abyss.uoregon.edu/~js/glossary/roche_limit.html

When two objects collide, they must do so NO SLOWER than the escape velocity of one relative to the other (remember, escape velocity is a scalar, not vector, quantity). If you approach your target at any speed, you fall towards it gaining even more speed, (relative to the target)regardless of the angle of approach. In the slingshot maneuver, the smaller object leaves the encounter faster than it approached (that’s the whole point). Momentum and energy of the entire system are conserved, of course, by the planet slowing down by a miniscule amount. This is transferred to the spacecraft, but there is no contact. Planetary accretion always involves a lot of fireworks, the planetesimals convert energy of motion to heat, that’s why there is so much evidence of massive lava flows on the moon. That comes from the gravitational energy lost by the collision.

The gravitational interaction of two approaching bodies offers only a limited number of alternatives. As they fall towards each other, the gravitational potential energy is converted to kinetic energy. They will then either collide inelastically (a lot of the kinetic energy is converted to heat)or they will swing around each other (if there is any lateral vector of velocity) and go slingshotting in opposite directions at speeds inversely proportional to their masses.
(If one body is much more massive than the other its peturbation may not be noticeable, as in the spacecraft example, but it is there, Newton demands it. There is no “glancing blow” although large amounts of debris may be ejected into orbit, as we suspect may have formed our own moon.

What you never get as a result of such an interaction is a mutual orbiting of two intact bodies. That must be done by adding or subtracting excess energy (aerodynamic braking, a powered rocket burn, or other masses nearby to provide or drain off excess energy and momentum. The latter is dynamically possible (its how you get ejections) but its very rare, things have to line up just right.

To put it another way, place two isolated stationary masses a large distance apart, and mutual gravitation will cause them to eventually collide at escape velocity. And that will not be the gentle kiss of snooker balls, it will be catastrophic.

Of course there can be. Orbital mechanics can be very counterintuitive–”go faster to go slower (higher)”. The example of the correct dynamic I had in mind is like the slingshot maneuver, in which one object approaches another from behind in its orbit, traveling at a higher velocity. As the overtaking object passes the other, the other’s gravity pulls the overtaker; at the correct angle for a slingshot it goes careening off in another direction, but at other angles it can slow down as the first object pulls it “back”. I’m pretty sure that most Mars orbital insertions have leveraged the effect to save energy.

Those kinds of maneuvers may seem improbable only because when we do them we have a goal in mind and it’s hard work to accomplish that goal. But stuff happens in Nature, and somewhere sometime some random collection of co-orbiting rocks can have all kinds of jolly fun a bumpin’ and a scrapin’. Planetary accretion can happen because not all collisions are high-velocity catastrophes that vaporize the participants.

Enough of that rant. But apropos the question of gravity, one thing I find fascinating about the full-globe Charon images is the high relief of the terrain and the jaggedness of the horizon. It’s a small body and it’s visibly lumpy. Its own gravity hasn’t smoothed out all its proud scars. You can’t think of Charon as a “billiard ball”.

And two large co-orbiting bodies with matching scars? Oh yes I think your theory is plausible, podrock. You may be on to something.

I’m just thinking out loud here. I’m relying on my astronomical intuition, not on a rigorous analysis. We are new to this system, and its located in a neighborhood we know little about. The system is full of surprises. I may be missing something, in fact I may be totally off-base. I’m just giving you my informed and educated opinion, but it is nonetheless just an opinion.

And then there’s this (from the Wpkipedia article on Charon)

Simulation work published in 2005 by Robin Canup suggested that Charon could have been formed by a collision around 4.5 billion years ago, much like Earth and the Moon. In this model, a large Kuiper belt object struck Pluto at high velocity, destroying itself and blasting off much of Pluto’s outer mantle, and Charon coalesced from the debris.[21] However, such an impact should result in an icier Charon and rockier Pluto than scientists have found. It is now thought that Pluto and Charon may have been two bodies that collided before going into orbit about each other.

Now maybe Charon is small enough and “sticky” enough that the tidal forces cannot overcome the tendency of its rocks to bind together, both gravitationally and mechanically. I really don’t know the answer to that.

But if that were the case, the two worlds would have highly elliptical orbits. Nice, round orbits strongly suggest the system is stable, and either formed from normal accretion during Pluto’s formation, or is the remains of an ancient collision between comparably massive bodies. Post-formation collisions and even near misses would leave evidence in the celestial mechanics of the system.

According to the Wikipedia entry on Charon, the eccentricity of its orbit is 0.00, almost perfectly circular to two sig figs! This strongly suggests a well-behaved origin to the system, from hierarchical condensation from some primordial nebula; or a collision scenario similar to the one proposed for our own moon, but no post-formation collisions or major disruptions from external third bodies. A near-circular orbit resulting from a collision is possible because only the fragments with low eccentricity orbits are available for accretion. The others either are ejected from the system or collide with one of the two principals

The matching red stains may be connected somehow, but I don’t thing a fender bender did it. The Pluto-Charon system is tidally locked, just like the earth-moon system. It would be very interesting if the two red spots faced each other continuously as the two worlds
circled each other.

EDIT: Link to earlier discussion

Do we know the orbital elements of the various satellites of Pluto? I know there are several others besides Charon. If they are all in neat, regular, near-circular orbits rotating in the same direction, this suggests an orderly accretion-driven formation to the system. OTOH, if they are in highly elliptical orbits, if any are retrograde or in different planes, perhaps locked into complex resonances, this suggests the system may have been formed under more catastrophic circumstances.

While the distance between these objects is not to scale, apparently the size is. Both have a scar. About the same size. Pluto is missing a spot of reddish tholin paint. Charon has a smear of tholins around its scar.

Thoughts?