Exploring Our Solar System

Dominic -   Now, we've been hearing about the challenges of landing rovers on Mars, but there have also been missions to land probes elsewhere in the solar system and each environment brings its own challenges. 

In 2004, the Huygens probe landed on Saturn's moon Titan, taking the first images of the surface of such a distant body.  And the key architect to that mission was the Open University's Professor John Zarnecki, who joins us now.  Now John, first of all, tell us a bit about Huygens and what it saw as it descended towards the surface of Titan?

John -   Well, you should realise that we'd never seen the surface of Titan because Titan is shrouded in this photochemical smog and as we descended through the clouds, for a long time, we saw nothing at all.  But then from about 15 km, we started to get an indication of the surface, and we saw what looked like river channels.  In fact, it's what's called a dendritic system.  So a system of rivers and tributaries, and so on. Then we landed, and we saw an undulating vista with pebbles and small rocks.  But what you have to understand is that Titan is an icy world, so we were seeing in fact things that looked familiar like hills and channels, and pebbles, and so on.  But these were not made of rocks and carved by water as on Earth, but the basic building block was ice, and the liquid is liquid methane and liquid ethane.

Dominic -   Like camping gas.  It sounds like you wouldn't want to drop a match into one of these lakes!

John -   Yes and no.  I mean, because of the temperature, it's very cold there, about minus 180 degrees so there isn't, we think, any free oxygen.  So, you'd probably be safe dropping a match, but it's not an experiment I'd like to do.

Dominic -   Now I remember following this mission in 2004 and it seemed rather the same after - I think it took you 7 years to get there - This only operated for 3 hours as it descended into the atmosphere.  Why couldn't you have made this a longer mission?

John -   When in fact, 3 hours was far, far longer than we expected.  We were sort of planning on 3 minutes, so 3 hours was a great luxury.  It was fundamentally an atmosphere probe.  So, it was a bonus getting any time, and any data on the surface.  You have to understand, this was incredibly challenging technically.  We didn't know if we'll be landing on ice, whether we'd be landing in a hydrocarbon sea. So it was very difficult to design for survival on the surface.  So for that reason, it was all focused on atmosphere measurements.  But now that we know much better what Titan is about, we want to go back and actually survive for a long time on the surface.

Dominic -   Now, I'll asked you in a moment about the time mission which I know you're working on at the moment, but first of all, in the nearer future, I know there are other interesting environments where the European Space Agency is hoping to land, including on the surface of a comet in 2014.  Tell me a bit about that.

John -   Yes, indeed.  Well, this is a mission called Rosetta which has been flying for 8 years already and it is again, a very ambitious mission to land on the surface of a cometary nucleus.  The comet, it's not a household name like comet Haley.  This is comet Churyumov-Gerasimenko.  We tend to call it comet CG and in 2 years' time, Rosetta is going to go and orbit around this comet and deliver a lander onto the surface.  Now that's a real challenge because a comet, it's a small object, maybe 10 km across.  So there's virtually no gravity.  So the real challenge there, apart from getting there and getting to the surface, is actually staying on the surface.

Dominic -   Now I guess as with your descent onto Titan with the Huygens probe, you have very little idea actually what you're going to find when you get that, what kind of surface you'll be touching down onto?

John -   That's a very good point indeed and in fact, the instruments and in particular the harpoon which is going to try and snare the probe onto the surface of the comet, is designed for a surface which has got anything which can vary between candy floss or concrete in terms of hardness and consistency.  We are that uncertain as to what we'll actually be landing on.

Dominic -   So you might even sink in.

John -   We could.  So, just put the date in your diary, November the 10th, 2014 about 10:30 in the morning.

Dominic -   Now I'm sure we'll look forward to that.  Now I know there were a couple of missions to land on icy moons in the outer solar system that you are interested in at the moment.  First of all, JUICE.  This is the Jupiter Icy Moons Explorer.  Tell me a bit about those moons.

John -   Okay, well, we're finding that actually perhaps that in the - as far as the giant planets are concerned - Jupiter and Saturn - it's the small icy moons which go around these giant planets that actually might be more interesting than the giant planets themselves.  These are small bodies, but it looks increasingly as if a lot of them actually have subsurface oceans.  And so, this mission JUICE as it's called - I hope we come up with a better name before we launch it - it's not going to land, but it's going to go into an orbit around Jupiter and it will  have several closed passes of several of these moons.  So, it's the start of our really deep investigations of these strange icy worlds.

Dominic -   Now I know you're interest is with the time mission which will hopefully take off and visit Titan again in 2016 or it will take off in 2016.  I gather there's a real problem on how to power something that far-out in the solar system?

John -   Well, yes that is always the case.  We're so far away from the sun that we can't really use solar cells.  So, we use plutonium to generate electricity. But this is a really exciting mission if it's selected, and we should know later this month, it's in competition with two others. The aim is to put a probe directly into one of the seas on Titan because now, we know where they are.  As with Huygens, we didn't know anything about the surface.

Dominic -   I'm sure we'll keep our fingers crossed with you with that selection process.  That was Professor John Zanecki of the Open University.