Popping Balloons in World War 1

Eddie Rickenbacker's Tribute to Balloon Busters

Apparently it's not just our generation; it's always been confusing why a balloon is not a big, fat, poppable target. Eddie Rickenbacker took time in his autobiography Rickenbacker and Fighting the Flying Circus to explain why "Balloon Buster" Frank Luke was something beyond even an ace, the "most daring aviator of the entire war."

Rickenbacker had gained the title of American Ace of Aces when he scored his 7th victory, but Luke took it away from him by shooting down ten balloons and four other aircraft in eight days, a record which Rickenbacker pointed out nobody, not even Richthofen, came close to. Luke became the first flyer to receive the Medal of Honor, but, as often happens, it was posthumous. Rickenbacker later reclaimed the American Ace of Aces title, but had already realized that "the honor carried the curse of death" as Rickenbacker was the only holder of the title to survive even America's short time in the war.

A Bullet to a Balloon is Relatively Smaller Than a Pin

So here's why shooting down a balloon was even harder than shooting down an airplane:

1. Anti-aircraft fire wasn't usually a big problem for airplanes, because the gunners had to time the fuses to explode at a  certain altitude, and the airplane didn't have to stay at that altitude. Except when attacking a balloon - then the gunners knew what altitude the balloon was at, and the airplane had to be close to attack. So they could, and did, set up a whole curtain of anti-aircraft fire around the balloon, which the attacking airplane had to both enter and exit to successfully attack.
2. The balloon didn't have to stay at altitude, and in good conditions could be reeled down about as fast as an aircraft could approach.
3. The balloon observer normally had a parachute, and so could jump to save his life. Allied airmen didn't.
4. The balloon was a desirable target (it was sitting up there watching troop movements many miles behind the lines and reporting to decision-makers on the ground in real-time; as compared to airplanes which could scout deep into enemy territory but had to get back home to deliver any news.) So enemy airplanes would "hide in the sun," waiting for an airplane to approach, and then pouncing.
5. A regular bullet was too small compared to the size of the balloon to make enough of a hole to deflate it. Like a birthday-party balloon, the material didn't have to be totally impermeable, it just had to let gas out slowly enough compared to its size that the balloon wouldn't change noticeably for hours or days. The way to shoot down a balloon was with incendiary bullets, if the gas would ignite, which wasn't certain (Rickenbacker mentioned a day when despite three fighters actually pumping bullets into balloons, nothing happened.)