How do you miss a stationary target?
Why do you think it was a stationary target?
Sidewinders targeting system uses infrared to locate an aircraft’s heat and exhaust. Assuming the balloon or Octagonal UFO had no heat signature, the pilot would have to manually line up the target and fire at a distance that did not put their own aircraft in peril by being too close to the explosion. I can see a miss with this approach quite easily infact.
I would imagine in the future they would want to use a laser guided projectile, or...why not just use the guns assuming its over remote areas.