Chris Smith first crossed paths with John Zarnecki in Cambridge during the 1990s, when Chris was immersed in his medical studies. It was there that John revealed his plan to land a probe on Titan – which is Saturn’s enigmatic and clossal moon…
John – There were six instruments, six scientific instruments on the Huygens probe, and I was one of the PIs, one of the six leaders. And of course, with all of these projects, there’s an enormous team involved. I mean, not full-time, you know, some come and go, teams in space agencies, in industry. In fact, one of the things that I loved about the work was this collaborative way of working and, you know, I mean, being a child of the 60s, a bit of a hippie, really, you know, this is an example of people working together for no reason other than pure science.
Chris – Why did you pick Titan?
John – It picked me, really. Science missions are selected competitively. It’s really very democratic. And so after Giotto, I got involved with doing studies for a mission called Vesta. This was a French-Soviet ESA mission to go to an asteroid and to fire a dart, if you like, a penetrator into the asteroid to try and make measurements, and this was in competition. I think there were five or six missions in the competition. And I remember going to Bruges in Belgium, where there were the presentations of each of the missions, and then the top committee in ESA would go away, deliberate and decide which one to choose. And to my horror, rather than choosing Vesta, which was clearly a magnificent project, they chose something which I think at the time was called the Titan probe. It was a project to work with NASA. NASA would provide a big spacecraft called Cassini, and Europe would provide a probe, a smaller probe, that would be detached and would land on Titan, the largest moon of Saturn.
Chris – Did you know much about Titan at the time? Or do you just sort of have this as an afterthought, “Oh, it’s a moon I’ve heard of”?
John – It was a moon I’ve heard of. I hadn’t published a single paper with the word Titan in it, but I remember going back to our lab, really deflated and saying to my colleagues, you know, we spent a year working on this Vesta project, we’ve done some interesting work, is it all wasted? Can we adapt any of it and use it for this dreadful mission, which ESA and NASA have chosen, which, you know, maybe we can get in on it? And so we sat down, and I literally remember it was a coffee break, and we had a new coffee room, and we went in there, and we got an enormous sheet of blank paper, and I said, “Right, if you land on a somewhat mysterious and unknown moon on the surface, what measurements would you like to make? And is there anything that we’ve done that could contribute to that?” So we wrote down just all of the physical and chemical properties that we could think of. Remember, we knew virtually nothing about the surface of Titan.
Chris – It’s shrouded in a very dense atmosphere and bloody cold. It’s what, minus 200 degrees or something, isn’t it?
John – Minus 180, yeah, something like that. And it’s covered, Titan, which is the second-largest moon in the solar system, but it’s shrouded in orange smog. So the Voyager 1 spacecraft had flown past in the early 1980s and produced a couple of thousand of images, all of which showed basically an opaque orange blob, no view of the surface.
Chris – Was that the attraction then? Because it was mysterious, people thought, let’s go there.
John – More than that, there was lots of data about the atmosphere from the Voyager flyby, and the inference from that was that the surface could actually be covered with liquid. Not water, of course, at minus 180, but hydrocarbon, so liquid methane or liquid ethane. So there was a real possibility that this was a really exotic place, the only other place in the solar system with liquids on the surface, and the fact it is also the only moon in the solar system with an atmosphere, a thick atmosphere. So really it stands out for all sorts of reasons, and that is why the target of the Cassini-Huygens mission, as it came to be called, was the Saturnian system, but with an emphasis on Titan.
Chris – And how did you get in on the action then? Was your pitch that, well, we actually know how to do some of this stuff then, and this is what we think we can deliver?
John – Yeah, sort of. Basically, ESA threw open – this is normally the way it works – the opportunity to propose an instrument. So they would say, we have committed to fly this mission, it’s got this much mass, volume, power, data available; we think we should fly these instruments, so respond to this AO, we call it, announcement of opportunity. So from that started over coffee, I put together a team – international team – and we proposed a collection of instruments called the Surface Science Package. Basically nine individual sensors, which would, each of which would try and make a particular measurement on the surface of Titan, and this was particularly difficult because, A, we didn’t know if the surface was, could have been hard ice, it could have been a gooey, tarry surface, it could have been liquid methane lake or sea. And also we had no idea how long we’d survive. ESA always told us we should aim for three minutes on the surface, so most of the measurements of the six instruments would be done during the descent and maybe we’d get three minutes on the surface. So we made a proposal to fly this instrument and we were selected.
Chris – That took off in 1997. It reached the Saturnian system in 2004. What was then the process to get to Titan? Because that was just Cassini-Huygens got there. How did you then get to Titan?
John – Once Cassini-Huygens had been captured into orbit around Saturn, Huygens was released just on a set of springs. It was essentially pushed away and it was then on a ballistic trajectory, so it had no rocket power, it was just on a path, on an orbit that would take it inexorably three weeks later to Titan. It would hit the atmosphere, had a series of three parachutes, so it would sail down to the surface in two and a half hours, something like that, land on the surface, and if we were lucky, it would survive for, well, we were designing for three minutes. As it turned out, we got 72 minutes on the surface.
Chris – Was your heart in your mouth? Because you must have been, because you went to America to be at mission control for this coming down the descent, didn’t you?
John – The descent was monitored and controlled from ESOC, we call it, European Space Operations Centre in Darmstadt in Germany. I mean, it was the highlight, I suppose, of my professional career. This was the culmination of 15 years of work. And I mean, you do have to be a little bit mad to do this sort of work because it could have been lost just like that. We could have ended up with zilch, no data whatsoever. And I mean, there are many bizarre aspects of it. One is that we were monitoring essentially what had happened four or five hours previously, because the data could not be transmitted straight to the Earth. It was transmitted from Huygens, our probe, up to the Cassini spacecraft, which was flying over Titan, you know, a few thousand kilometres away. Cassini would record the data, and then it would turn to point its large antenna to the Earth, and then it would squirt the data down to Earth. And so this all meant that we got the data four or five hours after whatever had happened had actually happened. So that was kind of bizarre.
Chris – I bet you were standing there watching the clock, thinking, “It’s now in the lap of the gods. This thing has, whatever’s going to happen has happened. Now I’m waiting for the radio signals to make it all the way back across the solar system.”
John – I mean, it was a highly emotional culmination of all these years of work. And we’d worked as a team pretty much very closely together.
Chris – What did you learn about Titan, though? What did that magic 72 minutes, that you were hoping for three and got 72, what did it reveal?
John – Well, that Titan is really a remarkable place. The surface is mostly icy, but about 10% of it is covered with lakes and seas of liquid ethane, mostly. The largest sea is larger than the largest of the Great Lakes, for example. And the atmosphere is thick; we know a lot about its composition, and we know a bit about what’s underneath the surface, the icy surface, and there’s a real possibility that there is a sea, a sub-surface sea. So it’s a truly remarkable place, Cassini-Huygens just scratched the surface, and this is why now NASA are building a mission to go back to Titan due to launch in, I think, in a couple of years.