During checks on the lander’s health, it was discovered that the active descent system, which provides a thrust upwards to avoid rebound at the moment of touchdown, cannot be activated.

At touchdown, landing gear will absorb the forces of the landing while ice screws in each of the probe’s feet and a harpoon system will lock Philae to the surface. At the same time, a the thruster on top of the lander is supposed to push it down to counteract the impulse of the harpoon imparted in the opposite direction.

“The cold gas thruster on top of the lander does not appear to be working so we will have to rely fully on the harpoons at touchdown,”says Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center.

“We’ll need some luck not to land on a boulder or a steep slope.”

“There were various problems with the preparation activities overnight but we have decided to ‘go’. Rosetta is lined up for separation,” says Paolo Ferri, ESA’s head of mission operations.

Thus despite the potential problem concerning the moment of touchdown, separation will proceed on the planned timeline.

Separation will occur in space at 08:35 GMT / 09:35 CET, but it will take the radio signals from the transmitter on Rosetta 28 minutes and 20 seconds to reach Earth and be transferred to the Rosetta Mission Control Centre at ESA’s Space Operations Centre in Darmstadt, Germany.

That means we must wait until about 09:03 GMT / 10:03 CET for confirmation the separation has happened correctly.

The Go/No-Go decisions leading up to this milestone began last night at 19:00 GMT / 20:00 CET, with the first confirming that Rosetta is in the correct orbit for delivering Philae to the surface at the required time.

The second Go was given at midnight (GMT), confirming that the commands to control separation and delivery are complete and ready to upload to Rosetta. The Go also confirmed that Rosetta’s overall health is good, and that the orbiter is ready to perform.

At 02:35 GMT / 03:35 CET the third GO was given after a final verification that the lander is ready for touchdown.

The final manoeuvre by Rosetta was conducted at 07:35 GMT / 08:35 CET, which is taking Rosetta to a point about 22.5 km from the comet’s centre for separation.

The manoeuvre was followed by the final Go/No-Go decision that verified the two spacecraft, the orbit, the ground stations, the ground systems and the teams are ready for landing.

After separation, we will not hear from Philae for some two hours until the lander establishes a communication link with Rosetta. Philae cannot send its data to Earth directly – only via Rosetta.

The descent to the surface of Comet 67P/Churyumov–Gerasimenko will take around seven hours, so confirmation of a successful touchdown is expected in a one-hour window centred on 17:02 GMT / 18:02 CET.

“We are anxious but excited,” said Jean-Pierre Bibring, lead lander scientist, during this morning’s press briefing. “It is not every day that we try to land on a comet.”

Follow the event live via www.esa.int/rosetta