The Huygens probe has been released and is heading for Saturn's largest moon, Titan, scientists have confirmed.
Huygens will now coast for three weeks before entering Titan
A signal that the robot lab had separated from its mothership, Cassini, was received by the US space agency at its Jet Propulsion lab in Pasadena.
Huygens is in a sleep mode and will take three weeks to reach the smog-shrouded satellite of Saturn.
When it enters Titan's atmosphere, the probe will have just a few hours to collect data before its batteries die.
This is the length of time the 2.7m-wide probe will be in view of Cassini, the relay station through which the data on the moon's environment will be sent back to Earth.
Researchers will be delighted with any information the US-European robot can return on the thick nitrogen-rich "air" that surrounds Titan; but the jackpot prize will be to get pictures back from the surface.
Such is the chemistry and temperature (-180C) on Titan that scientists suspect it may harbour lakes, even great seas, of methane or ethane.
In which case, there is every possibility that Huygens will make a splashdown and take the very first extraterrestrial oceanographic measurements.
So far, all efforts to get detailed pictures of the moon's surface have been frustrated by a photochemical smog that hides the true nature of its landforms.
Even Cassini's remarkable instruments have struggled to get at the facts. Scientists can see dark and bright regions on the surface, but quite what they represent no-one is really sure.
"What we're looking at is just two-dimensional patterns," explained Dr Carolyn Porco, the Cassini imaging team leader.
"Some of us still think the black stuff may be the equivalent of an ocean, a solidified ocean or viscous material; but that is by no means a conclusion at this stage."
Ground controllers received confirmation at 0324 GMT that the 319kg robot lab had ejected from Cassini.
The probe was released at a gentle, relative speed of 30cm/s and at a spin rate of seven revolutions per minute, which will help stabilise the craft when it enters Titan's atmosphere.
That entry is scheduled to begin at just after 0900GMT on the 14 January - although it will be some hours later before scientists on Earth learn if the mission has been a success or a failure.
1. HASI - measures physical and electrical properties of Titan's atmosphere
2. GCMS - identifies and measures chemical species abundant in moon's 'air'
3. ACP - draws in and analyses atmospheric aerosol particles
4. DISR - images descent and investigates light levels
5. DWE - studies direction and strength of Titan's winds
6. SSP - determines physical properties of moon's surface
The Saturnian system is so far from Earth that it takes over an hour for signals to be sent back, even at light-speed.
For many of the British researchers working on the project, it is another anxious Christmas and New Year. Last December and January they were waiting for news of their ill-fated Martian lander, Beagle 2.
It was eventually declared lost after repeated attempts to contact the robot drew a blank. They think it was probably destroyed when it entered the Red Planet's atmosphere too fast.
This year, many of the same workers are back and keeping their fingers crossed that Huygens fares better.
"I should be absolutely ecstatic; I've been waiting for this moment, really, for 15 years," Open University Professor John Zarnecki told BBC News. "But for the first time in that period, I'm starting to feel nervous."
Professor Zarnecki is the Principal Investigator for the Surface Science Package on Huygens. His team could get the most amazing images and data of the entire mission - they could equally draw a complete blank.
His OU colleague Professor Colin Pillinger was the lead scientist on Beagle and is philosophical about the outcome.
"The guys who work on these missions do everything they possibly can; they check through everything," he said. "Let's hope it's not the season for repeats on television."
Launched on 15 October, 1997, Cassini-Huygens went into orbit around Saturn on 1 July this year after a voyage of 3.5 billion km (2.2 billion miles).
The $3.2bn mission is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).
The adventure has already produced some extraordinary pictures of the ringed planet and its moons - the images surpass anything previously obtained.
The two spacecraft components of the mission are named after 17th-Century astronomers who made the first clear observations of Saturn and its moons, Italian Jean-Dominique Cassini and Dutchman Christiaan Huygens.
Although the short life of Huygens' batteries means nothing will be heard from the probe after 14 January, the mothership Cassini's mission around the Saturnian system will continue for at least another three-and-a-half years.