Fred Jansen: How to land on a comet

56,562 views ・ 2015-04-09

TED


Please double-click on the English subtitles below to play the video.

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I'd like to take you on the epic quest of the Rosetta spacecraft.
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To escort and land the probe on a comet,
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this has been my passion for the past two years.
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In order to do that,
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I need to explain to you something about the origin of the solar system.
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When we go back four and a half billion years,
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there was a cloud of gas and dust.
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In the center of this cloud, our sun formed and ignited.
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Along with that, what we now know as planets, comets and asteroids formed.
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What then happened, according to theory,
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is that when the Earth had cooled down a bit after its formation,
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comets massively impacted the Earth and delivered water to Earth.
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They probably also delivered complex organic material to Earth,
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and that may have bootstrapped the emergence of life.
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You can compare this to having to solve a 250-piece puzzle
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and not a 2,000-piece puzzle.
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Afterwards, the big planets like Jupiter and Saturn,
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they were not in their place where they are now,
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and they interacted gravitationally,
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and they swept the whole interior of the solar system clean,
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and what we now know as comets
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ended up in something called the Kuiper Belt,
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which is a belt of objects beyond the orbit of Neptune.
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And sometimes these objects run into each other,
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and they gravitationally deflect,
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and then the gravity of Jupiter pulls them back into the solar system.
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And they then become the comets as we see them in the sky.
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The important thing here to note is that in the meantime,
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the four and a half billion years,
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these comets have been sitting on the outside of the solar system,
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and haven't changed --
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deep, frozen versions of our solar system.
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In the sky, they look like this.
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We know them for their tails.
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There are actually two tails.
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One is a dust tail, which is blown away by the solar wind.
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The other one is an ion tail, which is charged particles,
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and they follow the magnetic field in the solar system.
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There's the coma,
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and then there is the nucleus, which here is too small to see,
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and you have to remember that in the case of Rosetta,
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the spacecraft is in that center pixel.
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We are only 20, 30, 40 kilometers away from the comet.
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So what's important to remember?
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Comets contain the original material from which our solar system was formed,
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so they're ideal to study the components
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that were present at the time when Earth, and life, started.
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Comets are also suspected
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of having brought the elements which may have bootstrapped life.
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In 1983, ESA set up its long-term Horizon 2000 program,
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which contained one cornerstone, which would be a mission to a comet.
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In parallel, a small mission to a comet, what you see here, Giotto, was launched,
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and in 1986, flew by the comet of Halley with an armada of other spacecraft.
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From the results of that mission, it became immediately clear
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that comets were ideal bodies to study to understand our solar system.
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And thus, the Rosetta mission was approved in 1993,
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and originally it was supposed to be launched in 2003,
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but a problem arose with an Ariane rocket.
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However, our P.R. department, in its enthusiasm,
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had already made 1,000 Delft Blue plates
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with the name of the wrong comets.
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So I've never had to buy any china since. That's the positive part.
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(Laughter)
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Once the whole problem was solved,
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we left Earth in 2004
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to the newly selected comet, Churyumov-Gerasimenko.
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This comet had to be specially selected
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because A, you have to be able to get to it,
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and B, it shouldn't have been in the solar system too long.
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This particular comet has been in the solar system since 1959.
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That's the first time when it was deflected by Jupiter,
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and it got close enough to the sun to start changing.
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So it's a very fresh comet.
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Rosetta made a few historic firsts.
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It's the first satellite to orbit a comet,
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and to escort it throughout its whole tour through the solar system --
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closest approach to the sun, as we will see in August,
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and then away again to the exterior.
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It's the first ever landing on a comet.
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We actually orbit the comet using something which is not
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normally done with spacecraft.
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Normally, you look at the sky and you know where you point and where you are.
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In this case, that's not enough.
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We navigated by looking at landmarks on the comet.
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We recognized features -- boulders, craters --
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and that's how we know where we are respective to the comet.
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And, of course, it's the first satellite to go beyond the orbit of Jupiter
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on solar cells.
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Now, this sounds more heroic than it actually is,
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because the technology to use radio isotope thermal generators
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wasn't available in Europe at that time, so there was no choice.
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But these solar arrays are big.
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This is one wing, and these are not specially selected small people.
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They're just like you and me.
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(Laughter)
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We have two of these wings, 65 square meters.
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Now later on, of course, when we got to the comet,
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you find out that 65 square meters of sail
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close to a body which is outgassing is not always a very handy choice.
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Now, how did we get to the comet?
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Because we had to go there for the Rosetta scientific objectives
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very far away -- four times the distance of the Earth to the sun --
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and also at a much higher velocity than we could achieve with fuel,
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because we'd have to take six times as much fuel as the whole spacecraft weighed.
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So what do you do?
