We talk to Rosetta’s lead scientist on eve of probe’s crash landing

The Rosetta mission to study a comet is about to come to a spectacular end with the spacecraft being sent to crash-land on the object it has been circling and studying closely for more than two years.

Rosetta's descent
Rosetta’s planned crash site, Ma’at, is indicated by the red circle, with the spacecraft superimposed to show its descent. Image credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0. Montage: Skymania

Tonight, ESA reported that Rosetta had completed its final manoeuvre and was now on a collision course with Comet 67P/C-G. A small thruster burn starting at 20:48:11 UTC and lasting 208 seconds set the craft on course towards its final destination. It is expected to crash at 11.20 UT on 30 September.

On the eve of the dramatic climax to the European Space Agency mission that has been such a stunning success, Skymania News spoke to its project scientist and heavy metal fan, Matt Taylor, about the decision to go out with a bang and what Rosetta has achieved at Comet 67P/Churyumov-Gerasimenko.

The adventure has been full of surprises, including the unexpected double bounce that saw Rosetta’s fridge-sized companion probe Philae end up lost in a crevasse after it was meant to make its own soft landing on a flat patch of the comet.

The mothership’s powerful OSIRIS camera finally found and photographed Philae, lying on its side with a leg in the air (we’ve all been there!) in early September. Tomorrow, 30 September, Rosetta will plunge onto a region called Ma’at, to be reunited with the probe it carried piggyback on a ten-year voyage from Earth.

Skymania’s Paul Sutherland is at mission control, at the European Space Operations Centre at Darmstadt, Germany for the big event, and caught up with Matt.

Skymania News: Why end the mission in this dramatic way?

Matt Taylor: We’re moving away from the Sun. We are in drastic power reduction, losing about 4 watts a day. It means we’ve had to switch off some parts of the spacecraft already and as we get further and further away, we’ll get that even more. We’re getting to the limit of what we can operate with the instruments.

We’re also about to go into solar conjunction, so the 1st of October we start to go behind the Sun with respect to the Earth, and that means the data rate will drop through the floor. We’re very limited in data rate already, probably less than 10 per cent of what we were getting at perihelion last year, so you’re in the situation where you’re getting to the end. We considered putting Rosetta back into hibernation, like we did before arriving at the comet, but it would have been a deeper hibernation, a colder hibernation because we’re now on the same orbit as the retreating comet. And the discussion was what was the best thing to do.

I think the best analogy I can offer is that with some rock bands from the sixties and seventies who persist in their touring, and maybe they can’t sing too well any more and maybe the guitars aren’t working quite so well. We didn’t want to get into that situation for the great mission that Rosetta is, to come back after another orbit and to find things are a bit cranky and some of the instruments don’t work that well, and maybe not at all. We’ve already lost a few parts of the systems, so its at the end of its optimum lifetime already and this seemed to be the best time to end it.

An ESA video visualising the descent. Credit: ESA

Now we could just have switched Rosetta off, but this plummet is the way to go. We wanted to go out with a bang, with the rock and roll scenario that Rosetta is. We could have just switched it off and left it there, but something we have never managed on this mission is to fly below two kilometres. We’ve been trying to get Rosetta as close as possible in its orbit since about the 9th of August. About the 5th of September we were starting to really feel the anomalous gravity when we were doing these overflights, about 2km from the surface, and we couldn’t get any closer because it was really perturbing the orbit by changing the orbital period in fact, up 17 hours in one direction and 10 hours in another.

We were stopping that orbit from changing by doing some manoeuvres because we have to keep the orbit at a 3-day period. The impact here is that when you have this gravitational pulling of the spacecraft, it means it puts in a place at a different time, so when you’re pointing, you’re looking at a different place than intended. And that was what happened with the searches for Philae, if you look at the image we’ve got, Philae is down here and we’re supposed to be in the middle, the location we’re aiming at, and this is due to this timing mismatch because we’re being perturbed by gravity.

