Difficulty with orbiting a SKFieldNode in SpriteKit

Question

I'm trying to get a simple SKSpriteNode to perfectly orbit an SKFieldNode.radialGravityField() once it comes into its defined SKRegion. I have the gravity of my scene set to zero via self.physicsWorld.gravity = CGVectorMake(0.0, 0.0).

I am trying to simulate real space. I have heard about using a joint, but that doesn't seem to be as smooth feeling as using real gravity calculations. Any ideas here?

EDIT: My Code so far does not work as desired. It will send an object into a radialGravityField() but does not orbit it perfectly (or even close to perfect). It does orbit, but in a wild ellipse. And when I say perfectly, I mean in the shape of a circle around the gravityField's center.

ADDITION: I was just thinking of an alternative to using a radialGravityField(). Maybe it would be easier to just calculate the position of the flying object in the update method. If it's position is within a planet's radius, then use an SKJoint and have it orbit. Anybody done that before?

Thank you in advance!

Here is my code so far...

   import SpriteKit

    class GameScene: SKScene {

    override func didMoveToView(view: SKView) {
        /* Setup your scene here */

        self.physicsWorld.gravity =  CGVectorMake(0.0, 0.0)
        self.view?.backgroundColor = UIColor.darkGrayColor()


        let circle = SKShapeNode(circleOfRadius: 30.0)
        circle.position = CGPoint(x: self.frame.width / 2 + 10, y: self.frame.height / 2)
        circle.fillColor = .whiteColor()
        addChild(circle)


        let gravityField = SKFieldNode.radialGravityField()
        gravityField.position = circle.position
        gravityField.region = SKRegion(radius: 100.0)
        gravityField.strength = 4.0
        gravityField.enabled = true
        addChild(gravityField)


    }

    override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
       /* Called when a touch begins */
        let p = SKSpriteNode(color: UIColor.purpleColor(), size: CGSize(width: 20, height: 20))
        p.physicsBody = SKPhysicsBody(rectangleOfSize: p.size)
        p.physicsBody?.dynamic = true
        p.physicsBody?.mass = 0.5
        p.position = touches.first!.locationInNode(self)

        addChild(p)

        p.physicsBody?.applyImpulse(CGVector(dx: 0.0, dy: 300 * p.physicsBody!.mass))

    }

    override func update(currentTime: CFTimeInterval) {
        /* Called before each frame is rendered */
    }
    }

Answer 1

Your object is in an ellipse because it's velocity does not perfectly match the gravity at that distance. If it initially shoots away from the gravity field then it is going too fast for a circular orbit, try reducing its velocity. If it initially falls towards the gravity field then it is going too slow, so try increasing the velocity.

Alternatively you could adjust the initial distance from the field or the strength of the gravity field.

It should be possible to make the orbit circular, but it will take a lot of fiddling around and should anything perturb the orbit (e.g. a collision) then that will throw it out again.

This is more about the physics of orbital dynamics than programming. Whether gravity is the right solution depends on what you are trying to achieve in the final app. Perhaps this SO question has some useful info.