What the OSIRIS Pictures have already shown us Print E-mail
Tuesday, 29 July 2014 17:04

The ESA/Rosetta space probe, after ten years of interplanetary cruise, has now reached the vicinity of its target: comet 67P/Churyumov-Gerasimenko. Day by day, it is coming closer and closer. The scientific camera, called OSIRIS, has already taken stunning pictures of the comet nucleus, and other instruments have been active too. Here are a few highlights.



(1) Detection of the comet. Rosetta had been "asleep" for the last part of its journey, and on

Jan. 20, 2014, it was successfully woken up, which means that radio communication was re-established and commands could be sent to the instruments and other parts of the spacecraft. Just like the entire set of instruments, OSIRIS proved to be in good shape, and already on March 21, it took the first picture of the comet.

Even though this was just a faint spot of light, it touched our hearts because we had been waiting so long, and finally, here was the proof that the target is in reach.




(2) The comet is active. The orbit of comet 67P around the Sun takes it, roughly speaking, between the orbits of the Earth and Jupiter. Currently, it is slowly approaching the Sun but remains at a distance of more than 650 million km.

When the comet comes closest to the Sun, which happens every 6.45 years, it always looks the way a comet should look. It is a roundish, diffuse patch of light, from which a tail extends. We know the reason. The nucleus contains a lot of ice, which means frozen water, that sublimates into vapor due to the heating by sunlight. This leads to a huge cloud of gas and dust (tiny grains), which emits the light we see.

However, at the current large distance, the temperature is so low that not much vapor can come out from the nucleus. The comet is expected to be "inactive" like most comets at such distances. But because of the Rosetta mission, comet 67P had been scrutinized at the previous passage through the outer parts of its orbit, and we had seen signs of some activity at the very place, where the comet was early this year. Thus, we were eagerly waiting to see, if the faint, star-like spot of light would shine up with a diffuse cloud around it.

This actually happened in late April. We are not taken by real surprise, but still, the way it happened was more impressive than we had imagined. In the course of a few days, the little spot changed into a full-blown comet, which even had a tail!

To many, this was unbelievable. Moreover, it soon turned out that we were witnessing an outburst of activity in this distant comet, since the cloud and its tail faded away, and the comet looked inactive again. But the activity was soon to return, this time, apparently, more persistently.

So, what was happening? Which gas was pouring out from the nucleus? It had been speculated that there could be special chemical compounds, so-called super-volatiles, that would sublimate at very low temperature and perhaps build up gas pockets in the nucleus, which would explode, as the pressure increased. Carbon monoxide is such a gas.

However, Rosetta also carries a little radio telescope called MIRO, and in connection with the outburst, MIRO detected water vapor emitted from the comet. At this moment, it appears that this vapor might have sublimated directly from the nucleus, but it's too early to say for sure. MIRO was not able to detect carbon monoxide even if it had existed, so the explosions are still possible. If so, lots of icy grains could have been expelled, and the observed water vapor could have come from those grains.



(3) The strange shape of the nucleus. Starting from the end of June, the OSIRIS narrow-angle camera was able to resolve the comet nucleus. As time passed, more and more details were seen, and those really took us by surprise!

Other spacecraft had imaged other nuclei before, and their shapes had been irregular but not extreme. They looked more or less like potatoes. But this one turned out to be very different. Its shape can truly be called weird. It gives the impression of two nuclei stuck together, a bit like Siamese twins. The whole aggregate measures a few km across, and it is slowly spinning around an axis that runs through the biggest piece. The length of the day on this little cosmic object is 12.4 hours -- about half that of the Earth.


How come? Why does the nucleus look like this? This is of course the most interesting question, but it is difficult to answer. The preliminary impression is that we are seeing the result of a very slow and gentle collision, which happened when the nucleus was forming.

Here, we come to the very goal of Rosetta, namely, to try to understand the circumstances around comet formation. The hope is that this will help humanity to understand how the whole solar system was formed more than 4.5 billion years ago. No doubt, comet nuclei were formed at the same time, so when we watch a comet nucleus close-up, we see one of the oldest objects in the solar system, typical of the things planets were built of.

The more comet nuclei we see, the more we will learn, but this particular one will be a milestone because of all the detailed studies that are now being planned for Rosetta during the coming year.


Hans Rickman

29 July 2014

Last Updated on Tuesday, 29 July 2014 17:33
Start News The ESA Rosetta Mission What the OSIRIS Pictures have already shown us
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