A new study spearheaded by researchers from the Rheinische Institut für Umweltforschung an der Universität zu Köln, Germany, has used data collected by ESA's Rosetta spacecraft to establish that the comet 67P/Churyumov-Gerasimenko (67P) is devoid of any large interior caverns. It had previously been theorized that the relatively low mass of the comet in regard to its volume may have resulted from cavernous hollows within the celestial wanderer.
Mankind has sent robotic pioneers to a grand total of eight comets since the advent of space exploration. Whilst these explorers have been successful in solving numerous mysteries regarding the nature of these celestial bodies, the riddle remains as to why comets possess such a low density.
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The new study looked to shed light on the question, making use of Rosetta's Radio Science Experiment (RSI) to probe the interior of 67P for large hollows by measuring the gravitational effect exerted by the comet on the probe's radio signal to Earth.
In order to accurately measure the comet's effect on the signal, the Rosetta team had to remove any extra gravitational influences and the influences of other forces in our solar system that could be distorting the signal. This involved subtracting the gravitational pull of our Sun and every other celestial body in our solar system, and even the pressures exerted by solar radiation and the outgassing emanating from 67P itself.
Thankfully, complicated processes such as this have become routine in space exploration missions, and with some work, the scientists were able to subtract the myriad of conflicting influences leaving only that of 67P.
Upon analysis of the radio signal, it was clear that 67P was devoid of any major caverns, which would have been evidenced by drops in acceleration of the signal. It was however discovered that the comet boasts a mass slightly below 10 billion tonnes, which, factoring in the lack of large caverns and an overall volume of roughly 18.7 km3 (4.5 cubic miles) places the density of the comet at 533 kg/m3.
With large hollows ruled out as a factor, it is now most likely that the low density of the comet is due to the properties of the dust particles that comprise the interior of the comet.
A previous study based on data collected by
Rosetta's COSIMA and GIADA instrument revealed that the particles
that mix with ice to make up the interior of the comet are far from
compact, instead taking on a "fluffy" aspect. This results
in a high volume, low density porous interior.