If you have a hankering to avenge the death of the dinosaurs by smashing apart a comet with your bare (spacesuited) hands, it might be easier than you think. An analysis of 67P/Churyumov-Gerasimenko suggests it is held together by forces a million times weaker than styrofoam. The work could shape how we tackle frozen threats to our planet.
Dr Nicholas Attree of Aix-Marseille Université, France, and scientists from 27 other institutions studied twenty overhanging cliffs seen by the Rosetta spacecraft as it circled 67P. Knowing the comet's gravity, they were able to calculate the minimum tensile strengths (resistance to lengthwise stress) required to prevent these cliffs collapsing. The gravity is so weak, just one ten-thousandth of Earth's, the ice and rock doesn't need to be bound tightly at all. Forces of around 0.3 Pascals would be sufficient.
If that was all we knew, it wouldn't tell us much. The calculated values are minimums, and the bonding could be much higher. However, in a paper to be published in Astronomy and Astrophysics (preprint on ArXiv.org), Attree and co-authors note many overhangs have eroded material at their base, indicating material frequently falls off. Unless the fallen ice once had radically different shapes to what remains, or was broken off by some more powerful disruptive force, it can be assumed the tensile strength of most, if not all of the comet's surface is not far from the calculated minimum.
The study also found 67P is quite homogenous, at least on this measure. Neither of the two great lobes that form its odd shape has significantly larger overhangs than the other, as would be expected if they were made of different material. Similarly, there is no trend for the overhangs to get larger or smaller towards the point where the lobes join.
“Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small (tens of km) body,” the authors note in their study. The rubble pile hypothesis, where comets form from small bits of rock and ice gently nudging into each other, has competed with models where they are formed by higher speed impacts, creating more solid objects.
When (not if) the Earth is again threatened by a comet, we might push aside a tightly bound object, but would need a different approach to a loosely bound rubble pile. Having only examined one comet in this much detail, we can't be sure that all such objects formed in the same way. It's possible there are more strongly bound “dirty snowballs” whizzing around the Solar System, and asteroids are a different matter entirely. On a sample of one, it's time to start planning.