Found this on Digg today and promptly wasted an hour playing around:

http://dan-ball.jp/en/javagame/planet/

Among other atrocities, I rammed a few large planets into each other to see how the debris fields would interact.

How the heck do you start it? Can't find a link or anything...all I see is the instruction text & comments!

Drag and click your mouse on the "space" and it will start a body in motion. It's really kind of mesmerizing to watch it and wait for "encounters"

It's diabolically addictive...almost hypnotic.

I've been hooked on it all night, after noticing babakm's note. It's simple...left click to add an object, and right click in the menu area to change options. I haven't found a limit to the number of objects the app can handle. I've been clicking until my finger wore out, creating a cloud of hundreds of objects.

I am having a difficult time getting small particles to fall into orbit around larger ones as moons that persisted very long, but my most recent simulation managed to create a system with a large body revolving wildly close to the sun and a small object locked into a resonance with it on the same orbit. I wonder if such a configuration could even be resolved with current techniques used to observe exoplanetary systems.

It was interesting to note that whether I used random or circular initial orbits, it didn't take long for the system to purge most of the chaos I imposed on it. It was fun to sit back and watch the carnage, with the occasional object ejected from the system as if it was fired from a rifle. You can change many of the parameters, and even insert additional, stationary stars, but then it gets really crazy.

This certainly is an improvement on the old DOS program that simulated orbits.

Just try adding more stars... you get some really interesting orbits when you have three or more gravity sources in the mix.

I placed three stars in a rough triangle, and found that I couldn't get a trajectory on any object that didn't impact one of the stars after only a few orbits. The planets tended to shoot up from one of the stars and then head in on a death-dive into one of the other ones.

-the other Doug

Set the speed to Zero, and then select the Random option. Cycling through the different random options gives faster random speed I think. Plus, with Random, you can hold the mouse button down and drop planets like crazy when paused. It is fun also to hold the button down non-paused, and with the trails sent to long lines. This makes it almost look like one of those magnetic line field maps.

I think what's educational about this is the encounters between bodies. We often hear how historical planetary enounters may have either captured a comet (planet or a sattelite) or ejected it from the solar system .

If you watch this long enough you can see those things happening. You can also observe the close encounters between objects with similar orbits such as goes on with Saturns shepherd moons.

This is really neat. Educational yet very easy to use compared to similar java games of this type.

I am starting to like the random option. Makes it easy to create double planets and satellite systems (create 3 large planets, two collide and the debris go into orbit around the third large planet). With that method, I managed to create a satellite system with 6 moons. Unfortunately, the hill spheres of even the large planets are pretty small when at the highest zoom level, so you have to do these at the farthest zoom level, but then you are likely to lose them to the simulation boundaries.