I’ve been helping my wife with some simple algebra problems and I’m amazed how much I’ve forgotten.

“It’s my mind, Dave. I’m losing it. I can feeeeeel it…”

Now it is certainly possible that an enormous cloud of debris was ejected after the collision which collected in a ring about the primary, and eventually a body coalesced from that. As I mentioned earlier, that’s probably how our own moon formed.

Its easy to think of this situation as if they were billiard balls striking glancing blows on the felt. But billiard balls don’t gravitationaly attract each other.

And it is dynamically impossible for two bodies to interact in such a way that they wind up in orbit about each other unless there is either a collision or some third body which provides the additional momentum and energy required. Two bodies approaching from infinity will either collide, or swing about each other and go their separate ways.

And don’t forget, at least one of the bodies will probably be tidally disrupted by tidal forces prior to the collision. It would be like a melon being hit by a cloud of buckshot.

Plus, both partners in the dance are large bodies with significant momentum. Rather than a small impactor with insignificant mass, in the case of Pluto and Charon the moon has roughly a tenth the mass of its primary–largest such ratio in the System.

If Charon was captured once upon a time by Pluto, then right there don’t we have an example of a non-catastrophic rendezvous between two large free-flying objects? How in the worlds did that happen if two objects in space can’t get close without being drawn into a deadly shattering embrace?

My final whack on the horse is to suggest that smaller lumpy bodies that haven’t been vulcanized into solid objects by gravitational heating, might be more resilient in an impact, and might give and crumble rather than shatter. A collision of two bean bags rather than two billiard balls. Or even a billiard ball and bean bag.

Consider, grasshopper, that paper beats rock by enfolding it. Perhaps Pluto captured Charon not by shattering it, but by cushioning it. You know, lithobraking. Or maybe I mean cryobraking.

So for example, if the earth were where Pluto is, an approaching meteor might come along at an orbital velocity comparable to Pluto’s, which is slow (about 0.16 that of earth).

The difference in orbital velocities between the two might be just a few meters per sec. But by the time that slow object fell down the gravity well, it would have sped up to 11 km/sec.

Now granted, Pluto is not as massive as earth, so it’s escape velocity is only 1.2 km/sec. But that is still faster than a speeding bullet. Seen from a distance, the colliding worlds would kiss in slow motion, but to an unlucky observer at the point of impact, the incoming would approach at the muzzle velocity of an artillery shell.

http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html

The first article was interesting for touching on that “cryogenic realm”, and it linked to another mind-blowing article: Could a Massive Collision Produce a Subsurface Ocean on Pluto? (io9′s a consistently good read, isn’t it?). Mlinar’s theories seem plausible to this layman, and although they’re still looking for the linear surface features that on Europa are believed to be a sign of a subsurface ocean, I have to think those polygonal markings in the smooth surfaces must be a sign of some fluidity underneath. Doesn’t have to much, not an actual ocean sloshing around, but even convection is very interesting.

Thanks for the links, podrock.

http://space.io9.com/how-much-do-we-really-know-about-collisions-in-the-kuip-1720637251?utm_source=taboola

I found this part interesting:

“Impacts on the terrestrial planets and the moons of the giant planets are “hypervelocity” impacts, that is, the impact speeds are far greater than the speed of sound in the planet or moon, so the impacts are essentially explosions. Very roughly, hypervelocity impacts typically produce craters about 10 times bigger than the size of the impactor. Impacts on Pluto and Charon occur at typical speeds of 2 km/s, which is comparable to the speed of sound. Thus the impacts are barely hypervelocity.”

As always, the Universe always seems to be much more complicated and interesting than it really needs to be.