a comet, starting a seven-hour descent that marks the most nail-biting phase of a ten-year mission.
Overcoming technical glitches, the European Space Agency craft jettisoned its lander on schedule at around 0400 ET to collect samples from the surface of 67P/Churyumov-Gerasimenko, which it has been orbiting since August.
"Now it's up to gravity to bring it down," Stefan Ulamec, Lander Manager from German Aerospace Center DLR, said.
The material that the lander, named Philae, analyses in the first contact of its kind will give insight into how Earth and other planets formed.
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.
The launch went ahead despite a problem with the thruster that was due to help stop the lander from bouncing back off the comet's surface, which means it may have to rely mainly on its harpoons to anchor it.
"There were various problems with the preparation activities overnight but we have decided to go. Rosetta is lined up for separation," Paolo Ferri, ESA's head of mission operations, said before the launch.
The team had to release the three-legged lander at exactly the right time and speed because there is no way of controlling it on its descent.
Images from deep space
After a period out of radio contact, mission control linked back up with both Rosetta and Philae as expected shortly after 0600 ET, the ESA said.
The probe is expected to touch down at around 1030 ET and confirmation of the landing is expected some 30 minutes later.
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.
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.