Comets can break apart and reform later, study finds

Despite the countless observations and massive amounts of scientific data the ESA’s Rosetta orbiter and Philae lander have collected from Comet 67P/Churyumov-Gerasimenko during the last several years, one great mystery remained: the object’s unusual rubber-duck shape.

Now, scientists from Purdue University and the University of Colorado-Boulder have found an explanation as to why the two-lobed comet has such a distinctive look: it, and other comets, can and often do break apart and reform as part of a process essential to the comet’s evolution.

The process takes place in periodic comets, or those that take less than 200 years to orbit the sun, and occurs when destructive forces cause the nucleus(the solid center of the comet) to split apart at a rate of about 0.01 per year per comet, Masatoshi Hirabayashi, a postdoctoral associate in the Purdue Department of Earth, Atmospheric, and Planetary Sciences, and his colleagues explained in the Wednesday, June 1 edition of the journal Nature.

While, as Space.com noted, previous research suggested that a comet’s nucleus can break apart and that such events occur in approximately one comet per century, little was known about the process. So Hirabayashi’s team analyzed data from Comet 67P and discovered that it (and other comets like it) may regularly experience cycles in which they break apart only to reform later.

Findings could represent a new step in the evolution of comets

While studying Rosetta’s comet, the researchers found a pair of cracks, each roughly the same size as a football field, that connected the two lobes of the so-called “rubber duck” comet. They then developed a numerical model where they increased 67P’s spin rate from one rotation per 12 hour period to one every seven to nine hours to see what this would change.

They discovered that the faster spin rate would cause more stress, and result in the formation of a pair of cracks similar in size and location to those along the neck of 67P. Based on their simulations, the study authors suggest that the spin rate of the comet’s nucleus would speed up when sunlight converted ice directly into gas plumes. The extra push the comet received from these jets could cause a strain, and that strain would ultimately lead to fractures.

By increasing the comet’s spin speed to less than seven hours per rotation, the researchers found the “head” of the rubber duck comet would pop off, co-author Daniel Scheeres, a professor in the CU-Boulder Aerospace Engineering Science Department, explained in a statement. At this point, the head and body will begin to orbit one another, and after a period ranging from a few hours up to several weeks, they will slowly collide and the comet will be reformed in its entirety.

“When we were looking at 67P, we sort of knew intuitively that if it spun fast enough, the head and body should just separate from each other,” Scheeres told Space.com. He and his colleagues examined four other comets with similar, two-lobed nuclei and discovered that the structures of these objects were consistent with this destruction-reformation cycle, suggesting that this sort of behavior is fairly common in comets that regularly approach the sun.

The professor added his team plans to analyze more comets to better learn about the shape and spin evolution of these objects in order to “better understand how this new evolutionary process we’ve identified can change comets over time” and to “explain unknown phenomena linked to comets, like how they can brighten very dramatically at random times in their orbits, or disappear from view. We think these sorts of phenomena may be linked to how they can spin fast enough to break apart.”

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Image credit: ESA

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