The European Space Agency says its unmanned Rosetta probe has successfully released a lander toward the surface of comet 67P/Churyumov-Gerasimenko.
The 100-kilogram lander will take seven hours to reach the comet's icy surface.
If successful, it will be the first time that a spacecraft has landed on a comet.
Mission controllers clapped and embraced as the Philae lander's separation was confirmed on Wednesday.
ESA announced early Wednesday that the lander's active descent system, which uses thrust to prevent the craft from bouncing off the comet's surface, could not be activated. Instead, the agency relied on ice screws and a harpoon system to secure the lander.
A simulation shows Rosetta's deployment of the Philae lander, with Comet 67P/Churyumov-Gerasimenko in the background. (ESA)
"The cold gas thruster on top of the lander does not appear to be working so we will have to rely fully on the harpoons at touchdown," said Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center.
"We'll need some luck not to land on a boulder or a steep slope."
During the descent scientists will be powerless to do anything but watch, because the vast distance to Earth — 500 million kilometres — makes it impossible to send instructions in real time.
The plan is that Rosetta and Philae will then accompany the comet as it hurtles toward the sun and becomes increasingly active as it heats up. Using 21 different instruments they will collect data that scientists hope will help explain the origins of comets and other celestial bodies.
The European Space Agency says that even if the landing doesn't succeed, the 1.3 billion euro ($1.62 billion US) mission launched in 2004 won't be a failure. Rosetta will be able to perform 80 per cent of the mission on its own.
