A mannequin of Alan B. Shepard, Jr., the first American in space, is part of an exhibit at the National Air and Space Museum in Washington, D.C.
Photograph by Joseph D. Lavenburg and Robert S. Oakes, National Geographic
the distance above sea level.
layers of gases surrounding a planet or other celestial body.
number of things of one kind in a given area.
region in Earth's atmosphere between the stratosphere and the thermosphere, about 50-80 kilometers (31-50 miles) above the Earth's surface.
space beyond Earth's atmosphere.
force pressed on an object by another object or condition, such as gravity.
vehicle designed for travel outside Earth's atmosphere.
level of Earth's atmosphere, extending from 10 kilometers (6 miles) to 50 kilometers (31 miles) above the surface of the Earth.
degree of hotness or coldness measured by a thermometer with a numerical scale.
layer of the Earth's atmosphere located between 80 kilometers (50 miles) and 550 kilometers (341 miles) above the Earth's surface.
lowest layer of the Earth's atmosphere, extending from the surface to about 16 kilometers (10 miles) above.