SOLUTION TO EXERCISE 6

The diameter of the cylinder-shaped tail is 10,000 km, so its volume
is 3.14 * (5000 km * 100,000 cm / km)^2  * (length of tail).  The number
density of the tail is 10,000 molecules per cubic centimeter, so another
way to express the volume is 3.1 * 10^34 / 10,000 = 3.1 * 10^30 cm^3.
Thus, the length of the tail is

          3.1 * 10^30 cm^2
   L =  -------------------- = 3.9 * 10^12 cm = 3.9 * 10^7 km.
        3.14 * (5 * 10^8)^2 

This is  39000000/12756 = 3100 times longer than Earth's diameter.  Very long.