Rubber duck comets 'decapitate' and reconnect


Comet 67P looks like two blobs stuck solidly together. But new research shows it may have broken up and rejoined multiple times over its life. Amy Middleton reports.


Splitting and reuniting may be fundamental to comet evolution.
ESA / ROSETTA / MPS FOR OSIRIS TEAM MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Comets, particularly those shaped like a rubber duck, may break apart and reconnect many times throughout their lifespan, much like a leading couple in a soap opera.

It sounds absurd, but this is the finding outlined in a new paper, which used data from the Rosetta mission and was published in the journal Nature.

Researchers at Purdue University and University of Colorado Boulder in the US wanted to explore the splitting that occurs in the nucleus, or solid nuggety part, of all comets.

Comets are objects made of ice, dust and rocks that orbit the Sun in less than 200 years. A "bilobed" comet is one of those shaped like two globes, separated by a neck of sorts.

This is a fairly common shape for comets – in fact, it describes five of the seven comets that have been photographed in high-res by astronomers, including Comet Halley.

To reach their finding, the researchers investigated cracks in the nucleus of a particular comet, discovered in 1969, named 67P/Churyumov-Gerasimenko (67P). 67P is roughly four kilometres long and orbits the Sun every 6.5 years.

Using modelling, the team showed the comet’s lifespan – around 5,000 years – including changes in its variable spin rate. Currently, 67P rotates roughly every 12 hours, but this may not always have been the case.

This process of make-up, break-up repetition may happen many times during the comet’s lifespan

The rotation rate of comets can be thrown off-kilter by the release of gases from its surface, or by gravity when the object passes close to the Sun or a planet.

A faster rate of rotation, the modelling suggests, would cause stress to the object – possibly enough to crack open the neck between the two separate lobes, breaking it into two. But not for long.

“The head and body aren’t going to be able to escape from each other,” says Daniel Scheeres, an engineer at CU-Boulder and co-author of the study.

“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.”

This process of make-up, break-up repetition may happen many times during the comet’s lifespan, and, according to the researchers, may be central to the evolution of these objects.

“Our spin analysis predicted exactly where these cracks would form,” explains Scheeres.

The researchers say the theory is supported by the lack of evidence of comets during an event known as the "Late Heavy Bombardment", in which an unusually high number of asteroids and other objects hit our Solar System around four billion years ago.

This instability of comets, according to the paper, may explain why these objects never made it to our inner Solar System during this galactic barrage.

  1. http://www.nature.com/nature/journal/vaop/ncurrent/full/nature17670.html
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