In the early days of space exploration and the moon program
in the 1950s and 1960s, the dilemma faced by NASA scientists
was weight. How could they send a rocket ship to the moon,
get it to land and then take off from the moon, and fly back
to earth? The problem was that if the rocket had enough fuel
to break loose of the earth’s gravity and land on the moon, it
would not have enough fuel to then break loose of the
moon’s gravity and return to earth