The European Space Agency early on Wednesday gave the final go-ahead for the Rosetta spacecraft, which has been orbiting comet 67P/Churyumov-Gerasimenko since August, to jettison a lander to collect samples from the surface.
They will give insight into how Earth and other planets formed, because comets are remnants of the formation of the 4.6-billion-year-old solar system. Scientists believe they may have brought much of the water in today's oceans.
A problem with the thruster that was due to help stop the lander from bouncing back off the comet's surface means it may rely solely on its harpoons to anchor it to the surface.
"There were various problems with the preparation activities overnight but we have decided to go. Rosetta is lined up for separation," said Paolo Ferri, ESA's head of mission operations.
The team must make sure to release the three-legged lander, named Philae, at exactly the right time and speed because they have no way of controlling it on its seven-hour descent.
Separation of Philae from Rosetta is due to be confirmed on Earth at around 0900 GMT, with news of the landing due at 1600.
Engineers designed the lander not knowing what type of terrain they would find on the comet's surface. Rosetta has been taking pictures of the comet and collecting samples from its atmosphere as it approaches the sun, showing it is not as smooth as initially hoped, making landing tricky.
The surface is also more dusty and porous than expected and the comet gives off gases with a sulphuric, rotten-egg smell.
The probe needs to land somewhere not too dusty or dark, so that light can reach its solar panels and power its instruments once its batteries run out after two and a half days.
If it does manage a smooth touchdown, it will complement studies already under way by Rosetta.
Philae includes experiments to test a molecule's symmetrical construction, or chirality. Amino acids on Earth are 'left-handed,' while DNA and RNA are 'right-handed.' Scientists are curious how the comet's samples compare.
© Thomson Reuters 2014