Inside the comet 67P / Churyumov-Gerasimenko there are no large voids. The Rosette mission of the European Space Agency made measurements that clearly demonstrated this and solved the old mystery. Comets are icy trash left over from the formation of planets 4.6 billion years ago. Only eight comets visited the spacecraft, and thanks to these missions we made a picture of the main properties of these space time capsules. Some questions were answered, others appeared.
Comets are known to consist of a mixture of ice and dust, and if they were compact, they would be superior in weight to water. However, previous measurements have shown that some of them have an extremely low density, much lower than that of water ice. Low density means comets must be very porous.
But what is this porosity: because of the large voids in the depths of the comet or because of the more homogeneous structure with low density?
In a new study published in the journal Nature this week, a group led by Martin Pazold of the University of Cologne, Germany, showed that comet 67P / Churyumova - Gerasimenko is also an object of low density, but does not have large cavities in its interior.
Hence, the most logical explanation would be that the porosity of the comet must be an internal property of dust particles mixed with ice, which makes up the inside of the comet.
This result is consistent with the earlier results of the CONSERT Rosetta radar experiment, which showed that the comet's “head”, consisting of two fragments, is fairly uniform on a scale of several tens of meters. Previous measurements of the apparatus have shown that comet dust is not compacted into a solid state, but rather “fluffy”, which ensures its high porosity and low density. The COSIMA and GIADA tools available on Rosette confirmed that it was precisely such dust granules that were observed at 67P / Churyumov-Gerasimenko.
The Pazold team made this discovery using the RSI experiment to study the process of attraction of the "Rosetta" by the gravity of the comet, which is generated by its mass.
The effect of gravity on the motion of the Rosetta was measured by changes in the frequency of the signals of the spacecraft arriving on Earth. This is a manifestation of the Doppler effect, which occurs whenever there is movement between the source and the observer. This effect changes the sound of emergency sirens passing by.
In this case, the Rosetta is attracted by the gravity of the comet, which changes the frequency of radio communications with the Earth. The 35-meter ESA antenna at New Norcia station in Australia is used to communicate with Rosetta during routine operations. Variations of the received signal were analyzed, and on their basis they made a picture of the gravitational field of the comet. Large internal cavities would be detected by a change in signal.
Such complex measurements of a comet were carried out for the first time precisely with the Rosetta mission.
“The law of Newton tells us that the Rosetta spacecraft, in fact, is attracted by all that it can,” says Martin Pazold, the main researcher of RSI. - From a practical point of view, this means that we must exclude the influence of the Sun, of all planets - from Jupiter to the dwarf planets - large asteroids from the inner asteroid belt, to the “Rosetta” motion, in order to leave the comet's influence entirely. Fortunately, these effects are well known, and this is the standard procedure currently in place for spacecraft operations. ”
Then it was necessary to eliminate the pressure of solar radiation and the gas tail leaving the comet. Both of these factors "blow off" the unit from the course. In this sense, the Rosetta ROSINA tool is extremely useful because it measures the gas flowing past the vehicle. Thanks to him, Patsold and his colleagues calculated and subtracted these effects.
The remaining motion is due to the mass of the comet. In the case of the comet 67P / Churyumov-Gerasimenko, this is slightly less than 10 billion tons. On the basis of the photographs, OSIRIS cameras formed a comet shape model and calculated a volume of about 18.7 km 3 , and with it a density of 533 kg / m 3 .
Determining the details of the comet's subsoil would not have happened if it were not for a fair amount of cosmic luck (even two shares!)
Given the lack of knowledge about the activity of the comet, for the safety of the device was chosen cautious approach trajectory. At best, the "Rosetta" along such a trajectory would not have come closer than 10 kilometers.
Unfortunately, until 2014, the RSI team said that it needed to bring the device closer than 10 kilometers to measure the internal distribution of the comet. This prediction was based on earth observations, which suggested that the comet would be round. From a distance of 10 kilometers and beyond, it would be possible to measure only the total mass.
Later it turned out that the shape of the comet is different. Fortunately for RSI, double fractions mean that differences in the gravitational field will be more pronounced and, therefore, easier to measure from afar.
“We have seen variations of the gravitational field already from 30 kilometers,” says Pazold. When the Rosetta entered a 10-kilometer orbit, the RSI team was able to make detailed measurements. She was absolutely confident in her results.
In September, Rosetta will perform a controlled collision with the comet's surface. This maneuver will provide a unique opportunity for flight dynamics specialists from ESA. As the “Rosetta” approaches, the complex gravitational field of the comet will impede the navigation process more and more. But for RSI, measurement accuracy will only increase. And allow the team to identify cavities the size of just a few hundred meters across.
The article is based on materials .
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