Does a sphere projected into 2D space always result in an ellipse?
Assuming a perspective projection and a view point external to the sphere, then the 'boundary' formed by the view point and the circle on the sphere which forms the horizon WRT the view point, will be a cone. 

Doing a perspective projection (onto a plane) is then equivalent to intersecting this cone with the plane which thus produces a conic section. FYI the four, non-degenerate, cases are shown in this image from Wikipedia  

An ellipse/circle is thus a possibility, but not the only one - unbounded parabolas or hyperbolas (and I guess if the plane passes through the eye, even degenerate cases ) are possible.