So anyway, we wanted to get close and this was the way. This kind of dive gets us below 2 km. We couldn’t get there by doing these overflights. We stopped trying those and said we couldn’t get any closer. We had already planned – and in hindsight it is good – we’d already planned to do this dive. It gets us to 2km and down to the surface. We’ve never been there. It is the acceleration region, where the comet’s material goes from ice to gas, where the comet’s coma becomes its outer atmosphere.

The latest in ESA’s charming animations to convey the mission to the public. Credit: ESA

There’s a thing called a Knudsen layer which is a transition region, and we hope to detect that, and we hope to see what is happening there with the dust and the molecules, plus the plasma interaction and also high resolution imaging of the surface. Basically it is unique science and we couldn’t do it without doing this landing. And it is basically maximising the science. As I was saying, when we get into October, we’re in conjunction and we couldn’t get any more data anyway, so we’re doing it right to the wire.

SN: So as we near the end, looking back, how do you see the mission? What have been the great successes from it?

MT: We’re only 10 per cent into the data analysis. We’ve only really started scratching the surface of the science. There’s so much stuff coming out already so we’re getting our first pictures of what we can see of the comet, what it is made up of, and what the importance of those small particles is. The fact that we’re seeing those organics and the importance of some of the silicate material and the dust. Basically the importance of the dust compared to the volatiles, the detection of molecular oxygen coming out. Some of the results are really constraining where the comet came from and how it’s evolving.

SN: Any particular highlight though?

MT: For me, the molecular oxygen I find fascinating, because oxygen is a friendly molecule which likes to get with other ones, but we found it by itself. That means it must have been trapped very early on and there are constraints and conditions for when the comet started to form.

It also has implications potentially for the presolar material as well, before the Sun was formed, so what the cloud or disk of dust was like then. So just by dissecting the abundance of that molecular oxygen gives you an idea of the situation and conditions that were there over 4.6 billion years ago. Just from measuring a molecule. So that to me is, Wow!

And there are a number of results like that, that say, OK, this means the comet must have been cold all the time; that the duck shape means two objects slowly colliding; and then subsequently getting more of the gases and finding glycine there. These things constrain how the comet was formed – glycine is important in terms of potentially delivering material to the Earth, and building blocks etc.

But we ‘re just scraping the top. It’s really then trying to combine all of these – saying, well we see this large scale structure, then we see smaller scale structures that may be the building blocks of the comet, and then you add that with all the volatiles that we’ve detected. All that really says, right, it must have have happened like this 4.6 billion years ago, here’s how the thing would have aggregated and stuck itself together.

SN: So you really feel we are beginning to unlock the secrets of the formation of the Solar System?

MT: Oh yes, we are already addressing this even now, even though we have just started touching the data. There’s so much of it and we’ve just started analysing it fully. And there will be decades of that to come.

SN: Rosetta has been a large part of your life for some time now. When this ends, how are you going to feel? And what next for Matt Taylor?

MT: Well, it continues for me. I stay on Rosetta. I stay on Rosetta for a couple of years to continue encouraging the scientists to work together to do the science, to get some journals together, to encourage more science to be done, set up workshops, that kind of thing, and also fundamentally to work with the teams in Spain, in the archive, to get that data in good shape for everyone to use.

SN: But do you feel, in a way, that you’re saying goodbye to an old friend?

MT: No, I don’t think so. But to be honest with you, you’ll have to ask me after the landing, I’m not sure at the moment. For now it is all about thinking forward, about how we’re going to do things with the data.

Actually, the only thing that came up was that I was talking to Fred Jansen, because he used to be the mission manager before he moved over to Gaia. And he said, you will feel there’s a hole, that there is something that’s not there. There is an adrenalin about it when Rosetta’s operating, because there is always something you might have to deal with, but that won’t be there any more. So that will be weirdly missed, I think.

So that aspect of having the phone next to the bed, and something happening – usually on a Sunday. My wife will at least be happy that that’s not happening any more!”

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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