Cassini-Huygens to land on Titan
The European probe Huygens is due to descend on the surface of Saturn's moon Titan on Friday, culminating a mission of more than seven years to explore one of Solar System's greatest enigmas.
Fitted with an array of ultra-sophisticated sensors, the 319-kilo (702-pound) craft was to plunge into Titan's atmosphere at a height of 1,270 kilometers (794 miles) at 1013 GMT, European Space Agency (ESA) Mission Control in Dermstadt said on Thursday.
For the next two and a half hours, initially protected by a heat shield and then slowed by successive deployment of three parachutes, the robot lab is to film Titan's surface, measure wind speed and pressure and analyse the atmosphere as it descended on the surface.
The data is to be relayed to an American orbiter Cassini, which carried Huygens in a seven-year trek from Earth to Saturn.
Cassini will then radio the information back to Earth, where it will be picked up by NASA's Deep Space Network of giant antennas.
At around 1234 GMT, Huygens is scheduled to make the farthest interplanetary touchdown ever attempted, meeting Titan's surface at about five metres (18 feet) per second, ESA said.
But so little is known about Titan that even on the eve of the long-awaited event, it was unclear as to when and where the site could be a hard surface of methane ice, or rock, or possibly a chemical sea.
Whatever the circumstances, the instruments are designed to carry on monitoring for another three minutes.
At 1444 GMT, about four and a half hours after the start, the circling Cassini will disappear beyond Huygens' transmission horizon. This is the end of Huygens' mission, for there is no more chance of receiving a signal even if the probe has continued to transmit beyond its designed life.
At 1524 GMT, if all goes well, the first data from Cassini should be received on Earth, says ESA.
The Cassini-Huygens mission is the most ambitious interplanetary exploration ever attempted.
Twenty years in planning and execution, 3.2 billion dollars in cost, it travelled 2.1 billion kilometers (1.3 billion miles) before reaching Saturn, the second largest planet of the Solar System, last July.
Cassini's four-year mission is to map Saturn and its leading satellites.
The interest in Titan comes from its strange, thick atmosphere, which is unique for a moon of the Solar System.
The atmosphere is believed to be mainly composed of nitrogen but also with abundant methane, a carbon-based chemical.
The suspicion is that Titan's atmosphere is undergoing the same type of chemical reactions that took place on the infant Earth, several billions years ago.
By understanding this process, scientists could shed light on the conditions that prevailed before life emerged on our planet.
But life -- or at least life as we understand it -- is unlikely to exist on Titan itself, given that it is so far from Sun, receiving negligible solar heat and light. The moon's surface temperature is estimated to be -180 C (- 292 F).