What would happen if Pluto's moon Charon's radius increased 10x?

You can’t assume they wouldn’t crash, because that is the most likely result due to gravity. They would either meld together, or they would each fragment.

What else are you changing or not changing? Does Charon increase in mass by 10x? How about the speed and/or distance they orbit each other?

Since you specify it changes such that they don’t crash, what other answer are you interested in besides Charon looks 10x wider, and they orbit each other?

I’m running a simulation right now. Actually I just started by increasing the diameter by a factor of 2, and keeping the density, so the mass went up quite a bit. This resulted in a dive towards Pluto that missed but then pulled in two of the other moons to Charon – crunch, crunch! Then antother swing around and a larger third moon hit Charon, changing its orbit a bit. Then I think the system stabilizes with a rather oblique orbit involving a near-miss of Pluto every three days. The surviving other moon, Nix, ended up launched out to a wide 265-day orbit of both of them. The results would vary depending on the starting positions when Gawd magically multiplied the size of Charon.

As for the 10x case you asked for, let’s see… I still don’t know whether you want the mass going up or not. If so, it’s clear… what happens is Pluto and the other moons get dragged into Mega-Charon like the Death Star pulling in the Millennium Falcon, only leaving huge glowing craters.

So for 10x the radius but the same mass… all the orbits are affected and it looks rather weird since Charon is now a giant puffball planet of very low density. The orbit of Charon relative to Pluto widens from a period of 6.4 days to 8.05 days, and the other moons’ orbits are altered. Kerberos gets close to Charon but doesn’t crash into it. The other three moons orbit further out and their orbits cross each other. I’m not sure if/when they’d collide or pass close enough to toss each other. But those specifics again depend on the starting point of the puff-i-zation of Charon.

Oh actually, on day 252 of the last scenario (puffball giant Charon), Charon overtakes Kerberos as their orbits come close, which draws Kerberos into a new steeper orbit that I think will lead to another encounter later… yep at about 279 days… near miss… even closer on day 300… then on day 324, Charon closes but is ahead in orbit… then they’re out of sync for a long time, so missing, and it looks like the diameter of Charon is a bit under Kerberos’ orbit, so they may be stable for quite a while…

Oh, I accelerated the simulation, and then about 2.25 years later, after it seemed stable, suddenly Charon flew away… I think it got too close to Pluto and was slingshotted away by the 8x heavier planet, but the simulation was running so fast I didn’t catch it.

I tried running it again, and yes the positions of the other moons matters. This time, Kerberos hit Charon pretty early, and the other moons got new more dispersed orbits that turned into slingshotted wide orbits eventually, but Charon and Pluto seemed pretty stable in a more regular orbit except for crazy fly-bys by two of the remaining moons.

Oh wow!

What type of simulation is it?

This was exactly the type of answer I was hoping for. Thank you!

Sure. I’m running Universe Sandbox 2 starting with their included data for Pluto and its moons, whose data I briefly checked matched this site . It allowed me to select Charon and change the radius, mass, and/or density, and then watch the results at whatever speed and from whatever position. It can create videos and screenshots, too.

Here is a gif showing what happens if you increase the radius by 10x and also increase the mass as if Charon had the same density as it actually has. Notice Pluto and the moons get yanked into it directly.

Is crashing the only possible scenario?
Could it be they might switch roles, and Pluto become a moon?

@Patty_Melt If the mass increases to match the 10x radius and the motion and distance aren’t increased, then yes they get sucked in very quickly. But if the moons were also given more velocity and distance, they could be fine.

If the mass doesn’t change, so you have a giant fluffball Charon or something, then it doesn’t take as much additional space or velocity. However there is still a bit of an issue in changing the mass distribution if you don’t adjust their orbits as well, because they’re in pretty close orbits and increasing the radius of Charon by 10x brings its surface close to the orbit of the innermost moon when they sync up.

I got some results where instead of crashing, near misses resulted in a new orbit for the smaller moons that sent them flying wide, which makes it less likely they’ll collide with anything after that happens.

As it is in reality, Pluto swings around due to Charon quite a bit. The lighter body will always be considered the moon, but their relative weights is reflected in how much one is affected by the other. Real Charon is a bit under 1/8 the mass of Pluto. If you multiply the radius of Charon by 10 and keep its density, it becomes vastly more massive than Pluto, and yes it’s a moon – and needs to be moving much faster/farther to not just be sucked directly onto the surface of MegaMassiveCharon.