Rapidly floating, pure silver object seen from an airplane window.Last summer, upon my flight from Mexico back into the United States, I was looking out the airplane window and saw what I initially thought to be a satellite--a silver, tilted rectangular object floating away from the airplane. I turned to a relative to ask if there happened to be satellites at this level, to find out that we were miles upon miles too low.I looked again and saw the object much futher away, yet still very visible. There were no markings whatsoever on it--in fact, it was perfectly smooth with no indentations, bolts, lights, exhaust, or anything of the ilk. It was floating rapidly in a straight line. No accelerations--just a steady fast, straight dash.A moment later, it was completely out of sight. I had a good 25 seconds to inspect this object, and I know very well that I was not hallucinating. There were no clouds at that level, and it was initially close enough to see with perfect accuracy.What it is, I don't know--but I do know that I've never seen (or honestly, heard of) anything like it before.