Rotating a sphere causes it to clip

Hi, I have a sphere which I wish to zoom in on and fly around. If I zoom in and then attempt to "fly" around the edge of the sphere, the middle of the sphere clips and is replaced with a grey circle.

The camera is setup as follows: -

SCNCamera *camera = [SCNCamera camera];

self.cameraNode = [SCNNode node];

camera.usesOrthographicProjection = YES;

camera.orthographicScale = 12;

camera.zNear = 0.00001;

camera.zFar = 600;

camera.xFov = 80;

camera.yFov = 80;

self.cameraNode.camera = camera;

self.cameraNode.position = SCNVector3Make(0, 0, 10);

[self.scene.rootNode addChildNode:self.cameraNode];

The Sphere is setup as follows: -

SCNSphere *sphere = [SCNSphere sphereWithRadius:kRadius];

sphere.segmentCount = 256;

self.sphereNode = [SCNNode nodeWithGeometry:sphere];

self.sphereNode.geometry = sphere;

self.sphereNode.position = SCNVector3Make(0, 0, 0);

self.sphereNode.geometry.firstMaterial.diffuse.contents = [NSImage imageNamed:@"Earth.png"];

self.sphereNode.geometry.firstMaterial.specular.contents = [NSImage imageNamed:@"specular.png"];

self.sphereNode.geometry.firstMaterial.emission.contents = [NSImage imageNamed:@"boundaries.png"];

self.sphereNode.geometry.firstMaterial.doubleSided = NO;

[scene.rootNode addChildNode:self.sphereNode];

Also, I don't seem able to zoom very close into the sphere. I'm clearly doing something wrong - any advice would be welcomed. Thank you 🙂