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, an attractive force which exists between any two objects, but is dependent 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 gravitational field. However, for a spacecraft to do so it must reach a velocity of 11 km/sec (7 miles/sec), or 39,600 km/hr (nearly 25,000 mph). It was only by the middle of the twentieth century that mankind finally understood enough about rocketry to make such high speeds attainable.
Today, spacecraft can also be carried into space by a shuttle, which releases them while in orbit. After leaving Earth's gravitational field, spacecraft then use small on-board thrusters to navigate through space and explore the universe.
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