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Comets regularly break and reunite again

They break up and then make up again – this sounds like a story, heard in a hallway of a high school. However, this is a scientific discovery about comets, made by Purdue University and CU-Boulder. It turns out some comets may regularly split into two and then reunite back together on their path around the Sun.

Comets, consisting of two huge pieces connected in the middle, can break when rotation speed increases, but later reunited again. Image credit: colorado.edu.

Comets, consisting of two huge pieces connected in the middle, can break when rotation speed increases, but later reunited again. Image credit: colorado.edu.

Team of scientists analysed several comets, but paid the largest portion of attention to a rubber duck-shaped object known as 67P/Churyumov-Gerasimenko (67P). It has been noticed that there are long cracks along the comet’s neck. Scientist wanted to see how they formed and what do they mean for the further life of the comet, so they used numerical models. They allow researchers to simulate a quicker rotation of the comet for the sake of research – they increased the speed of rotation from one rotation every 12 hours today to one rotation every 7 to 9 hours. Model revealed that a quicker spin would result in such cracks.

Professor Daniel Scheeres, one of the authors of the study, said: “Our spin analysis predicted exactly where these cracks would form. We now have a new understanding of how some comets may evolve over time”.

The comet 67P is characterized by two larger parts connected by a thinner neck and consists of ice, rocks and dust like all comets. Scientists say that such events as flybys of the sun or Jupiter can suddenly increase the spinning speed of the comet, and outgassing can have a similar effect too. The model also showed that further increasing the rotation speed to less than seven hours per rotation would cause the comet to split.

However, these two pieces would not go far away from each other and eventually would reunite. In fact, this breaking up and making up again could go regularly for entire lifetime of the comet. Scientists used the simulation to create 1,000 comet “clones” of 67P, testing various conditions that could have happened during the period of past 5,000 years. It showed that increase of the speed of the comet is chaotic, because of outgassing events that are unpredictable – they happen when icy compounds like carbon dioxide and ammonia shift directly from a frozen state to gaseous state and blow off the surface.

It is interesting that comets never split into two parts permanently. Even after the split eventually they slowly reunite until the next sudden increase of rotation speed. Even though it is not clear what implications this discovery has, it is very interesting to see the capabilities of numerical models used by astronomers.

Source: colorado.edu