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Rosetta’s comet touchdown

Rosetta’s comet touchdown

By Anita Heward on November 06, 2014

On 12 November, the European Space Agency’s Rosetta mission will deploy the Philae lander and attempt the first touchdown on the nucleus of a comet.  Rosetta has taken a decade to reach its target, Comet 67/P Churyumov-Gerasimenko (Comet CG), and will travel with this icy, dusty snowball for at least eighteen months as the comet approaches the sun and heats up, releasing gas and dust to form its tails. At Google Lunar XPRIZE, we are huge supporters of daring, innovative missions. But some of our team members and judges have an even more personal interest in the Rosetta mission and its audacious landing attempt.

 In the run up to the landing, we’ve spoken to Prof John Zarnecki from the Google Lunar XPRIZE judging panel about his involvement in Rosetta and what the mission means to him.  John has over 30 years of experience in space research, including developing instrumentation for the Hubble Space Telescope, the Giotto mission that flew past Halley’s comet in 1985, and the Cassini-Huygens mission to the Saturnian system. For the Huygens probe, he led the team that provided one of the scientific instruments that landed on Saturn’s largest moon, Titan, in January 2005.

You’re involved in a few of Rosetta’s instruments?  

I’m a co-investigator of GIARDA, the dust instrument, and also MUPUS -- I think that I invented the acronym there: MUlti-PUrpose SUbsurface Science -- which is an instrument on the lander and will make a variety of measurements of physical properties and thermal properties with a penetrometer that will be kind of hammered into the surface.

The instrument you built for the Huygens probe had a penetrometer. Are they related?

Yes, there was some cross-fertilization in the early days. My involvement [in Rosetta] was more in the early days, I suppose in the 90s. Then, personally, Huygens took over – there are so many hours in the day…

But when Huygens was selected back around 1990, we were backing another mission, a mission to an asteroid called Vesta, and we were doing some work on penetrators.  So our interest started with a failed mission to an asteroid and we then adapted that for Huygens and paved the way for that to be used in a modified form on a comet mission. So an interesting set of connections.

Your first involvement with planetary exploration was the European Space Agency’s Giotto mission?

Yes, that’s what got me into the Solar System. I was working on the Hubble Space Telescope on the Faint Object Camera, the European contribution for Hubble. And then I saw an advert to be a project manager to go to [comet] Halley, and I just couldn’t resist that. Giotto was such a temptation -- it was so daring and exciting!

Giotto gave us our first glimpse of a comet nucleus?

Yes, in fact that’s very important.  I think Giotto hasn’t had enough coverage.  By comparison with what we’ve had in recent times and obviously now from Rosetta, the images and data now seem relatively crude -- and that’s partly true.  But remember, in the beginning of March 1986, we didn’t know if comets even had a nucleus.  There was still a perfectly respectable theory that there was a flying gravel bank or sand bank -- a loose agglomeration of icy dust held together gravitationally rather than a single solid body.  I think it was those images from Giotto that said, “Hey, there is a nucleus!” and told us something about its properties. We saw jetting -- we saw definitely in the case of Halley that the activity is concentrated in certain areas. And it looks like this one – Comet CG – has got these active areas around the waist between the two lobes.

So Giotto, I think, is very important and, for Europe, I think we’re still proud that we had the first close flyby… and now we’re continuing that heritage at Comet CG.

 Now we have images of this very bizarre looking nucleus for Comet CG.

Yes, they’re absolutely fantastic looking images aren’t they? There’s so much variety – I think this is the sixth comet we’ve seen [up close] so far. At one level they’re similar but when you drill down and look at the detail we see tremendous diversity.  And this one, the shape is a bit different… Is it two objects that have got glued together somehow?  Or is it one object that has somehow is being preferentially worn away in the middle? Perhaps the area in the middle is being sculpted by the jets, which are producing these cliffs with fairly sharp edges? That’s one possibility…

 But this is one benefit, I think, of being able to fly along with the comet for so long – for over a year. It’s still early on, we’re nine months away from peak activity, so we might see much more active jetting and see what that does to the topography of the comet. We might even see it split in two!

 The excitement is building up for November 12th.

In a sense, people will think that’s the most important, critical part of the mission. But in fact it’s only a fraction of it.  Data from a year and a half of monitoring the changes and evolution and so on of the comet -- that’s powerful! There’s already been several months’ worth of data and papers are coming out. We’ve produced the first two papers on the dust, for example, with the first analysis of the dust that was detected in August in the first few weeks of operation. As we orbit around the comet you can see that you don’t detect the same number of particles uniformly as you go about. There are some areas where you get some more than others, reflecting the fact that the emission is far from uniform.  But we can see that anyway in some of the images -- you can see the jets. It’s important to realise that the landing’s not everything.  It’s the thing that catches the attention.

You are part of the Google Lunar XPRIZE judging panel.  Landing on a comet is obviously very different from landing on the Moon.  Is there anything that our teams should be looking out for or learning from Rosetta?

That’s an interesting question.  If nothing else, I would hope that they can take just a little bit of time off just to watch what’s going on, just to remind themselves of the excitement -- the excitement and terror -- of these sorts of occasions and maybe just try to imagine, when it’s all about to happen, that it’s their baby, rather than Philae, that is about to be deployed to land on the Moon. Think of it as a prelude to what we hope might happen in the coming months and years.  So, very different challenges, no gravity in this case, as opposed to one sixth [on the Moon] – which makes life easier in some respects but imposes new challenges.  Everywhere you go, there are different challenges.  But I think you can always learn from other people, so they should follow it with a keen eye – but not spend too much time on it because they’ve all got to work hard on their missions!  

Looking further ahead, down the line, assuming that the Google Lunar XPRIZE is won and commercial space exploration really takes hold, what will that do for planetary exploration? What opportunities might arise for the science community?

Planetary scientists are nothing else if not opportunistic and they’re very, very creative at finding ways of piggy-backing on other things, of making opportunistic measurements. So, if and when commercial activities – whether that might be asteroid prospecting or even comet prospecting if they find anything interesting… we will be there pestering people to give us a little bit of space, a little bit of aperture if they’ve got cameras, or a little bit of space to put our mass spectrometers or other such instruments. I would say that any time anybody goes somewhere interesting, all be it for commercial purposes, we can piggy back and find new measurements and interesting ways to do those measurements.  The opportunity for planetary science – still the number of missions are so, so few – so anything that gives us more opportunities, more chances, we’ll absolutely jump at them.

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