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SYFY WIRE Bad Astronomy

How to Make a Rubber Ducky Comet

By Phil Plait

Before we knew what comets and asteroids looked like up close, it was popular (at least in media) to imagine them as roughly spherical, maybe a bit lumpy.

The reality was way stranger. The first comet seen up close, Halley’s, was actually more elongated, like a rock you might find in your back yard. As we sent space probes to more of these celestial flotsam, we found most were oddly shaped, and some downright bizarre: a double-lobed bowling pin shape kept popping up. Hartley 2, Wild 2, Kleopatra (which looks like a cartoon dog bone!), and now, most recently, 67P/Churyumov-Gerasimenko, the new home of the Rosetta probe. The solid part of this comet looks like a rubber ducky, with a large, flattish lobe connected somewhat off-center to a smaller, more rounded chunk.

What gives?

I’ve written about this before. My original thinking was that these shapes are due to a slow collision by two bodies, which manage to stick together. 67P, however, has many features that look more like it was one object that has been eroding away in the middle, creating the double-lobed shape.

Now it looks like the former was more important than the latter. A new study has just been published showing that slow-speed collisions between two objects can create the shapes we see.

The researchers used three-dimensional modeling to determine how this could work, and the video above shows a representative example. A larger and small body collide slightly off-center, causing material from the smaller one to splash on to the bigger one, and slowing their relative speed. Their mutual (weak) gravity draws them together again about a day later, and they wind up sticking to each other, forming the familiar two-lobed morphology.

This also explains a peculiar layering seen in some comets, due to the splashing of material from one object to the other. It’s nice when a single model can explain more than one physical characteristic.

Collisions like this may have been common in the very early solar system, when there were a lot more objects out there; as the giant planets formed after a few tens of millions of years, their powerful gravity wound up eliminating many of those comets and asteroids (either by drawing them in and assimilating them, flinging them away and ejecting them from the solar system, or dropping them down in to the inner solar system in an event called the Late Heavy Bombardment, the scars of which are still visible on the Moon today). A collision like this would be rare now.

If this scenario is correct, then, looking at 67P, I have to think both processes are at work there. It originally formed as a slow speed collision, and then erosive processes have been at work for quite some time since. The cliffs on the smaller lobe appear to be due to cleaving or calving of the comet there, and you wouldn’t expect such large flat features after a collision.

That wouldn’t surprise me at all; a lot of the features we see in astronomical objects in the solar system today are the result of many processes, some of them ongoing. These things have been around a long time, after all, and there have been periods of fairly intense activity since the whole place formed some 4.56 billion years ago. It’s incredibly rare to find an intact, pristine time capsule from that time so long ago, and we have to be aware that, in simple terms, stuff happens. So to speak. 

If this model is true, it means that collisions like this were common (since we see double lobes so often) and that most would’ve happened a long time ago. This may be testable by examining these dog bone/rubber ducky/bowling pin objects and seeing if we can determine just when they may have assumed these shapes. If it happened right after the solar system formed, then great! If not, well, then we’ll have to modify the hypothesis or abandon it.

Such is science. We sometimes have to follow a path to a wrong idea to make sure it’s not right. But even then, it may be salvageable, and at worst we’ve learned something anyway. Science is pretty cool that way.

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