Comet relationships are rocky ones – these objects easily break up and eventually reunite.

Researchers from Purdue University and the University of Colorado recently found that some comets – particularly periodic comets, which orbit the sun in fewer than 200 years – may routinely split into two and then rejoin down the road. This may explain why many of them feature a number of lobes, calling a rubber duck shape to mind.

The team analyzed different comets, mainly the popular 67P/Churyumov-Gerasimenko (67/P), which revealed two lengthy cracks separating its two large lobes. They built a computer model to reconstruct the comet’s past life, finding that a spin rate twice as rapid as 67P’s would be enough for a similar object to crack and then break apart.

"Our spin analysis predicted exactly where these cracks would form. We now have a new understanding of how some comets may evolve over time," said co-lead author and CU-Boulder professor Daniel Scheeres.

Different factors can lead comets to spin faster, including the gravity of objects in transit or their “outgassing,” or when icy compounds go from frozen to gaseous and blow off the surface. Comet 67/P, for instance, can be affected by gravity during flybys of Jupiter, causing it to spin up or down.

Based on the numerical models, the comet's head will pop off if its spin is increased to less than 7 hours every rotation. Afterwards, the head and body, instead of escaping each other, will start to orbit each other and come back together in a gradual collision for hours or weeks.

This pattern likely goes on in the comet’s lifetime, Scheeres added.

According to previous studies, this cycle of destructing and reforming might explain why comets were not a huge part of the Late Heavy Bombardment, a massive onslaught of rock that struck Earth and the inner solar system around 4 billion years earlier. Many comets would have likely been destroyed as they roamed closer to the sun.

Scheeres said future research will probe the shapes as well as spin evolution of comets, looking at how the evolutionary process can alter the rocky bodies over time and help us understand mysterious comet-related phenomena, such as their random, significant brightening in their orbit.

The findings were published June 2 in the journal Nature.