and France. In 1942, the V-2 was launched to an altitude of 176 km (109 miles), making
it the first human-made object to travel into space. An altitude of 100 km (62 miles) is
generally considered to be the dividing line between Earth’s atmosphere and space.
Figure 23.19: V-2 Rocket: Explosions in a chamber create pressure that pushes gases out of
a rocket. This in turn produces thrust that pushes the rocket forward. ( 17 )
The leader of the team that built the V-2 rocket was a German scientist, named Wernher von
Braun. von Braun later fled Germany and came to the United States, where he helped the
United States develop missile weapons and then joined the National Aeronautics and Space
Administration (NASA) to design rockets for space travel. At NASA, von Braun designed
the Saturn V rocket (Figure23.17), which was eventually used to send the first humans to
the Moon.
Satellites
One of the first uses of rockets in space was to launchsatellites.A satellite is an object that
orbits a larger object. Toorbitsomething just means to travel in a circular or elliptical path
around it. This path is also called an orbit. When you think of a satellite, you probably
picture some kind of metallic spacecraft orbiting Earth, but the Moon is also a satellite.
Human-made objects put into orbit are calledartificial satellites. Natural objects in orbit,
such as moons, are callednatural satellites.
Newton’s Law of Universal Gravitation
Isaac Newton, whose third law of motion explains how rockets work, also came up with
the theory that explains why satellites stay in orbit. Newton’slaw of universal gravitation
describes how every object in the universe is attracted to every other object. The same
gravity that makes an apple fall to the ground, and keeps you from floating away into the
sky, also holds the Moon in orbit around Earth, and Earth in orbit around the Sun.
Newton used the following example to explain how gravity makes orbits possible. Consider
a cannonball launched from a high mountain, as shown inFigure23.20. If the cannonball