Say 12000 acft airborne; av acft surface area 500sq m; surface area of earth (sky) at av flight height 600,000,000 sq km (to make the maths easy), converting to sqm...
= 1/100,000,000
So with 1 piece of junk falling per day, = 1 hit on average every 274,000 years.
Gee, another thing to...