John Clear writes:

It is a real "whoa, what was that?" moment when the balloons streak
past you at 120kts.

Brian Shul tells a story about passing a weather ballon in an SR-71 at
70,000+ feet while flying at several times the speed of sound. He saw
something flash by and didn't even realize what it was until he had
passed it. He had a bit of a chill thinking about what might have
happened if he had actually hit the balloon. There was absolutely no
time to react. It particularly made him aware of just how fast he was
actually going. (Normally the aircraft flies so high when supersonic
that it's difficult to get a clear idea of how fast it moves from
aloft.)

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