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Published 27 August 2015 Referencing Hub media
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Rosetta Mission scientists used early images of Comet 67P to create a virtual model of the comet. This virtual model was used to pinpoint the best place to land Philae, the lander. Avionics engineer Warwick Holmes describes the process of landing Philae and the unexpected touchdown results.

Transcript

WARWICK HOLMES

We put many thousands of images together to make a simulated fly around the comet. This is about 20,000 images that have been stitched together to make a virtual model of the comet, and we used this then for planning our navigation.

Then we started looking for a landing site, and this is the difficult bit. The comet is incredibly rough and an extremely weird shape, and we had to try to get our lander, which is very fragile, onto an almost zero-G surface. And if we hit anything, it would tip over. This is the smoothest part we could find. And this was where we had to get as much sunlight as we could and a flat surface and also try to miss the boulders.

Well, 50% of the best place we found was still unsuitable, but there was nothing else, and we’d carried the lander all this way. Well, we had to put it down, so we decided this was where we were going to do the landing.

So this is a video generated by ESA showing what we hoped was going to happen during the landing sequence. The Rosetta is flying over the comet at about 18 centimetres per second, and the little lander on the back gets pushed off at exactly the same speed so it’s effectively stationary above the comet.

We open the landing gear, we open up the instruments and the antennas, and then we start drifting down about 1 metre per second from 22 kilometres high over about 7 hours. When we get down, we want to fire two harpoons to hold into the comet surface – there they are, the two red ones there were meant to fire, but they didn’t, and I’ll show you the consequence of that.

This is the real sequence using our high-resolution science cameras to film Philae as it was going down onto the surface of the comet. In that white square is a full-field view of the comet. Again, I’m trying to emphasise how dark the surface of the comet is. Because the little lander was lit by the Sun, we were interested in filming that – there’s no way we could see the surface.

So this is us coming down with the camera underneath the lander as it’s descending onto the surface, and this is part of the landing gear in the field of your camera.

Everything was looking good. We got to within half a metre of where we were aiming from 22 kilometres away. This was an unprecedented result. People just couldn’t believe how good this was. But unfortunately, of course, it was too good. We bounced off. We knew we hit the surface because we heard the microphones on the feet of the lander make the touchdown. This is what we heard.

That was it. That was the touchdown, OK? But it was a positive signal, and we knew we’d hit something. We were imaging the landing point, and all we could see was a puff of dust. So the green spot is where we were aiming, and this little light grey area here, this is the before and after of the touchdown. So this is before and after, and there was a dust cloud. Good news. We’d obviously hit somewhere close to where we wanted, but the lander wasn’t there. So we thought, well, where is the lander?

Well, then there was another image about the same time. And one of the flight dynamics people realised that, in this image, we could also see the little lander. This is from 22 kilometres above looking down for a half-metre height sized box of electronics – the lander. And we could see one pixel, which was the actual lander, and its shadow on the surface as it drifted away after it bounced off. And so this is an incredible image to get this. Again, I keep on emphasising how dark it is and how far away, but to get this kind of image is just unprecedented in space exploration history.

This was the area we wanted to land. Instead of landing here, the lander was still drifting over here, in this very rough, very dark craggy surface. After 1 hour and 50 minutes, it did actually stop bouncing around, and it came down.

And we started processing the six panoramic images, and this is what we saw. This is camera number 1. It was extremely dark, and we realised, well, this is really unexpected. We wanted to be in the nice sunny plain. Camera number 2 was looking straight out into space. Well, that’s strange. How on earth can that happen? Then camera number 3. We could see some sunlit surface again. Camera number 4, again sunlit, looking promising – 5 very dark, 6 very dark.

So we thought, well, what is all this telling us? Well, we did some processing, and we put the best images together – 3 and 4. And we got this, sort of, it looked like the wall of a cliff face, wall of a crevice, sunlit, with the lander leg here in the foreground. Then we added the colour and put some depth of field cues in, and we realised that we’re actually – we really were caught in some kind of a crevice. And this – it’s not a very good image, I’m sorry – but this little blue cube here, this is  the actual lander with its legs tilted on its side. And we realised that we’d bounced into a small crevice and tipped over on our side.

The Science Learning Hub would like to acknowledge the following for their contribution to this resource:
Warwick Holmes
Lecture video footage courtesy of the University of Waikato
All additional images and footage of Comet 67P, Philae and Rosetta courtesy of ESA – European Space Agency