Hey, Nexus!

1. The scroll bar on the right side of the screen adjusts the view plane. When it is centered, you are viewing edge-on. When it is on the top, you are viewing top-down. When it is on the bottom, you are viewing bottom-up.

2. I'm going to re-do this code because there's a bug that sometimes shows up when objects of different densities merge. But for bodies of the same density it works fine. Basically it works like this: From the size and mass of a body it determines its density. When two bodies collide, it computes their combined average density. This will be the density of the new object. From its density and mass it computes its volume, and from volume it computes its radius.

3. I didn't document this part well enough. The Create Objects window works fine for circular and elliptical orbits, but not for hyperbolic. For hyperbolic orbits, there's a few games you can play. Let me make up an example and step you through it.

In the simulation Sedna.gsim which is included in the install package, I wanted to re-create an experiment performed by astronomers. The simulation called for a brown dwarf to make a hyperbolic pass of the Sun at a distance of 200 km. The brown dwarf was to have a velocity at infinity of 1 km / second.

Step 1: Use Create Objects to create an object in circular orbit around the Sun at 200 AU. Semi-major axis should be 200 AU. You can adjust the other numbers if you like. Or you can leave every value except Semi-major axis at their default values.

If your body has significant mass, set it during this step so Gravity Simulator can take its mass into account when computing a circular orbit. Otherwise, leave it as a massless test particle.

Step 2: Open a Distance & Velocity box from the View menu. Set it to Sun and your newly-created object. This will tell you your circular velocity. Velocity and Tangental velocity should be the same, and Radial velocity should be 0 if your orbit is circular. In this particular example, I get 2.158 km/s.

Step 3: Determine how much the object's velocity should be to give it your desired hyperbolic orbit.

You can use the formula:

Sqrt((circularVelocity * Sqrt(2)) ^ 2 + additionalVelocity ^ 2)

Using this formula to compute a velocity which will yield 1 km / s of velocity at infinity I get:

sqrt((2.158 * sqrt(2)) ^ 2 + 1^2) = 3.212

You can also download this calculator which will do all the math for you:

http://orbitsimulator.com/gravity/OrbitVelocityCalc.exe Step 4: Using the View menu, open a Thrust Box and an Orbit Elements Box. Set the top object in the Thrust Box to your newly-created object, and the bottom object to the Sun. Do the same in the Orbital Elements Box.

Use the Prograde Boost buttons on the Thrust Box to increase the object's velocity to 3.212 km/s. If you overshoot your target velocity, use the Retrograde Boost buttons.

Look at the Orbital Elements Box as you are boosting your velocity. Notice that your eccentricity changes. Once it exceeds 1 you are in a hyperbolic orbit.

After following these steps, you will have an object in a hyperbolic orbit at its perihelion. I'd suggest you save the simulation at this point in case you make a mistake in the next few steps. Then you can go back to this starting point.

Step 5: In the case of the Sedna simulation I wanted the Brown Dwarf to approach the Sun rather than begin the simulation at closest approach. Using the Time Menu, set Time Backwards. Now increase the Time Step so you can see your object move. Zoom out using Screen Scale if it is not already in your view. Watch the Distance Box. Notice that the object's total velocity is decresaing. The number is slowly approaching 1 km/s.

When the object is at an acceptable distance from the Sun, set Time Forwards and save the simulation. In the Sedna simulation, it was at this point that I added the test particles orbiting Sedna. Now you can watch your object make a hyperbolic pass of the Sun.

Sorry for the confusing method. Next version may automate this for you.

Hope this helps.

--Tony