Photograph of Atlantis lift-off.
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Leaving Earth

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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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA