After a trek of more than a decade, covering 6.5 billion kilometers, a mini lab called Philae separated on schedule from its mother ship Rosetta.
Philae was placed on a trajectory to land on 67P/Churyumov-Gerasimenko, a comet now more than 510 million kilometers from Earth and racing toward the Sun.
“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 European Space Agency (ESA) said.
Scientists faced a seven-hour wait to see if the bet comes off, their nerves stretched by a non-fatal, last-minute glitch.
Comets are believed to be clusters of primordial ice and carbon dust left over from the building of the Solar System. They are doomed to circle the Sun in orbits that can range from a few years to millennia.
A 100-kilogram robot lab, Philae was designed to piggyback comet “67P” and probe its chemistry and structure with 10 instruments.
“The die is cast,” ESA director general Jean-Jacques Dordain told journalists. “Philae has to land — we can’t do anything more about it.”
“Now, it’s down to the laws of physics. We’re on the way to the surface,” added 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 that was supposed to fire at the same time and provide a downward push, said Stephan Ulamec with the German aerospace firm DLR.
“We are going to have to depend entirely on the harpoons,” which depending on the surface can penetrate to 2.5 meters, Ulamec said.
The $1.6-billion mission was approved in 1993.
Rosetta, carrying Philae, was hoisted into space in 2004, and took more than a decade to reach its target in August this year, having used the gravitational pull of Earth and Mars as slingshots to build up speed.
Turning slowly around “67P” ever since, Rosetta has made some astonishing observations.
The comet’s profile somewhat resembles that of a rubber bath duck —but darker than the blackest coal, and a surface gnarled and battered by billions of years in space.
It has a treacherous, irregular surface, with crags, cliffs and rocks—an extremely difficult target to land on.
The big test will be for Philae to settle down safely as Rosetta and 67P zip towards the Sun at a breakneck 18 kilometers per second (11 miles per second).