Lighting when lots of light sources creating polygon shape

Difficult to described but easily shown.

When I have lots of missile impacts on the right hand asteroid, the lighting it creating a sharp edged geometric shape rather than blending nicely.

Would really appreciate some thoughts on why this is happening and any steps I can take to make it look a little prettier.

Thanks in advance!

Well, the video is unavailable, but from the description it sound like you are experiencing vertex lighting. This is a good example of how to deal with vertex lighting: Pixel & Vertex Lights in Unity3D - Unity3D.College