As Philae descends, its historic comet landing offers a glimpse at Earth's beginnings

Alok Jha

Former Science Correspondent

Landing site chosen for Rosetta's robot lander, Philae, can be seen close to the top of the image above a large boulder-filled depression.

Not long ago, the Philae lander separated from its mothership, Rosetta, and began its drift towards the surface of the comet 67P/Churyumov-Gerasimenko.

In just a few hours we will know whether this audacious mission, more than two decades in the making, has passed its latest hurdle.

Rosetta has spent more than a decade closing in on its target. It arrived in August and began circling it and taking pictures - crucial information for scientists back on Earth to scout for a potential landing site.

They decided on Site J, later re-named Agilkia, in September. This morning, Rosetta moved to within 20km of the comet and pushed Philae away.

The gravity of the comet is minuscule in comparison to the Earth’s, so Philae will fall towards the surface at only a few centimetres per second. And gravity is the only force pulling the lander down right now.

There is no control possible from the European Space Agency’s flight control centre in Darmstadt and scientists will have to wait for signals from Philae and Rosetta at pre-determined times in order to know what is happening.

Philae’s path down to the surface has been modelled by ESA’s engineers based on what they have measured about the comet’s gravity field.

But models can’t predict everything and there are still many unknowns about the procedure.

There will, no doubt, be a lot of nerves right now in the control room.

So far the separation phase of the mission has gone precisely to plan, save for a minor glitch yesterday when it looked like Philae might not have warmed up properly in readiness for today’s mission - but that problem was soon resolved.

At 9:03am (GMT) - right at the minute it had been scheduled - the Rosetta control centre in Darmstadt confirmed that Philae had separated and was on its way. The actual separation had happened many minutes earlier but, because of the distance from the Earth to the comet, it took 28 minutes and 20 seconds for signals from the probes to arrive in Darmstadt.

The next milestone will occur at 10:53 (GMT), when flight operations should get a signal from Philae, via Rosetta. The “acquisition of first signal” means that the two probes have established a communications link during the descent and Rosetta can relay data from the lander to the Earth. That telemetry will include spacecraft health, status and any scientific measurements it has taken.

A model of the harpoon on the landing unit Philae. Credit: SVENÂ HOPPE/dpa

After that, we will wait for touchdown, expected at 16:02 (GMT). If Philae has landed without damage and anchored itself to the comet with its thrusters, harpoons and screws, this is when we will receive the signal - there is a 40-minute window to receive this message so the exact time might slip from the prediction. This signal would also confirm that communications to Earth, via the orbiting Rosetta, are working.

After that, the first thing Philae will do is take pictures of its surroundings - we should see those back on Earth by about 17:00 (GMT). An hour after that, the scientific work will start, including the main British instrument, the Open University's Ptolem, which will sniff the air around it and start to sample the ground beneath its legs in order to work out what the comet is made from.

xperts follow the flight path of robotic space probe 'Rosetta' in the ESA control centre in Darmstadt, Germany. Credit: BORIS ROESSLER/DPA

Philae will spend a year probing, sampling and imaging the surface of the comet. It’s aims are high - comets are leftover material from the earliest years of our solar system and the lander's experiments will give us insights into a time around our Sun before the Earth had formed. It will help to answer questions including how the planets formed and where the Earth’s water came from.?