Since Galileo's discovery of Venus's phases, the planet has been studied in detail, using Earth-based telescopes, radar, and other instruments. Over the centuries telescopic observers, including Gian Domenico Cassini of France and William Herschel of England, have reported a variety of faint markings on its disk. Some of these markings may have corresponded to the cloud features observed in modern times in ultraviolet light, while others may have been illusory. Important early telescopic observations of Venus were conducted in the 1700s during the planet's solar transits. In a solar transit an object passes directly between the Sun and Earth and is silhouetted briefly against the Sun's disk. Transits of Venus are rare events, occurring in pairs eight years apart with more than a century between pairs. They were extremely important events to 18th-century astronomy, since they provided the most accurate method known at that time for determining the distance between Earth and the Sun. (This distance, known as the astronomical unit, is one of the fundamental units of astronomy.) Observations of the 1761 transit were only partially successful but did result in the first suggestion, by the Russian scientist Mikhail V. Lomonosov, that Venus has an atmosphere.
The second transit of the pair, in 1769, was observed with somewhat greater success. Transits must be viewed from many points on Earth to yield accurate distances, and the transits of 1761 and, particularly, 1769 prompted the launching of many scientific expeditions to remote parts of the globe. Among these was the first of the three voyages of exploration by the British naval officer James Cook, who, with scientists from the Royal
Society, observed the 1769 transit from Tahiti. The transit observations of the 1700s not only gave an improved value for the astronomical unit but also provided the impetus for many unrelated but important discoveries concerning Earth's geography.
By the time the subsequent pair of transits occurred, in 1874 and 1882, the nascent field of celestial photography had advanced enough to allow scientists to record on glass plates what they saw through their telescopes. No transits took place in the 20th century; the first of the next pair was widely observed and imaged in 2004.
In the modern era Venus has also been observed at wavelengths outside the visible spectrum. The cloud features were discovered with certainty in 1927-28 in ultraviolet photographs. The first studies of the infrared spectrum of Venus, in 1932, showed that its atmosphere is composed primarily of carbon dioxide. Subsequent infrared observations revealed further details about the composition of both the atmosphere and the clouds. Observations in the microwave portion of the spectrum, beginning in earnest in the late 1950s and early '60s, provided the first evidence of the extremely high surface temperatures on the planet and prompted the study of the greenhouse effect as a means of producing these temperatures.
After finding that Venus is completely enshrouded by clouds, astronomers turned to other techniques to study its surface. Foremost among these has been radar. If equipped with an appropriate transmitter, a large radio telescope can be used as a radar system to bounce a radio signal off a planet and detect its return. Because radio wavelengths penetrate the thick Venusian atmosphere, the technique is an effective means of probing the planet's surface.
Earth-based radar observations have been conducted primarily from Arecibo Observatory in the mountains of Puerto Rico, the Goldstone tracking station complex in the desert of southern California, and Haystack Observatory in Massachusetts. The first successful radar observations of Venus took place at Goldstone and Haystack in 1961 and revealed the planet's slow rotation. Subsequent observations determined the rotation properties more precisely and began to unveil some of the major features on the planet's surface. The first features to be observed were dubbed Alpha Regio, Beta Regio, and Maxwell Montes.
By the mid-1980s Earth-based radar technology had advanced such that images from Arecibo were revealing surface features as small as a few kilometres in size. Nevertheless, because Venus always presents nearly the same face toward Earth when the planets are at their closest, much of the surface went virtually unobserved from Earth.
The greatest advances in the study of Venus were achieved through the use of robotic spacecraft. The first spacecraft to reach the vicinity of another planet and return data was the U.S. Mariner 2 in its flyby of Venus in 1962. Since then, Venus has been the target of more than 20 spacecraft missions.
Successful early Venus missions undertaken by the United States involved Mariner 2, Mariner 5 (1967), and Mariner 10 (1974). Each spacecraft made a single
Descent capsule of the Soviet Venera 4 spacecraft prior to its launch to Venus on June 12,1967. Tass/Sovfoto close flyby, providing successively improved scientific data in accord with concurrent advances in spacecraft and instrument technology. After visiting Venus, Mariner 10 went on to a successful series of flybys of Mercury. In 1978 the United States launched the Pioneer Venus mission, comprising two complementary spacecraft. The Orbiter went into orbit around the planet, while the Multiprobe released four entry probes— one large probe and three smaller ones—that were targeted to widely separated points in the Venusian atmosphere to collect data on atmospheric structure and composition. The Orbiter carried 17 scientific instruments, most of them focused on study of the planet's atmosphere, ionosphere, and interaction with the solar wind. Its radar altimeter provided the first high-quality map of Venus's surface topography. Pioneer Venus Orbiter was one of the longest-lived planetary spacecraft, returning data for more than 14 years.
Venus was also a major target of the Soviet Union's planetary exploration program during the 1960s, '70s, and '80s, which achieved several spectacular successes. After an early sequence of failed missions, in 1967 Soviet scientists launched Venera 4, comprising a flyby spacecraft as well as a probe that entered the planet's atmosphere. Equipped with a parachute and several instruments for measuring atmospheric temperature, pressure, and density, it reached its destination on October 18, becoming the first human-made object to travel through the atmosphere of another planet and return data to Earth.
Highlights of subsequent missions included the first successful soft landing on another planet (Venera 7 in 1970), the first images returned from the surface of another planet (Venera 9 and 10 landers in 1975), and the first spacecraft placed in orbit around Venus (Venera 9 and 10 orbiters).
