Simulations suggest that comets routinely split and reassemble
When you look at a comet like 67P, perhaps hoping for a glimpse of a derelict lander, one of the obvious traits that stand out is how little it resembles other objects we’re used to looking at in the sky. It’s got a battered, rocky surface, but the overall shape isn’t the sphere or potato-shape that most planets, moons and asteroids seem to end up in, instead being compared to hourglasses or rubber duckies. Astronomers from Purdue University and the University of Boulder suspect that this may all be explained with a some cracks they’ve detected, and may not be as odd as it first seems.
The cracks in question are over 300 feet long, crossing over the narrow part of the comet. They seemed to be signs of structural stress in the comet, possibly precluding an eventual split between the to larger ends of the object. While the Rosetta satellite is still in orbit around comet 67P, researchers tested their ideas about the comet’s structural integrity in a computer simulation with the accumulated data that’s been gathered over the last year.
Separated by spinning
They found that a key to the comet’s durability is the speed of its spin. Right now, 67P is rotating once every 12 hours, but if that speed increases a bit, say to a nine to seven hour rotation instead, it could stress the center portion of the comet enough to break apart. At that point, the two spheroid portions would likely enter a small orbit around each other before eventually rejoining, repairing the duck/dumbbell/hourglass shape it has now. The trigger for a change in spin may be an extra push from passing near larger objects’ gravitational influence, like the Sun or Jupiter. Or it may be that as the comet warms towards perihelion, it sometimes releases bursts of gas from once frozen material. These events are common enough that researchers think this temporary split may have happened before, and is likely to happen again.
Commonly shared shape
In fact, they’re wondering if this may be a feature of comets in general. While we don’t have a very large population of comets to compare, five of the seven we’ve closely observed are this shape, including Halley’s comet. To further test this model, simulations were run of 1,000 copies of 67P, each through 5,000 years of orbits. At this point it seems very likely that orbiting in and out of our solar system, past Jupiter and the Sun, is a strong influence on comet shapes, setting them apart from the more spheroid objects in their less eventful orbits.
Source: Study shows how comets break up, make up, Phys.org