Setting up for an International Space Station transit, this weather balloon was spotted through my LX200 finder scope. Difficult to keep balloon in small 11 x 9 arc-min FOV due to atmospheric currents and GUIDE speed on LX200 was no match for fast-moving balloon.

Back-of napkin calculation: angular size of balloon is .03 degrees; actual diameter of balloon is
~10 feet; altitude is thus 19,100 feet (3.6 miles).

YouTube Video:
http://uk.youtube.com/watch?v=ym2ju4RFLe4