Travelling at a speed relative to the sun of more than 66,000km/h , the Philae lander is now riding piggy-back on a 4km-wide lump of rock, ice and dust that some have likened to a rubber duck due to its odd, double-lobed shape.
Nine minutes after 3am, Rosetta's flight manager Andrea Accomazzo announced to the world that the probe had landed safely. "We see the lander sitting on the rock," Dr Accomazzo said.
Jean Jacques Dordain, the director general of ESA, quickly followed up with a comment on just how momentous was the achievement: "We are the first to do this - and that [achievement] will stay forever."
David Parker, chief executive of the UK Space Agency, told the waiting audience gathered at the European Space Operations Centre in Darmstadt, Germany: "Hollywood is good, but Rosetta is better."
ESA engineers said the landing was softer than expected judging from the 4cm depression of Philae's legs, but there was some concern over the failure of the probe's anchors to fire into the surface, suggesting the lander may not be fixed as tightly to the comet as ESA would have liked.
"Apparently the anchors didn't deploy, so there's a concern about the stability of the lander," warned an ESA engineer.
Initial data from the spacecraft indicated that it lifted off again, turned and then came to rest.
"Today we didn't just land once; we maybe even landed twice," said Stephan Ulamec, head of the lander operation.
Nevertheless, the mission's main goal of landing a probe softly on the surface of a distant comet appeared to have been a success.
"This is the most difficult landing in space history, like landing a balloon in a city centre on a windy day with your eyes closed," said Matthew Genge, lecturer in earth and planetary sciences at Imperial College London.
The day started with high drama as Philae failed to switch on properly during the preparations for its separation from Rosetta. ESA engineers solved it by the old trick of turning it off and on again.
The separation went as planned and mother ship and lander took parting shots of one another as they slowly drifted away. Philae finally landed just after 2.30am but it took another half hour for the probe to confirm its presence on the comet's surface.
Martin Barstow, professor of astrophysics and space science at the University of Leicester, said: "The Rosetta mission has been a tremendous adventure for ESA and the scientists involved. It has already proved to be a scientific success and promises to deliver much more over the next months and years."
The Philae lander will now travel with the comet as it continues its journey around the Sun. It will drill into the comet's surface to analyse samples of ancient dust and rock left after the creation of the Solar System.
The Rosetta will orbit the comet, taking measurements as the trio continue to orbit the Sun.
Following the mission closely are engineers at New Zealand-based tech company Rakon, whose special frequency-control technology is a crucial component of Rosetta's mission.
The company's crystal oscillators and crystal filters are used in Rosetta's Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT), which is designed to probe the comet's interior by using radio waves transmitted through the nucleus between the Philae lander and Rosetta orbiter.
One question Rosetta hopes to answer is whether comets contain the organic building blocks of life, Dr Genge said.
Q&A Rosetta's stone
How difficult was it to land on a comet?
Nothing like it had been attempted before. Nasa landed a probe on an asteroid in 2001, but comets are much more volatile destinations because they continually release dust and gas that can harm a spacecraft. Also, because this landing was taking place 500 million kilometres from Earth, the Rosetta spacecraft and the Philae lander it carried were designed to perform the landing autonomously. Once they received the go-ahead, there was nothing scientists at mission control in Darmstadt, Germany, could do to adjust the lander's course.
Paolo Ferri, head of mission operations for the space agency, said Philae's landing was almost perfectly on target - save one unplanned bounce.
Were there any problems with the landing?
Several. First, the lander failed to wake up properly. Scientists fixed that with a trick familiar to anyone who has used a computer. They rebooted it.
After the separation from Rosetta it became clear there was a problem with Philae's downward thrusters. They were designed to press the lander on to the surface during the final touchdown, to help it get a grip using harpoons and screws. The thruster failure wouldn't have been so dramatic if the harpoons had fired, but they didn't. So Philae bounced, once, before coming to rest again on the comet.
What data are scientists seeking?
Scientists have likened the trillion or so comets in our solar system to time capsules that remain virtually unchanged since the earliest moments of the universe. One of the things they are most excited about is the possibility that the mission might help confirm that comets brought the building blocks of life - organic matter and water - to Earth.
How will the lander collect data?
"The science starts the minute we get down to the ground," said Mark McCaughrean, a senior scientific adviser on the mission. Philae has 10 instruments on board - including devices to measure light, electrical magnetism and heat - and flying up above the comet the Rosetta spacecraft has another 11 instruments. The Philae lander will provide plenty of images of a world no human has seen close up and will drill below the surface to extract a sample of the comet that will be analysed onboard.
The lander's batteries are expected to last just 64 hours. But that should be enough for scientists to gather a huge wealth of data. In addition, the lander has a solar panel that should provide an hour's worth of battery life a day for up to five months.
What challenges lie ahead?
The comet is on a 6-year elliptical orbit around the sun. At the moment it is getting closer to our star. This makes for great science, since Philae and Rosetta will be able to observe the comet becoming more active.
As 67P approaches the sun, the amount of matter it sheds will greatly increase, posing a potential risk to the lander and even to the Rosetta spacecraft flying in tandem with the comet. As the comet reaches perihelion, the closest point to the sun, the rising temperatures could also damage Philae. But the lander should remain stuck to the comet forever, even after its systems have shut down. Rosetta will continue to fly alongside the comet until the end of its useful lifetime in about two years. Scientists have suggested a fitting end would be for it, too, to land on the comet.
- AP
