Venus is the brightest planet in the sky, which, with the naked eye, can be seen for a few hours near sunrise and sunset. Mayan priests (ca. 200–1500) associated its movements with astrological omens, and developed a Venus-based calendar. Motivated by astrology, Babylonian priests (ca. 1600–400 bce) applied sophisticated mathematical techniques to predict the first appearances and disappearances of Venus. The Greek philosopher Heracleides (fl. 300 bce) suggested that Mercury and Venus orbited the Sun as the Sun orbited the Earth. Claudius Ptolemy's (fl. 150) Earth-centered theory had the movements of Mercury and Venus centered on a line connecting the Earth and Sun, but not orbiting the Sun.
In 1610 Galileo Galilei's early telescopes revealed that Venus showed phases like the Moon. Its full illumination provided evidence against Ptolemy's theory, and for Nikolaus Copernicus's Sun-centered theory (which had been foreshadowed by Aristarchus) and Tycho Brahe's Earth-centered theory (which had been foreshadowed by Heracleides). In 1761 Mikhail Lomonsov concluded that Venus had an extensive atmosphere, when he observed a thin, luminous ring around the planet when Venus was backlit by the Sun. In the 1920s, photographs taken in ultraviolet light showed variable cloud features. Timings of transits of Venus, when Venus passes in front of the Sun, were crucial in determining the distance from the Sun to Earth, the astronomical unit. Astronomers traveled globally to observe these events, starting with the transit of 1761.
Venus's total cloud cover frustrated attempts to determine the rotation period, which yielded estimates ranging from 22 hours to 225 days. In 1958 the Lincoln Laboratory of Massachusetts Institute of Technology made the first attempts to probe through the Venusian atmosphere using ground-based radar. In March 1963 Richard Goldstein and Roland Carpenter of Jet Propulsion Laboratory's Goldstone tracking station announced that Venus had a retrograde rotation period of about 240 days.
Spectroscopic measurements of Venus's atmosphere, which began in the late nineteenth century, were difficult to interpret because of the intervening Earth's atmosphere. In 1932 astronomers at Mount Wilson Observatory, using a near-infrared spectrograph, identified significant quantities of carbon dioxide. In 1939 Princeton University's Rupert Wildt linked this to greenhouse heating and potentially high surface temperatures. By the late 1950s, optical, infrared, and radio telescope observations yielded estimates of surface pressure of 5–100 bars (5–100 times that of Earth), and surface temperature estimates from well under the boiling point of water to more than 300°C. If surface temperatures were lower than water's boiling point, then scientists believed life was possible on Venus, feeding speculation of hot jungles under the clouds.
With the instigation of the space race in the late 1950s and early 1960s, a politically driven robotic spacecraft race targeted Venus as the nearest objective for U.S. and Soviet planetary missions. The U.S. Mariner 2 flew within 35,000 km of Venus in 1962, determining the cloud tops to be 60–80 km above the surface, whose temperature measured 425°C. It found no magnetic field. After failed attempts in 1962, 1964, and 1965, the Soviet Union's Venera 4 reached Venus in 1967. It found a weak magnetic field, and hence a bow shock (where Venus's magnetic field deflected the solar wind) only 500 km above the planet's surface. Venera 4 also carried a lander, which measured atmospheric constituents of 96 percent carbon dioxide, and traces of nitrogen, water vapor, and oxygen. It also measured a temperature of 270°C and pressure of 20 bars when its transmissions ended, which contradicted U.S. results, including that of Mariner 5, which arrived at Venus one day after Venera 4. Actually, the Venera 4 lander's last transmissions had stopped 25 km above the surface.
For the next decade, the Soviet Union had Venus to itself while the United States focused on Mars and the outer planets. Its Venera 7 and 8 spacecraft of 1970 executed soft landings and found what appeared to be volcanic basalt rock. They measured high-altitude wind speeds up to 225 mph and surface winds of 2 mph, and found that the atmosphere shielded Venus's surface from cosmic radiation and from 98.5 percent of the Sun's light. The Venera 9 and 10 landers of 1975 transmitted the first surface images, showing little erosion and virtually no sand. The Venera 10 orbiter altimeter measured a relatively flat surface across the planet, as compared to Earth.
In 1978 the U.S. launched the Pioneer Venus Orbiter (PVO) and the Pioneer Venus Multiprobe, which released four probes into the Venusian atmosphere. The probes confirmed sulphuric acid in Venus's clouds, which had been suggested early by Godfrey Sill, and Andrew and Louise Young, while the orbiter's radar mapped 93 percent of the planet to 20 km surface resolution and 100 m altitude resolution. The PVO measured a mainly flat surface, with two major high plateaus and a number of shield volcanoes. The Soviet Union continued with Veneras 11 and 12 in 1978, Veneras 13 and 14 in 1981, Veneras 15 and 16 in 1983, and Vegas 1 and 2 in 1984. The Vega spacecraft flew by Venus on the way to Halley's Comet, dropping modules with balloons that measured atmospheric circulation. The Veneras discovered chlorine, carbonyl sulphide, and hydrogen sulfide in the atmosphere. The Venera 13 landing site showed more sand and dust than other locations, but all landing sites exhibited volcanic compositions. Veneras 15 and 16 radar provided resolution to 2 km and provided data on the north polar region skipped by PVO. The U.S. Magellan spacecraft, launched in May 1989, used synthetic aperture radar to provide surface resolution mapping to 120 m resolution of 98 percent of the planet.
Magellan’s high-resolution images showed the limited occurrence of folded mountains, but no mountain systems as extensive as those on Earth, and no indications of planetwide plate tectonics. The relative lack of craters showed that Venus's surface is young, averaging some 400 million years old. Magellan also imaged unique Venusian features, such as “pancake domes” of hardened lava. On Venus, less dense rocks, such as the granites on Earth, have not generally “floated” at higher levels than denser ones, such as terrestrial basalts. On Earth the lighter continents are on average several kilometers higher than the denser ocean floors, resulting in two-level topography. A very large portion of Venus's surface consists of low-lying plains, while extensive elevated regions are fewer and smaller in area compared to Earth. The cause of this difference between the two planets is uncertain.
In April 2006, the European Space Agency's Venus Express spacecraft was inserted into Venus orbit, primarily intending to study the mechanisms of Venus's atmosphere. It also provided the first images of the Venusian south pole, which showed a double vortex, as previously seen at its north pole. As of April 2010 its mission was continuing.
See also: National Aeronautics and Space Administration, Russia (Formerly the Soviet Union)
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