The Rosetta spacecraft has successfully released its on-board lander, Philae, which has now begun a seven-hour descent to become the first object to land on a comet.
Just after 8pm AEST, the European Space Agency's ground control staff received the crucial signal that confirmed Philae, a 100-kilogram science lab, had detached from its mothership about 22.5 kilometres above the comet's surface.
It takes 28 minutes and 20 seconds for messages from Rosetta to travel 500 million kilometres back to Earth.
Australian avionics engineer Warwick Holmes, who helped build Rosetta, was ecstatic when he heard the ejection had worked successfully.
"I saw Philae being connected to the spacecraft 12 years ago. Now it has separated for the first time and it's free flying," Mr Holmes told Fairfax Media from mission control in Darmstadt, Germany.
"People who are normally very serious are extremely emotional, myself included," he said.
"We've waited so long for this event."
Seven-hour descent:
After ejection, Philae's three legs will unfold in preparation for landing.
Philae has no control system and so will drop unguided towards comet Churyumov-Gerasimenko.
The comet's weak gravitational force (about 100,000 times weaker than Earth) will pull Philae towards the surface.
The lander's attitude will be stabilised by an internal flywheel (gyroscope).
Philae's cameras will take regular images of the comet's surface, and a goodbye picture of Rosetta.
Several of Philae's 10 science instruments are now active, and will begin measuring dust, plasma and the comet's magnetic field.
Touchdown sequence:
Philae landing's speed will be about one metre per second. At touchdown, Philae's three legs will absorb most of the shock of the landing, using that energy to drive three ice screws into the surface.
Two harpoons will also fire to anchor Philae firmly to the ground. The lander has a small thruster that is supposed to help push it into the ground as the harpoons expel, but pre-separation checks found the active descent system could not be activated.
"It may work still work. We just don't know what the status is," said Mr Holmes.
This may make the already complicated landing even more fraught because there is a chance Philae may bounce off the comet never to be heard from again.
There's also a reasonable chance the lander may hit a boulder or slope and tip over so its legs can lift, rotate or tilt if it needs to be repositioned to upright. Confirmation of a successful landing is due about 3am and the first images from the comet's surface should be released by ESA soon after.
Landing site:
Philae will aim to land at a spot called Agilkia, named after an island on the River Nile, an area a few hundred metres wide on the smaller of the comet's two lobes. While it was selected because it a "minimum risk" to the lander compared with other sides, the terrain is still covered with large boulders, crevasses, cracks and cliffs. The area also receives sunlight, allowing Philae to recharge its solar panels to power its science instruments.
Let the science begin:
Once firmly anchored to Comet 67P, Philae's science instruments will begin work immediately.
- Six micro-cameras will take panoramic pictures of the surface (CIVA)
- One instrument will measure the electrical and mechanical properties of the surface (SESAME)
- A drill will take sub-surface samples from 22 centimetres and feed them to the on board laboratory (SD2)
- Another camera will take a close-up of the surface (ROLIS)
- Gas measurements will be taken (COSAC and PTOLEMY)
Philae's on-board batteries are only expected to last about 64 hours. Later, it will switch to solar power.
