Venus
The second planet from the Sun, Venus is often called Earth's "sister planet" because it is the closest planet to Earth and because the two planets are nearly identical in size and mass. Venus gets its name from the Roman goddess of love and beauty and, viewed from Earth, it is indeed one of the most brilliant objects in the sky.
The first to see Venus with a telescope was Galileo. He immediately recognized and recorded its different phases, announcing his findings in a letter written in 1610. Followers of Nicholas Copernicus and his heliocentric theory (that the Sun was the center of the solar system) had earlier predicted the phases of Venus and Galileo's discovery gave conclusive evidence in support of the Copernican theory.
Scientists set to work to learn more about Venus. By 1640 there was no longer any doubt that Venus orbited the Sun closer than the Earth did; the orbital period was established as 224.7 Earth days. But a major problem that occupied astronomers for centuries was measuring the period of Venus's daily rotation. In 1667 Gian Dominico Cassini followed an especially bright spot emanating from Venus and concluded (erroneously) that its daily rotation was twenty-three hours.
In 1761 astronomers observed during a transit (when a planet passes in front of the Sun) that Venus had a dense atmosphere of thick, luminous clouds. May scientists erroneously believed that these thick clouds were composed of water vapor and that Venus, therefore, must be a swamp-like planet overrun with vegetation.
This image survived until the twentieth century, when a radically different view of Venus emerged. In the 1920's astronomers Charles Edward St. John and Seth Barnes Nicholson, using a spectroscope, determined that the clouds surrounding Venus did not contain water, and that, instead of being a highly vegetative planet, Venus had a dry, desert-like surface. This claim was supported by Rupert Wildt (1905-1976) who in 1937 discovered that Venus's atmosphere contained high amounts of carbon dioxide but very little oxygen or water vapor.
Scientists today recognize Venus as a supreme example of the greenhouse effect. Energy from the Sun becomes trapped by the dense carbon dioxide gasses in the atmosphere, heating the surface temperature of Venus to well over ten times that of Earth.
In the early 1960s, both the Soviet Union and the United States began extensive space exploration of Venus. In 1961, the Soviet Union launched Venera 1 to Venus, the first spacecraft to be sent to another planet. Unfortunately, radio contact was lost after two weeks and the Venera 1 mission failed. The United States spacecraft, Mariner 2, launched in 1962, was the first successful flight to Venus. Although it only came within 22,000 miles of the planet, Mariner 2 as able to send back important information. It found cold, dense clouds in the upper atmosphere, a surface temperature of 800°F (426°C), and a water vapor level of only 1/1,000 of the Earth's. It also determined that Venus had no magnetic field.
The Soviet Union succeeded in sending probes to the planet's surface in the 1970s that measured a surface temperature of 900°F (482°C) and a pressure of ninety atmospheres, which is equal to a crushing pressure of 2/3 ton per square inch. The first photographs of Venus were taken in 1975 when Venera 9 and Venera 10 landed on Venus and sent back pictures of its surface. Venera 9 transmitted images back to Earth for approximately fifty minutes, revealing sharp, angular rocks all around the landing site. Venera 10, launched less than a week later, transmitted for sixty minutes, sending back photographs that shower older, more weathered rock formations.
in 1978, the United States launched Pioneer Venus 1 and Pioneer Venus 2 which, up to that time, were the most sophisticated spacecraft to be sent to Venus. Venus 1 was instrumental in mapping the surface area of the planet while Venus 2 was equipped to measure atmospheric conditions. The Venus 1 orbiter successfully mapped 90 percent of Venus's surface in less than two years, revealing great mountain ranges, vast plateaus, and expansive plains.
In the early 1980s, the United States laid the groundwork for the Magellan program, the most extensive space mission to Venus yet. Magellan was launched aboard the space shuttle on May 4, 1989 and began orbiting Venus fourteen months later. Using a sophisticated radar system, Magellan was able to send back images of Venus that have three times the resolution power of earlier orbiters. Whereas previous photographs could only show surface features 180 miles across, Magellan returned images as small as one mile across. These images revolutionized our understanding of Venus, revealing its topography, evidence for recent (within 500 million years) volcanic activity, the nature of its gravitational field, and no evidence of the earthquake-inducing tectonic activity so common on Earth.
Magellan ended its mission in spectacular fashion in October 1994, as NASA controllers ordered it to brake and descend into Venus's atmosphere. The spacecraft returned some final information about the nature of Venus's clouds, but before long contact was lost as Magellan descended through the atmosphere, part of it probably being incinerated and part of it eventually crashing on the planet's surface.
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