A successful landing of the Philae probe on Comet 67P would mark the culmination of a mission that began with the launch of Rosetta 10 years ago.
The spacecraft finally caught up with the fast-moving comet in August after an epic four billion-mile journey that took it across the asteroid belt.
Information from the mission is expected to shed new light on the origin of the Solar System, the Earth and possibly even life.
#COMETLANDING: Full Coverage
European Space Agency (ESA) controllers will be holding their breath during the seven hours it will take for the spider-like probe to reach the surface of comet 67P/Churyumov-Gerasimenko, a 2.5 mile-wide lump of ice and dust hurtling through space at around 40,000mph.
Touchdown is expected at 4.02pm UK time but will not be confirmed until half an hour later. It will take that long for a radio signal from the lander to travel 316 million miles to Earth.
Before it does, here's a handy guide to the mission's history and some of its best moments so far: