# S.2 IS Space Travel !!easy

Why is the position of the point of "zero-gravity" closer to the Moon, but not at the middle of the Moon and the Earth?

The force of gravity of the Moon acts on the space shuttle resulting in an increasing speed. If you were the astronaut, would you choose "parachutes" or by "propulsive lift" to slow down the space shuttle?

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• 最愛解答

The mass of the moon is about 1/80 of that of the earth. Since gravitational force is proportional to mass (Neton's Law of gravitation), the earth's gravitational force is thus 80 times stronger than that of the moon. At the mid-point of the moon and earth where distances from the moon and earth are equal, the force of gravity due to earth is apparently stronger than the force due to the moon. As such, the point of "zero gravity", where the gravitational forces from the earth and moon balance one another, is expected to be at location close to the moon.

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Clearly, propulsive lift should be used, because the moon does not possess an atmosphere, thus parachutes, which utilizes the resistance of air for decceleration, cannot work on the moon.