The moon is likely to become the industrial（工业的） hub（毂） of the solar（太阳的） system, supplying the rocket（火箭） fuels for its ships, easily obtainable from the lunar（月亮的） rocks in the form of liquid oxygen. The reason lies in its gravity（地心引力）. Because the moon has only an 80th of the Earth's mass（块）, it requires 97% less energy to travel the quarter of a million miles from the moon to Earth orbit than the 200 mile journey from Earth's surface into orbit. This may sound fantastic, but it is easily calculated（计算）. To escape from the Earth in a rocket, one must travel at 7 miles per second. The comparable（可比较的） speed from the moon is only 1.5 miles per second.

  Because the gravity on the moon's surface is only a sixth of Earth's, remember how easily the Apollo astronauts bound it along.