Comets break up, make up during journey

 Periodic comets - objects that orbit the Sun in less than 200 years - may regularly split in two, then reunite down the road, a new study has found.

This may be a repeating process fundamental to comet evolution, according to the study led by Purdue University and the University of Colorado Boulder in US.

Researchers studied several comets, primarily a bizarre rubber duck-shaped object known as 67P/Churyumov-Gerasimenko (67P). Images of 67P show two cracks on the comet's neck that connects its two larger lobes.

In order to reconstruct the past life of 67P, the team used numerical models in which the spin rate was cranked up from its roughly one rotation every 12 hours today to one rotation every 7 to 9 hours.

The models showed the faster spin would lead to more stress and the formation of two similar cracks on the neck of 67P in the same location.

"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 Daniel Scheeres from CU-Boulder.

Often referred to as 'dirty snowballs,' comets are made of ice, rocks and dust. Comet 67P is "bilobed" meaning it has two larger parts connected by a thinner neck.

Scheeres said there are several factors that can cause comet nuclei to spin faster. During flybys of the Sun or Jupiter, for example, periodic comets like 67P can get torqued by gravity, causing them to either spin up or spin down.

The spin also can be affected by periodic comet "outgassing," when icy compounds like carbon dioxide and ammonia shift directly from a frozen state to gaseous state and blow off the surface.

The models run by the team showed that if 67P's spin is increased to less than seven hours per rotation, the head will pop off, said Scheeres.

"The head and body aren't going to be able to escape from each other," he said.
"They will begin orbiting each other, and in weeks, days or even hours they will come together again during a slow collision, creating a new comet nucleus configuration," he said.

This pattern could go on for the life of the comet, said Scheeres.
Bilobed comets may turn out to be fairly common. Of the seven comets that have been imaged in high resolution by astronomers, five of those - including P67 and Comet Halley - are bilobed, said Scheeres.

Studies of the bilobed comets by the team indicate they all are similar in their volume ratios between each lobe, meaning they probably go through the same break-up/make-up cycles as 67P.

Discovered in 1969 and visited by the European Space Agency's Rosetta spacecraft in 2014, 67P is roughly four kilometres on a side and orbits the Sun every 6.5 years.
The study was published in the journal Nature.