Speeding towards the Sun at 65,000km/h, a lab called Philae will detach from its mother ship Rosetta and head for a deep-space rendezvous laden with risk.

The 100kg probe will seek out a minuscule landing site on the treacherous surface of an object darker than coal, half a billion kilometres from home.

"It's not going to be an easy business," was the understated prediction of Philippe Gaudon of France's National Centre for Space (CNES) as the mission prepared to enter countdown mode.

The stakes facing Rosetta managers in Darmstadt, Germany are daunting as the 1.3 billion euro ($A1.95 billion) project reaches a peak.

Two decades of work have been poured into what could be a crowning moment in space exploration.

The goal: the first laboratory research into the primeval matter of the Solar System - ancient ice and dust that, some experts believe, may have helped to sow life on Earth itself.

According to this theory, comets pounded the fledgling Earth 4.6 billion years ago, providing it with complex organic carbon molecules and precious water.

Rosetta has already sent home fascinating data on the comet, but Philae will provide the first on-the-ground assessment, using 10 instruments to study the comet's physical and chemical composition.

Like Rosetta, it will wield a mass spectrometer, a high-tech tool to analyse a sample's chemical signature, aimed at drawing up a complete carbon inventory.

The showstopper find would be molecules known as left-handed amino acids, the European Space Agency (ESA) says.

"These are the bricks with which all proteins on Earth are built," it says.

But getting Philae into position will be a white-knuckle ride.

After its launch in 2004, Rosetta spent 10 years zig-zagging around Earth and Mars, using the planets' gravitational pull as a slingshot to build up speed to reach its prey, Comet 67P/Churyumov-Gerasimenko.

No one knows what a comet's surface is like.

Is it hard and crusty, like a shell? Crumbly? Slippery? Is it brittle - will it crack, causing Philae to sink into some fudgy or spongey substance below?

Seeking to cover all the possibilities, Philae's designers have equipped the lander with three outstretched legs designed to dampen the impact.

AFP