Large white sphere floating between clouds near Boeing field..As I was driving north on Airport Way S. I witnessed one very large oval shaped sphere with what appeared to be a small triangular tail wing on the back and a small dark oval shape on the surface. The object was floating slowly from behind one cloud into another, and although it was overcast I could see a bit of the sun shining on this creme/white frosty surface. It seemed too big and round to be a blimp,I knew right away that it was something I've never seen before, it made no sound. Can someone tell me if what I saw sounds like an experimental craft that maybe NASA or Boeing is working on? Before I could think about pulling over to really look at this thing, it disappeared into a cloud and was gone..