When the Voyager 2 spacecraft flew by Neptune in the autumn
of 1989, it was travelling at a speed of 40000 km/hr away from
Earth, at a distance of 35 AU.  Remember that one AU is equal
to 1.5 * 10^11 meters.

(a) Radio waves travel at the speed of light, which as you know
is 300000 km/sec.  How long did it take radio signals to travel
between Voyager 2 and Earth?

(b) During that flyby, many radio signals were transmitted by 
Voyager 2 and received at Earth.  How far did the spacecraft
move during the travel time of a single signal?  Compare this
distance to the distance between Earth and the Moon, which is
384000 km.

(c) If the transmitter on Voyager 2 had a power output of 550
watts, what was the flux from that transmitter at Earth's surface?