Leaving the Earth
Before reaching space, scientists had to solve the problem of escaping from the Earth's gravity--the force that pulls objects toward Earth and prevents them from floating off into space. A spacecraft leaving Earth must travel fast enough to overcome this strong gravitational pull. The speed needed to overcome the Earth's gravity, called escape velocity, is about 7 miles (11 kilometers) per second, or 25,000 miles (40,000 kilometers) per hour. Reaching escape velocity does not mean that a spacecraft has freed itself completely from the Earth's gravitational pull, which extends far out into space. But it does mean that the spacecraft will not fall back to Earth even if no additional power is used. As the spacecraft continues to move away from the Earth, the gravitational force weakens until it no longer has a significant effect on the spacecraft.
For a spacecraft to enter orbit around the Earth, it must reach a speed called orbital velocity. The orbital velocity will depend upon how far above the Earth the craft is supposed to orbit. For example, a spacecraft must attain an orbital velocity of about 17,500 miles (28,000 kilometers) per hour to orbit the Earth at a distance of 100 miles (160 kilometers). A slower orbital velocity is needed to keep a spacecraft in orbit farther from Earth.
A spacecraft is sometimes put into a temporary, or parking, orbit before it is sent farther out into space. There are two reasons for doing this. A spacecraft launched directly into space would need more powerful, more expensive rockets. Scientists have also found that it is easier to aim a spacecraft toward its destination if it is put into a parking orbit first.
Rockets and the Space Age
It was the development of rockets that enabled spacecraft to escape Earth's gravity and travel into space. The first rockets were invented by the Chinese who, as early as A.D. 1000, put gunpowder into sections of bamboo and used them as weapons. Later, in the 1600's, the English scientist Sir Isaac Newton formulated the laws of motion that explain the principle of how rockets work. But it was not until the 1900's, with the development of liquid propellant rockets, that spaceflight became a possibility. The first long-range rockets were built in the 1940's during World War II. After the war, rocket research led to even more powerful rockets, such as the Saturn V, that were capable of launching a spacecraft from the Earth's surface and putting it into orbit around the planet.
The space age truly began on October 4, 1957, when what was then the Soviet Union sent the first artificial satellite, Sputnik 1, into orbit. On April 12, 1961, the Soviet cosmonaut Yuri Gagarin became the first person to travel in space. His spacecraft, Vostok 1, made one orbit around the Earth and then returned home. But perhaps it was the first landing of human beings on the moon that has been the most exciting event in space travel so far. On July 20, 1969, U.S. astronaut Neil Armstrong climbed down from the lunar landing module of his spacecraft, Apollo 11, and placed his foot on the surface of the moon. For the first time, a human being was standing on ground other than that of the Earth.
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