other instruments to see the panorama around the descending spacecraft.
12.34 p.m. - Surface touchdown. Huygens will hit the surface at a speed of 5 m per second - the equivalent of dropping it from a kitchen table. The probe could land on a hard surface of rock or ice or possibly splash down into an ethane sea. In either case, Huygens is designed to survive three minutes on the surface.
"Huygens is an atmospheric probe, not a lander," says the chief scientist of the European Space Agency, Dr David Southwood. "This mission will have been a success if we get information all the way through the descent. Surface science will be a terrific bonus."
Today, tomorrow and Saturday at the Discover Science and Engineering stand at ESAT Young Scientist Exhibition in the RDS, Discover Science and Engineering (www.science.ie) and STEPS will host a spectacular multimedia event to mark the arrival of Europe's Huygens probe at Saturn's cloud-draped moon called Titan.