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OU Lecture 2007: The Penetrometer

Updated Tuesday 26th June 2007

Seventeen years of work, seven years of space flight, all for one second of data from the Huygens Penetrometer. John relives the white knuckle ride to Titan, Saturn's largest moon.


Copyright The Open University


Copyright The Open University

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Let me now go on to what is possibly my most favourite space instrument of them all, and it's this – or, at least, this is the first prototype that we made of it. This is the Huygens Penetrometer. And I will explain in a minute why I did that.

Now what is Huygens and what is Cassini? It was a joint mission of Europe and the US named after Cassini, who studied Saturn’s rings and Huygens who discovered Titan, Saturn’s largest moon, in the seventeenth century. This was a very ambitious mission. Two spacecraft to go to Saturn, to study Saturn itself, the rings and the many satellites of Saturn but in particular Titan. Titan is the only moon in the entire solar system with an atmosphere. It’s a very, very thick atmosphere and one that has some similarities to the primitive atmosphere that Earth had maybe four billion years ago. Now Titan was visited by Voyager 1 and Voyager 2 – two spacecraft which flew past in the early 1980s. And they produced about two thousand images, which in some senses were quite disappointing. And this is probably the most exciting picture that they produced. They all showed this body completely shrouded in this orange photochemical smog or haze. Not a single gap in the haze to enable us to see to the surface.

Titan is a world of ice. It's not a rocky body like our own Earth.

Nitrogen, like our own atmosphere, and methane. The surface was about minus one hundred and eighty degrees Centigrade, really, really cold. So with all of this information Titan clearly stood out as a target for special study, so that’s why the Huygens probe was chosen to eventually land on Titan. Cassini-Huygens travelled for seven years to go to this place, to find out what was going on below the haze. That’s Huygens – released on a collision course with Titan. It travelled for twenty-two days, slowed down by the very thick atmosphere, and then it deployed a series of parachutes to float down for two and a half hours through the atmosphere down to the surface. I was particularly interested in the surface. One of the things that Voyager had hinted at was that perhaps the surface was really rather exotic, partially covered by lakes or even seas of liquid methane. So my instrument, one of the six scientific instruments, was the Surface Science Package. We had nine sensors, some designed to work if we landed on a solid surface and some a liquid. And this one – the Penetrometer – this was the first prototype we made. It's three times the actual size. It was instrumented with a force transducer here, literally to measure the force of impact. And from a simple measurement like that we were hoping we would say something about the nature of the surface.

Here is a schematic. You can see Huygens under the parachute and here circled is the Penetrometer, sticking out through the front of the probe. Now the trouble is, of course, we didn’t know what we were going to hit. The camera might not have been working. We might have had no idea what it was we were landing on, not even whether it was liquid or solid. We might well have been destroyed on impact. There was no guarantee that the probe would survive so we had to send the data back instantaneously. Also, when we talk about impact data you have to remember that impact would have lasted for less than a second. So we had to sample it very quickly and send it back straight away. In fact, think about it. We are talking about seven years of designing and building the instruments; seven years of travelling to Titan and then maybe less than a second of data. Now that’s not crazy – that really is bonkers!

Let me take you now to January 14th, 2005 – probably the most important day of my professional life. So this was the day that Huygens arrived at Titan. Remember, we had been working on this, or I had been working on it, for seventeen years, not a single piece of scientific output to show from it yet. So you can imagine it was just a little bit tense that day. There I was in the same European Mission Control Centre. The carpets had changed.The computers were a bit flashier but it was the same place that we had been before. We were waiting, waiting to see whether those seventeen years were wasted or not. I really don’t have the words to tell you how I felt – what the atmosphere was like. I won't even try. Then – 15.26 Central European Time – the screens filled with green figures. That told us that Huygens was transmitting from under its parachute, sending the data to Cassini and then back to the Earth. As before, cheers, hugs, screams, shouts. You know. You have heard it all before. The data was coming through.


With thanks to:

  • NASA
  • ESA

The Open University Lecture 2007

This is part eight of ten

Next: Crème brûlée


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