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“Actually, we would like to carry on!”

An interview with Holger Sierks from the Max Planck Institute for Solar System Research on the end of the Rosetta mission

On 30 September, the Rosetta mission to the comet 67P/Churyumov-Gerasimenko came to an end. Felicitas Mokler spoke about this with Holger Sierks from the Max Planck Institute for Solar System Research, who headed the consortium for the Osiris camera system on board the space probe.

Mr. Sierks, the Rosetta cometary mission has come to an end. Doesn’t this make you feel a little sad?

Holger Sierks: The mission lasted around 30 years: starting with the orientation phase at the scientific level, then the planning and construction phase, and finally the travel time to the destination comet. During the past two and a half years, Rosetta has accompanied the comet at a close distance. The end is therefore certainly very emotional for all those involved. Only a very small number of colleagues remain from the pioneering phase; I myself came on board 20 years ago. Rosetta is therefore a good example of intergenerational work in space research. What’s more, the space probe had been still functioning perfectly. Actually, we would have liked to carry on!

But would that have been possible?

Sierks: The alternative would have been to again put the probe into hibernation and to reactivate it again after the comet had reached its furthest point away from the Sun. But the fuel would then not have been sufficient for the comet to re-approach the Sun and observe the next cycle of activity. That’s why we decided to land the space probe on the comet now. This allowed us to approach the comet’s surface closer than ever before in the mission and obtain images from a distance of 1000 metres during the descent – this is something we’ve always dreamed of – and then from 500, 200, 100 metres...

What do you hope to see in these close-ups?

New insights into the dust structures on the surface of 67P/Churyumov-Gerasimenko. We expect grain sizes from a few millimetres to a few centimetres. The size distribution of these materials tells us something about the activity of the comet. We want to know: Is the grain size determined by the physics on the surface? How does the dust extricate itself from the surface? We would also love to find out something about the comet’s formation phase. Is there a connection between the size of the dust grain which we see today and that which predominated at the time in the Solar System when the comet formed? These images would certainly increase our knowledge.

How did Rosetta land? Was there no danger of the probe bouncing back into space because of the low gravity? It could not have clung to the comet, it wasn't built for that.

Rosetta reached the surface as slowly as Philae did. There was no crash. The probe did certainly bounce back slightly, but it did not reach the escape velocity and finally, it remained on the surface. But we were not able to observe whether and how it tumbled because when it touched down, its radio antennae no longer knew where Earth was. The space probe then switched off automatically.

Rosetta is a European mission with other international partners. Which projects is the Max Planck Institute for Solar System Research involved in?

We have made key contributions to many of the instruments. We have the system management and head the consortium for the Osiris camera system, we contributed the cameras and are in charge of the scientific evaluation. The consortium includes around 90 scientists from twelve institutes in eight countries – not only from Europe, the USA and Taiwan are involved as well. The Cosima dust experiment is also under the auspices of our Institute.

Furthermore, we contributed to the Miro and Rosina instruments. For the Philae lander, the Max Planck Institute is in charge of the Cosac gas analyzer. We have provided contributions to the Romap, Sesame and Consert instruments and also supplied crucial input for the construction and development of the lander.

What was the most interesting aspect of the mission for you personally?

I was moved most by the discussion about the origin of the comet. We hope to gain some insight into what the Solar System looked like during the first few million years. The cometary nucleus which we see today is thought to have formed from two smaller ones. In the gas phase of the accretion disc around the young Sun, these nuclei decelerated and collided with each other at very low speed.

What I also found exciting are the cylindrical sinkholes, where we look down from the surface almost 200 metres into the inner structure of the comet, and this on a cometary nucleus with a radius of only 2,000 or 1,000 metres! Although the material there has certainly been processed somewhat by the solar radiation, we were looking into the depths of the comet and thus maybe back into its history over the last 4.5 billion years. The inner walls of these sinkholes are by no means smooth and homogeneous. They have very sharply defined structures on the two to three-metre scale, which resemble a number of oranges in a crate of oranges. This wants to tell us something, and we would like to understand it.

I expect that quite a lot of data is still waiting to be evaluated. How long, in your estimation, will it keep you and your colleagues busy?

Collaborations from the Giotto mission, which flew past Halley’s comet 30 years ago, still exist today. I assume that we will need 20, 30 years for Rosetta as well. What I mean here is not only the analysis of the Osiris image data, but also the global analysis of the spectrometer data, the thermal, millimetre and sub-millimetre data of the structures close to the surface from Miro and the other instruments aboard the space probe.

As far as the Osiris images are concerned, we initially have three years to collate a comprehensive archive. This procedure is new in the research community and for the European Space Agency ESA as well. This work is normally completed when the data is handed over after 12 months. We will calibrate the images, put together mosaics and terrain models, and then make the products available to the public and the scientific community.

Your bottom-line at the end of the mission?

In 2014, Rosetta managed to appear on the front cover of Science with the caption “Breakthrough of the Year”. I believe that the mission really must be classified as a breakthrough in cometary research.

What will be the next step in cometary research after Rosetta?

I think the scientific community agrees that the next step has to be to fetch cometary material to Earth, and analyze it in laboratories here, especially the organic components. We are already considering how we would design such a Sample Return Mission.

We look at the comet and ask ourselves: Where would even sample the material: at the surface? Or would we dig slightly deeper? And what laboratory equipment would be needed on Earth to be able to analyze this material? It will certainly take another 30 years until such a Sample Return Mission is realized, maybe slightly less. We thus come full circle in setting intergenerational, scientific goals.

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