Reflective solid object hung motionless in the sky before shooting off at an extremely high rate of speed.

Around 12:30 in the afternoon, I and one other person noticed a round metallic object hovering in the sky.  I'm making an educated guess based on the cirrus clouds in the sky with the object, that it was around 17000 feet in the air.  
Although it was so high it was very easy to notice because it had a very shiny, metallic surface (like tin foil or stainless steel) and reflected the sunlight.  It appeared to be a single, solid object.  I am unable to to determine how large it was, only that it was very easy to see, as the sky was almost totally clear.  
The object made no sound as it hung there motionless for about 1 minute.  I had enough time to point the object out to my friend and give him at least 20 uninterrupted seconds to observe it too.  
The object then took off so fast that it almost seemed to have disappeared.  It left so quickly that I would have missed it had been blinking.  
At that point I realized what we saw wasn't an aircraft.