Landing on a comet: Philae probe begins slow descent

It's now a tense wait for scientists in Europe as robot lab Philae separated from mothership Rosetta Wednesday and began a crucial 7-hour descent to land on a comet for the first time in history.

This-image-obtained-from-ESA-on-November-10-shows-the-Agilkia-landing-site-on-this-image-of-Comet-67P-Churyumov-Gerasimenko-taken-with-Rosetta-s-navigation-camera-on-6-November-AFP-Photo-ESA-Rosetta-NavCam-HANDOUT

It's now a tense wait for scientists in Europe as robot lab Philae separated from mothership Rosetta on Wednesday and began a crucial 7-hour descent to land on a comet for the first time in history.

Half a billion kilometres from Earth, it is now descending towards Comet 67P/Churyumov-Gerasimenko, racing towards the Sun.

If all goes well, Philae will touch down in a few hours before carrying out scientific experiments on its surface - a double first in space history.

'Landing is the most critical moment of all,' said Stephan Ulamec, in charge of Philae at the German aerospace firm DLR.

'Everyone's nervous, everyone's on tenterhooks, but we know the risk is worth taking. The rewards are enormous,' ESA senior science advisor Mark McCaughrean said.

'You won't get anything without taking risks. Exploration is all about going to the limits.'

'We know that it's a risky business, but the potential gain is so great,' said Jean-Pierre Bibring, in charge of Philae's scientific operations.

All is in order so I'm GO for separation from @ESA_Rosetta! Just a few hours left to #CometLanding

— Philae Lander (@Philae2014) November 12, 2014

And now, my little buddy @Philae2014 is GO for separation and #comelanding :-) pic.twitter.com/nvltoAKkbU

— ESA Rosetta Mission (@ESA_Rosetta) November 12, 2014

I'm so ready! MT @ESA_Rosetta: Imagining the final stages of descent to the landing site: http://t.co/hPcWYrrzj0#CometLanding#67P

— Philae Lander (@Philae2014) November 9, 2014

Philae is meant to settle down at a gentle 3.5 km per hour, firing two harpoons into a surface that engineers fervently hope will provide enough grip.

Ice screws at the end of its three gangly legs will then be driven into the low-gravity comet to stop the probe bouncing back into space.

A confirmation signal about touchdown is expected on Earth at about 1600 GMT would have it in association with the NCP.

'Conceived in the 1980s, the Rosetta mission seeks to reveal secrets about the origins of the Solar System and maybe even life on Earth.

The 1.3-billion-euro ($1.6-billion) project was approved in 1993. Rosetta, carrying Philae, was hoisted into space in 2004, but needed more than a decade to reach its target in August this year -- a six-billion-kilometre (3.75-billion-mile) trek around the inner Solar System.

Turning slowly around '67P' since its rendezvous, Rosetta has made some astonishing observations.

The comet 67P/Churyumov-Gerasimenko somewhat resembles that of a rubber bath duck -- but a 'toy' darker than the blackest coal, and gnarled and battered from billions of years in space.

It has a treacherous, irregular surface, with crags, cliffs and rocks -- an extremely difficult target to land on..

Any error in its course will widen during descent -- the probe could miss its landing site and smash into rocks or cliffs nearby.

Comets are believed to be balls of primordial ice and carbon dust left over from the building of the Solar System.

Some scientists theorise they may have 'seeded' Earth 4.6 billion years ago with life-giving carbon molecules and water paving the way for life to evolve.

Video: Philae probe begins 7-hour descent to land on comet