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You use gravitational flybys, slingshots,
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where you pass by a planet at very low altitude,
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a few thousand kilometers,
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and then you get the velocity of that planet around the sun for free.
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We did that a few times.
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We did Earth, we did Mars, we did twice Earth again,
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and we also flew by two asteroids, Lutetia and Steins.
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Then in 2011, we got so far from the sun that if the spacecraft got into trouble,
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we couldn't actually save the spacecraft anymore,
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so we went into hibernation.
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Everything was switched off except for one clock.
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Here you see in white the trajectory, and the way this works.
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You see that from the circle where we started,
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the white line, actually you get more and more and more elliptical,
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and then finally we approached the comet
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in May 2014, and we had to start doing the rendezvous maneuvers.
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On the way there, we flew by Earth and we took a few pictures to test our cameras.
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This is the moon rising over Earth,
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and this is what we now call a selfie,
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which at that time, by the way, that word didn't exist. (Laughter)
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It's at Mars. It was taken by the CIVA camera.
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That's one of the cameras on the lander,
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and it just looks under the solar arrays,
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and you see the planet Mars and the solar array in the distance.
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Now, when we got out of hibernation in January 2014,
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we started arriving at a distance
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of two million kilometers from the comet in May.
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However, the velocity the spacecraft had was much too fast.
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We were going 2,800 kilometers an hour faster than the comet, so we had to brake.
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We had to do eight maneuvers,
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and you see here, some of them were really big.
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We had to brake the first one by a few hundred kilometers per hour,
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and actually, the duration of that was seven hours,
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and it used 218 kilos of fuel,
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and those were seven nerve-wracking hours, because in 2007,
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there was a leak in the system of the propulsion of Rosetta,
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and we had to close off a branch,
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so the system was actually operating at a pressure
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which it was never designed or qualified for.
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Then we got in the vicinity of the comet, and these were the first pictures we saw.
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The true comet rotation period is 12 and a half hours,
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so this is accelerated,
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but you will understand that our flight dynamics engineers thought,
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this is not going to be an easy thing to land on.
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We had hoped for some kind of spud-like thing
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where you could easily land.
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But we had one hope: maybe it was smooth.
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No. That didn't work either. (Laughter)
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So at that point in time, it was clearly unavoidable:
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we had to map this body in all the detail you could get,
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because we had to find an area which is 500 meters in diameter and flat.
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Why 500 meters? That's the error we have on landing the probe.
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So we went through this process, and we mapped the comet.
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We used a technique called photoclinometry.
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You use shadows thrown by the sun.
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What you see here is a rock sitting on the surface of the comet,
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and the sun shines from above.
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From the shadow, we, with our brain,
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can immediately determine roughly what the shape of that rock is.
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You can program that in a computer,
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you then cover the whole comet, and you can map the comet.
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For that, we flew special trajectories starting in August.
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First, a triangle of 100 kilometers on a side
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at 100 kilometers' distance,
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and we repeated the whole thing at 50 kilometers.
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At that time, we had seen the comet at all kinds of angles,
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and we could use this technique to map the whole thing.
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Now, this led to a selection of landing sites.
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This whole process we had to do, to go from the mapping of the comet
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to actually finding the final landing site, was 60 days.
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We didn't have more.
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To give you an idea, the average Mars mission
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takes hundreds of scientists for years to meet
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about where shall we go?
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We had 60 days, and that was it.
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We finally selected the final landing site
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and the commands were prepared for Rosetta to launch Philae.
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The way this works is that Rosetta has to be at the right point in space,
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and aiming towards the comet, because the lander is passive.
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The lander is then pushed out and moves towards the comet.
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Rosetta had to turn around
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to get its cameras to actually look at Philae while it was departing
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and to be able to communicate with it.
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Now, the landing duration of the whole trajectory was seven hours.
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Now do a simple calculation:
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if the velocity of Rosetta is off by one centimeter per second,
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seven hours is 25,000 seconds.
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That means 252 meters wrong on the comet.
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So we had to know the velocity of Rosetta
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much better than one centimeter per second,
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and its location in space better than 100 meters
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at 500 million kilometers from Earth.
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That's no mean feat.
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Let me quickly take you through some of the science and the instruments.
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I won't bore you with all the details of all the instruments,
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but it's got everything.
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We can sniff gas, we can measure dust particles,
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the shape of them, the composition,
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there are magnetometers, everything.
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This is one of the results from an instrument which measures gas density
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at the position of Rosetta,
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so it's gas which has left the comet.
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The bottom graph is September of last year.
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There is a long-term variation, which in itself is not surprising,
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but you see the sharp peaks.
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This is a comet day.
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You can see the effect of the sun on the evaporation of gas
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and the fact that the comet is rotating.
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So there is one spot, apparently,
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where there is a lot of stuff coming from,
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it gets heated in the Sun, and then cools down on the back side.
