Rosetta paints a new picture of comets


A completely changed understanding of comets. This may be the outcome of space probe Rosetta’s twelve-year journey through the solar system, which came to an end on Friday 30 September. Now, researchers continue to discuss how to interpret all the data Rosetta has collected over the years and sent back to earth.

One of the many amazing images produced by the Osiris camera on Rosetta. Hans Rickman was part of the development of filters for the camera before its journey through space. Photograph: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Hans Rickman, Professor
Emeritus of Astronomy.

Rosetta’s journey through space came to an end in a crash landing on the comet 67P/Churyumov-Gerasimenko, which Rosetta has studied for several years. Hans Rickman, Professor Emeritus of Astronomy, has been part of the European Rosetta project since it began. The purpose was to learn more about comets.

“We wanted to know more about the whole solar system and how it came about. Thirty years ago we studied Halley’s comet and began to view comets in a new way, as witnesses to the formation of the solar system. Comets were there from the start and probably hadn’t changed since. This was something new, when we realised their important role.”

What are your feelings now that the project is coming to an end?

“Every day, e-mails about Rosetta have poured in when I have opened my inbox. It’s been that way for several years and now it’s over. What remains now is to try to agree on how to interpret the images. We are still trying to arrive at what we have learnt about comets. I look forward to seeing the outcome.”

Does the data tally with the current understanding of comets?

“No, it doesn’t. Not at all. That is the most prevailing impression. We have seen so many important things that seem to be contrary to our preconceptions. Our fundamental knowledge of comets is now being questioned.”

Comets with no ice

For instance, comets have previously been thought to be made up largely of ice. This is now being questioned.

“I have had the view that a comet is like a dirty snow drift, but more recently I have begun to regard the dirt as more important and the ice as less important. One possibility is that the comet core almost entirely consists of rock and that the proportion of water or ice is smaller than we could ever have guessed. These are still only indications, but some of us argue quite strongly that the ice only makes up a smaller part of a dominantly rocky material.” 

A more theoretical question is how primordial the comet core is. Hans Rickman and a researcher colleague have investigated if it really is possible for comets to have spent the first half billion years in space without crashing into each other and breaking up. Are comets really completely intact, or are they actually fragments? 

“The researchers today belong to two camps: Those who believe 67P/Churyumov-Gerasimenko is a fragment, and those who firmly say it is a primordial comet. These are interesting times”, says Professor Rickman.

Developed filters for the camera

Professor Rickman’s part of the Rosetta project was to help develop the Osiris camera and order Swedish, custom made filters.

“We have worked with different filters where we have isolated light and certain wavelengths to get more detailed information about which elements are whizzing around and what colour they are.” 

The filters were made by a firm in Täby (Stockholm) and Hans Rickman was their main contact. 

“One of the challenges was that the camera would sit on the space probe for ten years before reaching the comet. It’s a tough environment with energy-rich particles bombarding it with cosmic radiation which can damage the filters.” 

Because of this, the filters were first tested in the Tandem Accelerator at Uppsala University. The filter substrates were bombarded with particles to make sure they would not be damaged.

Incredible images

The camera did in fact survive the journey through space, as did the other instruments within the space probe – all results of the European cooperation. Apart from the comet landing in 2014 when the lander Philae first bounced back and then touched down in a completely different spot, most things have gone according to plan.

“What fascinates me is that things have gone so well. There are so many things that could have gone wrong. We, the camera team, have produced loads of incredible images and what you see in those pictures has nothing to do with that snow drift I was imagining,” says Hans Rickman, who also has contributed to the interpretation of the images from Osiris.

Background facts

In March 2004 the European space probe Rosetta was launched from French Guiana.

The space probe is equipped with two 14 metre long solar panels with a total surface of 64 square metres. Its weight is some 3,000 kg (of which the lander Philae made up 100 kg) and its size is 2.8 x 2.1 x 2.0 metres.

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Last modified: 2023-08-04