One of the most dramatic moments of the twentieth century occurred on October 4, 1957. The former Soviet Union,
using an R-7 Semiorka rocket built for its weapons program, sent
a small shiny sphere with four long antennas into space. They
called it Sputnik I. Sputnik is a Russian word that means “traveling
companion.” The satellite traveled so fast that its ballistic flight
continued all the way around Earth. A radio transmitter on board
sent a simple beeping signal that could be heard everywhere as it
passed overhead. It was an artificial moon.
Any satellite—natural or artificial—travels in a path called an orbit. The same thing applies to the planets of our solar system: they all orbit the Sun. Because outer space is a vacuum with nothing to slow down the moons and planets, they keep orbiting forever. The most important influence on their motion is the force of gravity. In the seventeenth century, along with the laws of force, Sir Isaac Newton also explained how gravity works: It is a force of attraction between any two objects in the universe. Larger objects have stronger forces, and the closer two objects are, the stronger is the force between them. Newton figured out a formula to calculate the strength and direction of the force.
Any satellite—natural or artificial—travels in a path called an orbit. The same thing applies to the planets of our solar system: they all orbit the Sun. Because outer space is a vacuum with nothing to slow down the moons and planets, they keep orbiting forever. The most important influence on their motion is the force of gravity. In the seventeenth century, along with the laws of force, Sir Isaac Newton also explained how gravity works: It is a force of attraction between any two objects in the universe. Larger objects have stronger forces, and the closer two objects are, the stronger is the force between them. Newton figured out a formula to calculate the strength and direction of the force.
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