Chasing a comet

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My speciality for the last twenty years or so has actually been going to other planets and other objects in the solar system rather than merely, if I might say that, observing from orbit around the Earth. The first such object that I went to was Halley’s Comet – Halley, named after Sir Edmund Halley, the famous astronomer. And this was the destination for Europe’s kamikaze mission, the Giotto Project. It was called Giotto because the Italian artist Giotto di Bondone was the first one to give a realistic representation of a comet in his Adoration of the Magi. Now I had been working at British Aerospace. It was a wonderful experience but I actually missed the research environment in Academia. So I saw an opportunity to work on this project at the University of Kent as project manager for one of the ten scientific instruments on Giotto and I took that opportunity.

Now, what was Giotto going to do? Well, this is a typical image of a comet from the Earth. We see a head or a coma, as it's called – a bright condensation of light – and the characteristic feature of a comet, which is this tail which spreads thousands, tens of thousands, hundreds of thousands of kilometres across the sky. Gas and dust reflecting light. But what we didn’t know is what's at the heart of it all? What creates this? What causes this? What is at the centre of the comet coma? We can't tell from the Earth. We are blinded by the light, the scattered light, and we just don't have the resolution, the ability, to peer in there. So the answer was to send a spacecraft to have a look. Now our instrument, the one that I was helping to design and build, was actually to look at the dust – the dust making up much of the structure. And we wanted to measure the size of the dust particles; its mass and also how it was distributed about the comet.

Now there was a problem. The closing speed, the relative speed between the spacecraft and the comet, was sixty-eight kilometres per second. That’s one hundred and fifty thousand miles an hour or from here to the centre of London in 1.2 seconds. I wonder how that would look on the speed cameras? Now, at that speed, a tiny particle, I mean a thousandth of a gram, had the ability to destroy the spacecraft. There is so much energy at those incredible speeds. So that was why I called it a kamikaze mission. The likelihood was the spacecraft would be destroyed on its mission and it did also mean that all the data that we collected had to be transmitted back absolutely in real time. And we also had a dilemma. What to go for? What miss-distance to go for? I mean, should we go right for the centre, give us the chance to get us the best quality data, but almost certainly destroy the spacecraft – or should we take the more conservative approach, maybe go for a miss-distance of several thousand kilometres? Presumably we wouldn’t get such good data but at least we would have a chance of surviving or at least getting closer. Well, I can tell you the answer. And the answer, ladies and gentlemen, is five hundred and ninety four – kilometres that is. That was the miss- distance that we chose by committee, three nights before the closest approach. We had all ten teams meeting. This was the last chance that we had to make a fine correction to the trajectory of the spacecraft. Some of us, our team included, wanted to go absolutely for the centre. We wanted to go for broke. Others wanted to stand off a bit. I can't remember why or how we came up with five hundred and ninety four but it was the compromise.

Anyway, how are we going to collect, with our instrument, our dust impact data? Well this is a cartoon of Giotto travelling through space. And you might be able to see that, at the front of the space craft, there is something called a bumper shield. Here we can see it in more detail. It had never been tried before in space. At the front there was a shield, quite thin aluminium, a few millimetres thick, that would protect us against the very smallest particles. But the larger ones (large, remember, is thousandths or even less of a gram) would punch straight through and we had then a second shield here made of Kevlar, that’s the material that you make bullet-proof vests out of. The act of going through the front shield would spread the particles out and they would impact over a wider area and that, we thought, would give us a chance to protect ourselves against all but the worst particles. Here we can see the team working on a flight spare of the front shield and what we did was to put our sensors on the rear side of that front shield. Here you can see some of them. And what these are essentially is very sensitive microphones. Every time a dust particle hit the shield it essentially set up an acoustic wave, a sound – it created a sound and our very sensitive detectors would pick up that sound. We also put some sensors on the rear shield to pick up those ones that punched through the front and got through the rear shield.

Well, the launch was in 1985, summer 1985. Nine month journey to get to Halley's Comet. So I would like you to imagine the scene. We are now March 1986. We are in Mission Control, Europe’s version of Houston. It's in Darmstadt in Germany. It seemed that half of the world’s press is there. Science Ministers from all over Europe. Anybody who is anybody in the Space business was there. Frankly it was a bit of a circus. Two hours from closest approach we are closing in. All ten instruments are on. The camera picks up in the distance a fuzzy blur. In fact we are just beginning to see a solid object at the centre of the comet, the first time that this had ever been seen – a comet nucleus. The magnetometer is picking up magnetic anomalies. The plasma instruments are detecting gas flowing out of the comet. One by one the instruments are detecting data. We are on a corridor, ten different rooms with each instrument team. We hear the cheers, the shouts, as the instruments start to detect their data. All of the instruments are working – except for one. You can guess which one it was!

Credits

With thanks to:

• ESA
• David Malin and the Anglo-Australian Observatory