A key milestone has been reached in the quest to land a spacecraft on a comet.
Controllers say they have now made radio contact with the descending Philae robot, meaning they should be able to receive pictures from it.
Philae was dropped towards Comet 67P/Churyumov-Gerasimenko by the European Space Agency’s Rosetta satellite at 08:35 GMT.
Early on Wednesday (GMT), the third “go/no-go” decision was delayed. The thruster system used to push the robot into the surface of the comet at the moment of touchdown could not be primed.
“We will just have to rely now on the harpoons, the screws in the feet, or the softness of the surface. It doesn’t make it any easier, that’s for sure,” said lander chief Stephan Ulamec, from the German Space Agency.
The comet’s tricky terrain means that Philae could bash into cliffs, topple down a steep slope, or even disappear into a fissure.
Analysis by Science editor David Shukman
The handshakes are warm but the smiles are brittle here at the European Space Agency’s mission control in Darmstadt. As the clock ticks down to the unprecedented attempt at landing on a comet, there’s real nervousness in the air.
Key timings for landing effort (GMT)
• Rosetta delivery manoeuvre – shortly after 06:00
• Latest Go/No-go decision – before 07:35
• Philae separates from Rosetta – 08:35
• Confirmation signal at Earth of separation – 09:03
• Rosetta’s post-delivery manoeuvre – 09:15
• Radio connection established – after 10:30
• First data from descending Philae – after 12:00
• Landing of Philae on 67P – after 15:30
• Confirmation signal at Earth – around 16:00
Esa’s detailed landing timeline
Following separation, the 100kg robot has no means of adjusting its descent; Philae will go where the comet’s gravity pulls it.
Controllers in Darmstadt will want to hear not only that Philae landed in one piece but that it is securely fastened to the comet.
The nature and strength of the surface materials are unknown, however.
Philae could alight upon terrain whose constitution is anything between rock hard and puff-powder soft.
If it can, the robot will endeavour to lock itself in place with screws in its feet and harpoons that fire from its underside.
Not only is landing on a comet an untried technique, but Wednesday’s effort is also having to rely on some relatively old technologies.
Rosetta was despatched from Earth to catch 67P in 2004. That means it and Philae were designed and built in the 1990s.
And given the conservatism of space engineering, a number of its onboard systems will therefore undoubtedly be 1980s vintage.
But even if the landing attempt fails, the pictures and measurements of 67P acquired by the Rosetta mothership in recent weeks will be enough to re-write the textbooks.
“The real scientific value of this mission is spread all over Rosetta and its instruments, and the lander is just a part of that,” explained Esa flight director Andrea Accomazzo.
“The lander is obviously spectacular; it’s the thing the public recognise. But already, even before the landing, the scientific return of Rosetta is orders or magnitude above what we knew about comets previously.”