The European Space Agency has landed a probe on a comet, a first in space exploration and the climax of a decade-long mission to get samples from what are the remnants of the birth of Earth's solar system.
The lander, named Philae, touched down at around 3.30pm this afternoon and confirmation of the landing came as expected around 30 minutes later.
The one-metre square robotic probe landed on the surface of 67P/Churyumov-Gerasimenko, situated 509,000,000km from Earth.
On landing, Philae was supposed to fire two harpoons into the ground, and screws on the end of its three legs were also supposed to wind into the surface.
However, the ESA has said the harpoons did not fire on contact as planned, perhaps because the lander touched down in a soft area of sand.
Stefan Ulamec, Lander Manager from German Aerospace Center DLR, said a subsequent fluctuation in the radio link may have been caused by the lander slowly lifting off the surface again and turning because the gravity is so slight.
He said two hours later the fluctuations stopped, implying the lander may have landed a second time.
Mr Ulamec said images and plenty of technical and scientific data has already been received from the lander.
As expected, the radio link between Philae and Rosetta was broken when the orbiter went beyond the lander's horizon, said Mr Ulamec.
More would be known when communications return on tomorrow, he added.
"We definitely confirm that the lander is on the surface," said Andrea Accomazzo, flight operations director this afternoon. "We can't be happier than what we we are now."
"This is a big step for human civilization," said the agency's director general, Jean-Jacques Dordain, as a crowd of scientists, guests and VIPs cheered and applauded in relief.
The ESA said Philae may have bounced slowly and landed a second time after its first touchdown on the comet.
The material that Philae analyses in the first contact of its kind will give insight into how Earth and other planets formed.
Comets are remnants of the formation of the 4.6 billion-year-old solar system.
Scientists believe they may have brought much of the water in today's oceans.
The launch went ahead this morning despite a problem with the thruster that was due to help stop the lander from bouncing back off the comet's surface.
"There were various problems with the preparation activities overnight but we have decided to go. Rosetta is lined up for separation," Paolo Ferri, ESA's head of mission operations, said before the launch.
The team had to release the three-legged lander at exactly the right time and speed because there is no way of controlling it on its descent.
After a period out of radio contact, mission control linked back up with both Rosetta and Philae as expected shortly after 11am, the ESA said.
Engineers designed the lander not knowing what type of terrain they would find on the comet's surface. Rosetta has been taking pictures of the comet and collecting samples from its atmosphere as it approaches the sun.
The surface is more dusty and porous than expected.
The probe needed to land somewhere not too dusty or dark, so that light can reach its solar panels and power its instruments once its batteries run out after two-and-a-half days.
It will complement studies already under way by Rosetta.
Philae, a dishwasher-sized lander packed with electronics, descended slowly down onto the comet. It includes experiments to test a molecule's symmetrical construction, or chirality.
Amino acids on Earth are 'left-handed,' while DNA and RNA are 'right-handed.'
Scientists are curious how the comet's samples compare.
Comet 67P/Churyumov-Gerasimenko is a 4km-wide sooty lump of ice and dust weighing 10 billion tonnes, left over from the birth of the solar system 4.6bn years ago.
Irish firms behind build
Two Irish companies were involved in the building of the craft. Space Technology Ireland Ltd, led by Prof Susan McKenna Lawlor, designed the Electrical Support System (ESS) processor unit.
When Philae is on the comet, this system will play a key role in passing the streams of commands and data between the Rosetta spacecraft and the instruments on the lander - making it "mission critical hardware".
Prof McKenna Lawlor is also on the project steering committee and is involved in one of the experiments on board Philae.
CAPTEC - a software development specialist company in Malahide - developed the Electronic Support System software for the Philae lander's interface with the satellite and also carried out validation and engineering support work on systems built by other companies.
Meanwhile, Laurence O'Rourke from Killucan Co Westmeath, is ESA Lander System Engineer and one of two science operations coordinators for the project.