The European Space Agency says it has succeeded in landing a spacecraft on a comet for the first time.
The successful landing capped a 4-billion-mile (6.4-billion-kilometer) journey by the European Space Agency's Rosetta spacecraft, launched a decade ago to study the 2.5-mile-wide (4-kilometer-wide) 67P/Churyumov-Gerasimenko comet. The Philae lander was the first spacecraft ever to land on a comet.
Scientists have likened the trillion or so comets in our solar system to time capsules that are virtually unchanged since the earliest moments of the universe.
"By studying one in enormous detail, we can hope to unlock the puzzle of all of the others," said Mark McCaughrean, a senior scientific advisor to the mission.
ESA controllers clapped and embraced at mission control in Darmstadt as they got confirmation that the unmanned Rosetta, racing through space in tandem with the comet at 41,000 mph (66,000 kph), had successfully released the 220-pound, washing-machine-sized Philae lander.
"Philae has gone. It's on its path down to the comet," Rosetta flight director Andrea Accomazzo said. "We are all glad that it worked flawlessly in the past minutes."
Philae was supposed to drift down to the comet and latch on using harpoons and ice screws. ESA announced hours before the release that a third component - an active descent system that uses thrust to prevent the lander from bouncing off the surface of the low-gravity comet - could not be activated. It wasn't clear how big of a setback that was.
"We'll need some luck not to land on a boulder or a steep slope," Ulamec said.
During the descent, scientists were powerless to do anything but watch, because the vast distance to Earth -- 311 million miles (500 million kilometers) -- makes it impossible to send instructions in real time. It takes more than 28 minutes for a command to reach Rosetta.
Two hours after the lander separated, scientists re-established contact with it.
"Now we can follow it on its descent," said Paolo Ferri, head of mission operations at ESA.
Rosetta, which was launched in 2004, had to slingshot three times around Earth and once around Mars before it could work up enough speed to chase down the comet, which it reached in August. Rosetta and the comet have been traveling in tandem ever since.
"The science starts the minute we get down to the ground," McCaughrean said.
Tantalizingly, the mission will also give researchers the opportunity to test the theory that comets brought organic matter and water to Earth billions of years ago, said Klim Churyumov, one of the two astronomers who discovered the comet in 1969.