The European Space Agency wrote a new chapter in the history of space exploration Wednesday by landing a probe on the surface of a comet for the first time, marking the climax of a decade-long mission.

But the mission encountered a problem after the lander failed to deploy anchors to keep the craft tethered to the comet.

The Philae lander touched down on Comet 67P/Churyumov-Gerasimenko shortly after 1600 GMT, seven hours after separating from its mothership Rosetta around half a billion kilometres from Earth.

The agency's control centre in the western German city of Darmstadt erupted in celebration when the small robot sent its first signal from its destination.

"We're on the comet!" Andrea Accomazzo, the spacecraft operations manager in Darmstadt, announced to loud applause.

ESA director general Jean-Jacques Dordain hailed the development as "a big step for human civilization."

The landing elevates Europe to the major league of space exploration - an area previously dominated by the United States and Russia and in which China has recently emerged as a rising star.

Speaking at an event in Paris, French President Francois Hollande called the landing a "victory for Europe, progress and humanity."

Charles Bolden, the administrator of US space agency NASA, welcomed the European accomplishment as "a breakthrough moment in the exploration of our solar system and a milestone for international cooperation."

He noted three NASA instruments aboard the craft and said the US would build on the findings with its own planned mission to return a sample from an asteroid.

But the euphoria became tinged with concern after it emerged the lander had not deployed harpoons designed to anchor it to the comet's surface, possibly jeopardizing its work.

"I'm on the surface, but my harpoons did not fire. My team is hard at work now trying to determine why," ESA tweeted from its Philae account.

The use of the harpoons was seen as crucial to prevent the 100-kilogramme lander bouncing off the comet, which has extremely low gravity.

"As long as we're not certain that it is properly anchored, we can't deploy the drill (used by the probe to collect samples)," French astronomer Jean-Pierre Luminet, who has been following the landing for local media, told BFM TV.

Launched in 2004 at a cost of about 1 billion euros (1.24 billion dollars), the Rosetta mission aims to shed light on the origins of the solar system about 4.6 billion years ago.

Comets are primitive building blocks of the solar system and, as such, serve as its historical record.

By studying samples collected by Philae, the ESA aims to help determine, for example, whether comets supplied Earth with water.

Comet 67P/Churyumov-Gerasimenko is named after two Ukrainian astronomers who first spotted it in 1969. Klim Ivanovych Churyumov, one of its co-discoverers, was present in Darmstadt to witness the landing.

Rosetta went into orbit around the comet in August and will continue to accompany it for another year as it moves closer to the sun, to observe how the mass of frozen gases, rock and dust is transformed by the sun's warmth.

When a comet nears the sun it spews dust and gases that form a luminous tail that stretches for millions of kilometres and can be seen in the night sky.

Rosetta is named after the Rosetta stone, which helped to decipher Egyptian hieroglyphics almost 200 years ago.

PICTURE STORY OF SPACE HISTORY, 317 MILLION MILES AWAY

By Joachim Baier and Clare Byrne