The attempt to land the Philae space probe on a comet is "the most difficult landing in space history" experts have said.
The landing - which is currently in progress - is the culmination of a ten-year, six billion kilometer journey by the Rosetta space craft.
If successful it could lead to important new insights into what comets are made of, how they were created and what role - if any - they may have played in creating the oceans (and life) here on Earth.
But it is by no means a sure thing that the landing will even take place - even as the hours tick down to the supposed launch.
Dr Matthew Genge, senior lecturer in Earth and planetary science at Imperial College London, said: "If today's landing goes well, Rosetta will tell us if comets made our planet blue by providing the oceans.
"[But] 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.
Genge said that the landing could prove to be a transformative moment in space history - but that its fate comes down to "the final few seconds".
"Did comets deliver the building blocks of living things and start life on Earth? We may soon know with the help of Rosetta," he said.
"Some scientists have spent 20 years working on this mission. The fate of all that work lies in the final few seconds of touch down."
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.