Lunar Footprints. Micrometeorites will eventually erase tl1e footprints left by the Apollo astronauts. Assume that the ~'loon is hit by about 25 million micrometeorite impacts each day (this number comes fro111 observations of meteors in Earth's atmosphere) and that these impacts strike randomly around the Moon's surface. Also assume that it will take about 20 such impacts to destroy a foot print. How long would it take for a footprint to be erased? (Hints: Use the Moon's surface area to determine the impact rate per square centimeter, and estimate the size of a footprint.)