It's the first time a man-made craft has touched down on a comet - and this groundbreaking moment could herald a torrent of fascinating data.
In its decade-long journey across the solar system, Rosetta has already gathered more data than has ever been collected before about a comet and its mission is expected to continue until at least December 2015.
Since it was launched in 2004, Rosetta has travelled four billion miles in its quest to find out whether comets could have sparked life on Earth.
If the landing probe Philae has successfully touched down at the Agilkia landing site on the surface of Comet Churyumov-Gerasimenko, it will begin to test samples for amino acids, which could show that similar comets once seeded Earth with the chemicals needed for life.
There will be considerable excitement among the scientific community back on Earth if they find left-handed amino acids, as these are the type that make up most of life on Earth.
It could show that life on Earth originated in space and Earth-like life is capable of existing on other planets.
John Plane, professor of atmospheric chemistry at the University of Leeds, said: "One of the great mysteries is whether life came from comets. And if these left-handed amino acids are found, then clearly these comets will be seeding other planets as well."
Another theory suggests that icy comets may have brought huge amounts of water to Earth during a period of bombardment four billion years ago.
As Rosetta approached the comet, scientists from the European Space Agency detected an unusual phenomenon broadcasting a low bubbling 'singing' thought to be created by the release of a stream of electrically charged particles as the body raced through space.
Once the initial battery power carried by Philae runs out, about 40 hours after landing, it will switch to rechargeable batteries powered by sunlight and could continue gathering data until March next year.
The comet is losing up to five litres of water a second, which blasts from its icy body. As the comet gets closer to the sun, the jets of water vapour grow ever more intense. If all goes well, Rosetta may deliberately fly through one of the jets next summer to gather more information.
For a moment it seemed that the whole world held its breath. Then the anxious flight directors glued to their monitors at the European Space Agency (ESA) in Darmstadt, Germany, gave the thumbs-up and everyone was on their feet applauding.