After more than 10 years chasing a comet through deep space, the spacecraft Rosetta will on Wednesday night attempt one of its greatest feats - to drop a small lander on the comet's surface.
Since August the European Space Agency's Rosetta has been orbiting comet 67P/Churyumov-Gerasimenko, inching ever closer to its surface for the landing.
Just after 7.30pm (AEST) the probe will eject its washing-machine sized lander Philae, having received its final set of signal commands from ground control 18 hours earlier via a radio antenna north of Perth.
Philae will detach about 20 kilometres above the comet's surface and take seven hours to make its uncontrolled descent. News of a successful touchdown, which is far from guaranteed, is expected at about 3am.
Landing on a giant mass of dust and ice was always going to be a risky manoeuvre, but the comet's rough surface - covered with crevasses, craters, giant boulders and cliffs up to 150 metres high - will make it especially challenging.
"There's a 50 percent chance that the landing is going to fail because the surface is so ridiculously rough, much rougher than anyone predicted," said Warwick Holmes, an Australian avionics engineer who helped build Rosetta.
Analysis of the selected landing site, named Agilkia after an island on the River Nile and in keeping with the mission's Egyptian theme, suggests up to half of the surface area would be unsuitable for landing.
"Even we get a perfect ejection, and everything works perfectly on the way down, we've only got a 50 percent chance of getting a good landing."
Philae has no control system of its own, except for a gyroscope, which will stabilise the craft after ejection to ensure its orientation remains vertical as it floats towards the surface.
When the lander touches down a small thruster will fire to hold it in place while two harpoons and three ice screws twist and lock the lander into the ground.
But there is a reasonable chance the lander will strike a boulder and tip over, or fail to attach itself to the ground and bounce off the surface never to be heard from again.
Mr Holmes said a failed landing would not mean a failed mission. "Eighty percent of the science of Rosetta is coming from the orbiter and that is successfully in orbit since August 6," he said.
"The lander is a huge bonus. It's the icing on the cake."
As comets are some of the oldest material in the universe, studying them promises to unlock secrets about how the planets formed and water came to cover the Earth.
