This showed up when I looked at our APRS tracks after a recent ride:
Poking around a bit showed the target:
Contrary to what I thought, it didn’t come up the Hudson River from West Point:
Knowledge of the Universal Law of the Conservation of Perversity informs you a balloon will never land in the middle of a putting green:
Apparently the launch is part of a regular class project at West Point. Good clean fun!
The Smell of Molten Projects in the Morning 
It’s wonderful that you “saw” the balloon that day.
Here are my slides about the mission (and the one the year before it): http://www.spy-hill.net/~myers/talks/USMA-HAB-2018.pdf
And now we know the rest of the story!
Apropos of space exploration: I got The Thousand Yard Model a zillion years ago, collected a box of planets and a bowling ball sun, set it up at several family gatherings, and never took space travel for granted ever again.
That’s great. I was set my model up on the Walkway Over the Hudson two weeks ago, but then had to stow it immediately when they spotted approaching lightning. I hope to have a few more shots at it this summer.