The European-made space probe Huygens headed Saturday for a historic up-close encounter with Titan, the only celestial body of the Solar system with an atmosphere resembling that of Earth, after successfully separating from the US spacecraft Cassini.
The mission may help prove, or disprove, a long-held hypothesis that Titan may even have rain as well as lakes and rivers, albeit containing not water by unknown muddy substances.
"The probe successfully detached from the Cassini orbiter," Rosemary Sullivant, a spokeswoman for the Jet Propulsion Laboratory in Pasadena, California, told reporters in a brief telephone interview.
"They received the signal, I think, at 7:24 (0324 GMT Saturday) this evening Pacific time," she said. "All systems performed as expected, and no problems were reported."
It will take Huygens 20 days to reach Titan. The European probe is scheduled to arrive at its destination on Jan. 14.
At separation time, specially designed springs gently pushed Huygens away from Cassini onto a ballistic four-million-km path toward Titan, officials said.
And then, it was on its way.
"We wish to congratulate our European partners as their journey begins and wish them well on their descent to Titan," Robert Mitchell, NASA's Cassini program manager, said in a statement.
"We are very excited to see the probe off and to have accomplished this part of our job."
David Southwood, director for science at the European Space Agency, thanked his American colleagues for their assistance in the project and expressed confidence in continued fruitful cooperation between the two sides.
"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," Southwood said.
The European probe will remain dormant until the on-board timer wakes it up shortly before it reaches Titan's upper atmosphere.
Then it will begin a dramatic plunge through Titan's murky atmosphere, tasting the chemical makeup and composition as it descends, with the help of parachutes, to touch down on the moon's surface.
The data gathered during this descent, which is expected to take two and a half hours, will be transmitted from the probe to the Cassini orbiter.
Cassini, in turn, is scheduled to perform Monday a deflection maneuver to keep it from following Huygens into Titan's atmosphere.
The move will also help establish the required geometry between the probe and the orbiter for radio communications during the European probe's descent.
Then the US orbiter will point its antenna to Earth and relay the data through the US National Aeronautics and Space Administration to the European Space Agency's operations center in Darmstadt, Germany.
Huygens will be the first human-made object to explore up-close the unique environment of Titan, whose chemistry is assumed to be very similar to that existed on early Earth before life formed.
Titan, the largest of Saturn's satellites, has intrigued astronomers for a long time.
Its atmosphere, like that of Earth, is composed mostly of nitrogen, yet appears to have few clouds. However it contains significant quantities of aerosols and organic compounds, including methane and ethane. At minus 180 degrees Celsius, the temperature on the Titan's surface is too cold for liquid water, and the atmospheric pressure is 1.6 times greater than that on Earth.