Yesterday (Wednesday the 12th of November) the European Space Agency (ESA) successfully landed a tiny space-probe called Philae on to the surface of a comet. This is a remarkable achievement, and one which has clearly captured the imagination of the public and was the top story on the news here in the Disunited Kingdom yesterday. I was on the BBC talking about this just before 8am, with Philae detaching from its mother ship Rosetta just over half an hour later, at 08:35 UTC.
This remarkable photograph was taken of Philae by Rosetta’s cameras as Philae descended towards the surface of the comet. You can see in the photograph that Philae has deployed its landing legs, but unfortunately just before its release from Rosetta it was realised that a thruster on top of the washing-machine sized lander was not working. They decided to drop it anyway.
Philae descended very slowly towards Comet 67P. A common misconception seems to be that the comet does not have any gravity, but this is not true. It does, but its gravity is very very weak. Philae has a mass of 100kg, but on the surface of the comet this would feel more like a few grams. If you were on the surface of Comet 67P and you were to jump gently upwards, you would probably not return back to the surface, so weak is its gravity. This photograph was taken by Philae as it slowly descended towards the surface.
ESA finally reported that Philae had landed soon after 16:00 UTC, and here is a screen capture of the robot’s Twitter feed (@Philae2014). As you can see from the announcement of its landing, it also says that the harpoons did not fire. These were designed to secure the robot to the surface.
As of my writing this on Thursday morning, we still do not know how securely Philae is on the surface of the comet. The latest reports are saying that Philae bounced a few times, but that it is now stationary on the surface. My understanding is that ESA are now trying to decide whether to fire the harpoons to secure it better to the surface. The danger of doing this is by firing them Philae will be sent in the opposite direction, ie. away from the surface, due to Newton’s 3rd law (to every action there is an equal and opposite reaction). If the harpoons manage to get a grip on the comet then firing them is not a risk, as then the motors can winch Philae back to the surface. But, if they fail to grip, Philae may float away from the surface not to return.
Whether ESA gets Philae secured to the surface or not, this landing has still been a remarkable success. Many of the instruments on Philae are working and taking data, and of course we also need to remember that the Philae landing was just a part of a larger mission for Rosetta. Rosetta will stay in orbit about Comet 67P and observe and study it up close as it passes at its closest to the Sun, and is scheduled to continue sending us data until December 2015.
So, whether Philae does manage to get a firm grip on the comet or not, let us remember that this is one of the greatest achievements in our history of space exploration – to land a robotic probe onto the surface of a comet which is hurtling through space some 500 million km away from Earth. Well done ESA!