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And we can see the density variations of this.
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These are the gases and the organic compounds
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that we already have measured.
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You will see it's an impressive list,
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and there is much, much, much more to come,
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because there are more measurements.
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Actually, there is a conference going on in Houston at the moment
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where many of these results are presented.
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Also, we measured dust particles.
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Now, for you, this will not look very impressive,
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but the scientists were thrilled when they saw this.
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Two dust particles:
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the right one they call Boris, and they shot it with tantalum
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in order to be able to analyze it.
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Now, we found sodium and magnesium.
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What this tells you is this is the concentration of these two materials
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at the time the solar system was formed,
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so we learned things about which materials were there
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when the planet was made.
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Of course, one of the important elements is the imaging.
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This is one of the cameras of Rosetta, the OSIRIS camera,
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and this actually was the cover of Science magazine
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on January 23 of this year.
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Nobody had expected this body to look like this.
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Boulders, rocks -- if anything, it looks more like the Half Dome in Yosemite
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than anything else.
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We also saw things like this:
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dunes, and what look to be, on the righthand side, wind-blown shadows.
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Now we know these from Mars, but this comet doesn't have an atmosphere,
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so it's a bit difficult to create a wind-blown shadow.
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It may be local outgassing,
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stuff which goes up and comes back.
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We don't know, so there is a lot to investigate.
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Here, you see the same image twice.
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On the left-hand side, you see in the middle a pit.
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On the right-hand side, if you carefully look,
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there are three jets coming out of the bottom of that pit.
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So this is the activity of the comet.
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Apparently, at the bottom of these pits is where the active regions are,
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and where the material evaporates into space.
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There is a very intriguing crack in the neck of the comet.
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You see it on the right-hand side.
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It's a kilometer long, and it's two and a half meters wide.
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Some people suggest that actually,
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when we get close to the sun,
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the comet may split in two,
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and then we'll have to choose,
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which comet do we go for?
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The lander -- again, lots of instruments,
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mostly comparable except for the things which hammer in the ground and drill, etc.
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But much the same as Rosetta, and that is because you want to compare
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what you find in space with what you find on the comet.
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These are called ground truth measurements.
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These are the landing descent images
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that were taken by the OSIRIS camera.
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You see the lander getting further and further away from Rosetta.
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On the top right, you see an image taken at 60 meters by the lander,
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60 meters above the surface of the comet.
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The boulder there is some 10 meters.
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So this is one of the last images we took before we landed on the comet.
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Here, you see the whole sequence again, but from a different perspective,
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and you see three blown-ups from the bottom-left to the middle
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of the lander traveling over the surface of the comet.
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Then, at the top, there is a before and an after image of the landing.
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The only problem with the after image is, there is no lander.
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But if you carefully look at the right-hand side of this image,
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we saw the lander still there, but it had bounced.
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It had departed again.
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Now, on a bit of a comical note here
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is that originally Rosetta was designed to have a lander which would bounce.
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That was discarded because it was way too expensive.
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Now, we forgot, but the lander knew.
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(Laughter)
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During the first bounce, in the magnetometers,
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you see here the data from them, from the three axes, x, y and z.
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Halfway through, you see a red line.
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At that red line, there is a change.
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What happened, apparently, is during the first bounce,
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somewhere, we hit the edge of a crater with one of the legs of the lander,
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and the rotation velocity of the lander changed.
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So we've been rather lucky
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that we are where we are.
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This is one of the iconic images of Rosetta.
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It's a man-made object, a leg of the lander,
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standing on a comet.
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This, for me, is one of the very best images of space science I have ever seen.
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(Applause)
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One of the things we still have to do is to actually find the lander.
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The blue area here is where we know it must be.
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We haven't been able to find it yet, but the search is continuing,
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as are our efforts to start getting the lander to work again.
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We listen every day,
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and we hope that between now and somewhere in April,
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the lander will wake up again.
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The findings of what we found on the comet:
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This thing would float in water.
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It's half the density of water.
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So it looks like a very big rock, but it's not.
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The activity increase we saw in June, July, August last year
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was a four-fold activity increase.
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By the time we will be at the sun,
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there will be 100 kilos a second leaving this comet:
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gas, dust, whatever.
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That's 100 million kilos a day.
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Then, finally, the landing day.
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I will never forget -- absolute madness, 250 TV crews in Germany.
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The BBC was interviewing me,
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and another TV crew who was following me all day
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were filming me being interviewed,
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and it went on like that for the whole day.
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The Discovery Channel crew
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actually caught me when leaving the control room,
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and they asked the right question,
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and man, I got into tears, and I still feel this.
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For a month and a half,
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I couldn't think about landing day without crying,
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and I still have the emotion in me.
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With this image of the comet, I would like to leave you.
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Thank you.
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(Applause)
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