The balloon has a lighter than air gas in it. It flies at 30 km above sea level. It is spherical, with a radius not exceeding five hundred meters. How long can it fly before a meteor pokes a hole in it?
Here's a rough estimate. From the curves at top left of the plot in figure 1 in this paper, we can expect on average, per year per million km2 of earth surface, 4.5 meteors, weighing 31 grams ("log m" -- base 10, not base e, which would mean 7 grams). Heavier ones are rarer.
Your 500 m diameter balloon's projected area is about 0.2 km2.
So the average duration between meteors hitting it is
1,000,000 / 0.2 / 4.5 = about a million years.
Even if I've horribly misinterpreted the plot, and even if meteors arrive in bursts (as they do), the balloon will very likely succumb to something else before any meteor hits it.
EDIT: As Peter Martyn commented, the paper gives another estimate, at the end of p. 875:
log N = 2.14 - 0.49 log m.
For m = 31 grams, this gives N = 48 instead of the plot's 4.5.
An order of magnitude more, but close enough for our purposes.
For m = 0.1 grams, still big enough to damage the balloon, N = 794.
Then the average duration between meteor hits is 1,000,000 / 0.2 / 794 = 6300 years.