Scientists are at the start of a nerve-racking seven-hour wait before knowing if they have made history by landing a spacecraft on to a comet more than 300 million miles away.
The spider-like probe, Philae, parted company with its orbiting Rosetta mothership at 9.05am and is now descending to the surface of comet 67P/Churyumov-Gerasimenko.
It is due to touch down on the comet, a 2.5 mile-wide rugged lump of ice and dust hurtling through space at around 40,000 mph, just after 4pm.
Mission controllers will not know if they are successful until half an hour later. It will take that long for a radio signal from the lander to travel 316 million miles to Earth.
The European Space Agency (ESA) controllers have already received a radio signal from the descending lander relayed from Rosetta.
Dr Paolo Ferri, head of ESA mission operations, said: "This is a very, very important moment. It's established (that) the link is there ... Now we can sit down and wait for the initial data to come, like pictures, over the next five hours.
"Rosetta has confirmed that it is receiving data from the lander."
Assuming the probe does not crash or miss its target, it will be the first time any man-made object has made a controlled landing on a comet.
Nine years ago the US space agency Nasa's Deep Impact mission smashed a projectile intocomet Tempel 1 to study debris from the blast. Philae, in contrast, will float gently down at walking pace.
News of the successful separation was broadcast from the Rosetta mission control centre in Darmstadt, Germany.
Flight director Andrea Accomazzo said: "We've been living and flying together for 10 years now. Philae is on its way down to the comet. We can see it on telemetry."
The probe had travelled with Rosetta on a decade-long journey that took it across the asteroid belt, a distance of four billion miles.
Scientists had a heart-stopping moment earlier when the lander's active descent system, which uses thrust to prevent the craft bouncing off the comet's surface, could not be activated. But Philae also has two harpoons and ice screws attached to each of its three legs to keep it anchored down.
Philae lander manager Stephen Ulamec said: "The cold gas thruster on top of the lander does not appear to be working so we will have to rely fully on the harpoons at touchdown.
"We'll need some luck not to land on a boulder or a steep slope."
During the descent scientists have no control over the probe's trajectory, relying on automated systems, careful programming and the pull of gravity.
Part-way through the descent a radio link will be established to confirm that all is well. The lander may also snap a farewell image of Rosetta.
The separation involved a complex manoeuvre in which Rosetta effectively "dive-bombed" the comet to get the lander on target, before veering away.
Philae is due to land on the smaller of two lobes that make up the comet, giving it the appearance of a "rubber duck".
The chosen landing site, named Agilka after an island in the Nile, was the least hazardous of five possible candidates. The whole of the comet is covered in deep pits, towering cliffs and peaks, craters and boulders - some the size of houses.
Philae is equipped with cameras, a suite of 10 instruments, and a drill that can bore out samples to a depth of 23cm.
One British-led instrument, Ptolemy, will be used to analyse the composition of samples in the craft's on-board laboratory.
The lander is designed to collect data for just two-and-a-half days, but Rosetta will remain with the comet as it flies past the Sun and heats up, approaching as close as 118 million miles.
Scientists hope the £1 billion mission will yield valuable information about the origin of the Solar System, the Earth, and possibly life.
Comets bombarded the Earth early in its history, helping to fill the seas with water and depositing complex organic chemicals which may have contributed to the birth of living things.