It will see the Rosetta satellite, which is currently orbiting the huge “ice mountain” known as 67P, drop a small robot from a height of 20km.
If all goes well, the lander will free-fall towards the comet, making contact with the surface somewhere in a 1km-wide zone at roughly 15:35 GMT.
The European Space Agency (Esa) says the challenges ahead are immense.
Imagine pushing a washing machine out the back of an airliner at twice cruising altitude and expecting it to hit Regent’s Park in London – all while the ground is moving underneath.
Although not really analogous for many reasons, this scenario does give a sense of the difficulties involved. The chances of failure are high.