Photograph of Atlantis lift-off.
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The average person can jump about 2 feet off the ground, and few of us can throw a baseball higher than even fifty feet in the air. It seems as if there is some force that "pulls" everything on the Earth down to the ground.
This "pull" is due to gravity, a force of attraction which exists between any two objects, but depends on their size. Since our planet is so massive, we experience a strong pull toward its center.
Space travel is only possible is if we can escape Earth's gravity. However, for a spacecraft to do so it must be launched with a speed of 39,600 km/hr (nearly 25,000 mph). .
It was only by the middle of the twentieth century that mankind finally understood enough about rockets to be able to reach such high speeds. After leaving Earth, spacecraft use small on-board thrusters to navigate through space and explore the universe.
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