The European probe Philae on Wednesday was on course to make the first-ever landing on a comet after separating from its mother ship, mission control said.
Philae ejected at around 3:35 a.m. ET (08:35 GMT), said Andrea Accomazzo, flight operations director at the European Space Agency (ESA).
“We see it in telemetry,” he said, amid cheers and relieved applause from members of the team.
“The Philae lander has separated from the Rosetta orbiter, and is now on its way to becoming the first spacecraft to touch down on a comet,” the agency said.
Touchdown by the 100-kilogram (220-pound) robot lab is expected around seven hours later.
A robot lab bearing 10 instruments, Philae is designed to carry out experiments on Comet 67P/Churyumov-Gerasimenko, racing towards the Sun at a distance of more than 500 million kilometres (300 million miles) from Earth.
Comets are believed to be pristine clusters of ice and carbon dust, holding secrets on the origins of the Solar System — and possibly of life on Earth — 4.6 billion years ago.
If all goes well, Earth will receive a signal at about 11:00 a.m. ET (1600 GMT) saying Philae has landed.
“Now, it’s down to the laws of physics. We’re on the way to the surface,” said ESA’s senior science advisor, Mark McCaughrean.
“I don’t have finger nails, so I won’t be biting them,” he quipped.
Philae is meant to settle down at a gentle 3.5 kilometers per hour, firing two harpoons into a surface that engineers fervently hope will provide enough grip.
Ice screws at the end of its three legs will be driven into the low-gravity comet to stop the probe bouncing back into space
A final check found an apparent malfunction with a small gas thruster on top of Philae which is supposed to fire at the same time, providing a downward push, said Stephan Ulamec with the German aerospace firm DLR.
“We are going to have to depend entirely on the harpoons,” which if the surface is right can penetrate to a depth of 2.5 meters (more than eight feet), Ulamec said.