I don’t think those are a “marching ants” selection around the shuttle, I can think of about three different reasons that ranges from unlikely to impossible. What it looks like to me is that the shuttle layer was rendered with an unpremultiplied background, but it was accidentally set as having a premultiplied background in the compositing program.
“Premultiplied” means that, even when the background is transparent, the rendered element’s edges are blended with a background color (usually black), whereas unpremultiplied backgrounds have the rendered object in total isolation, but it causes that sort of aliasing artifact when viewed with the wrong setting (either without the transparency layer, or with the compositing program set to expect a premultiplied layer, so it includes more of the edges around the objects than it should).