In terms of the advances they provided in the global understanding of Venus, the most important Soviet missions were Veneras 15 and 16 in 1983. The twin orbiters carried the first radar systems flown to another planet that were capable of producing high-quality images of the surface. They produced a map of the northern quarter of Venus with a resolution of 1-2 km (0.6-1.2 miles), and many types of geologic features now known to exist on the planet were either discovered or first observed in detail in the Venera 15 and 16 data. Late the following year the Soviet Union launched two more spacecraft to Venus, Vegas 1 and 2. These delivered Venera-style landers and dropped off two balloons in the Venusian atmosphere, each of which survived for about two days and transmitted data from their float altitudes in the middle cloud layer. The Vega spacecraft themselves continued past Venus to conduct successful flybys of Halley's Comet in 1986.
The most ambitious mission yet to Venus, the U.S. Magellan spacecraft, was launched in 1989 and the next year
Magellan spacecraft and attached Inertial Upper Stage (IUS) rocket being released into a temporary Earth orbit from thepayload bay of the space shuttle orbiter Atlantis on May 4, 1989. Shortly afterward, the IUS propelled the spacecraft on a Sun-looping trajectory toward Venus, where it arrived on Aug. 10, 1990. NASA/JPL
entered orbit around the planet, where it conducted observations until late 1994. Magellan carried a radar system capable of producing images with a resolution better than 100 metres (330 feet). Because the orbit was nearly polar, the spacecraft was able to view essentially all latitudes on the planet. On each orbit the radar system obtained an image strip about 20 km (12 miles) wide and typically more than 16,000 km (almost 10,000 miles) long, extending nearly from pole to pole. The image strips were assembled into mosaics, and high-quality radar images of about 98 percent of the planet were ultimately produced. Magellan also carried a radar altimeter system that measured the planet's surface topography as well as some properties of its surface materials. After the main radar objectives of the mission were completed, the spacecraft's orbit was modified slightly so that it passed repeatedly through the upper fringes of the Venusian atmosphere. The resulting drag on the spacecraft gradually removed energy from its orbit, turning an initially elliptical orbit into a low, circular one. This procedure, known as aerobraking, has since been used on other planetary missions to conserve large amounts of fuel by reducing the use of thrusters for orbital reshaping. From its new circular orbit, the Magellan spacecraft was able to make the first detailed map of Venus's gravitational field.
In 1990, on its way to Jupiter, the U.S. Galileo spacecraft flew by Venus. Among its more notable observations were images at near-infrared wavelengths that viewed deep into the atmosphere and showed the highly variable opacity of the main cloud deck.
The U.S. Cassini-Huygens spacecraft flew by Venus twice, in 1998 and 1999, on the way to its primary target, Saturn.
During its brief passages near Venus, Cassini failed to corroborate signs of the existence of lightning in the planet's atmosphere that had been observed by previous spacecraft. This suggested to some scientists that lightning on Venus is either rare or different from the lightning that occurs on Earth.
The European Space Agency's Venus Express, which was launched in
Deep-level clouds on the nightside of Venus, from an image made by the Galileo spacecraft during its gravity-assisted flyby of the planet in February 1990. In a view that penetrates 10-16 km (6-10 miles) below the cloud surface visible to the human eye, the image shows the relative transparency of the sulfu-ric acid cloud deck to the radiant heat emanating from the much warmer underlying lower atmosphere. NASA/JPL
2005, entered into orbit around Venus the following year, becoming the first European spacecraft to visit the planet. Venus Express carried a camera, a visible-light and infrared imaging spectrometer, and other instruments to study Venus's magnetic field, plasma environment, atmosphere, and surface for a planned mission of more than two Venusian years. Among its early accomplishments was the return of the first images of cloud structures over the planet's south pole.
Our planet, Earth, is the third from the Sun and the fifth largest in the solar system in terms of size and mass. Its single most outstanding feature is that its near-surface environments are the only places in the universe known to harbour life. It is designated by the symbol ©. Earth's name in English, the international language of astronomy, derives from Old English and Germanic words for ground and earth, and it is the only name for a planet of the solar system that does not come from Greco-Roman mythology.
Since the Copernican revolution of the 16th century, at which time the Polish astronomer Nicolaus Copernicus proposed a Sun-centred model of the universe, enlightened thinkers have regarded Earth as a planet like the others of the solar system. Concurrent sea voyages provided practical proof that Earth is a globe, just as Galileo's use of his newly invented telescope in the early 17th century soon showed various other planets to be globes as well. It was only after the dawn of the space age, however, when photographs from rockets and orbiting spacecraft first captured the dramatic curvature of Earth's horizon, that the conception of Earth as a roughly spherical planet rather than as a flat entity was verified by direct human observation. Humans first witnessed Earth as a complete orb floating in the inky blackness of space in December 1968 when Apollo 8 carried astronauts around the Moon. Robotic space probes on their way to destinations beyond Earth, such as the Galileo and the Near Earth
The planet Earth, as photographed from the Galileo spacecraft during its December 1990 flyby en route to Jupiter. The predominance of water on Earth is apparent, both as ocean and in the form of swirling clouds. The landmass at centre right is Australia, and the bright white patch at the bottom is the South Polar ice cap covering Antarctica. NASA/JPL
Asteroid Rendezvous (NEAR) spacecraft in the 1990s, also looked back with their cameras to provide other unique portraits of the planet.
Viewed from another planet in the solar system, Earth would appear bright and bluish in colour. Easiest to see through a large telescope would be its atmospheric features, chiefly the swirling white cloud patterns of midlatitude and tropical storms, ranged in roughly latitudinal belts around the planet. The polar regions also would appear a brilliant white, because of the clouds above and the snow and ice below. Beneath the changing patterns of clouds would appear the much darker blue-black oceans, interrupted by occasional tawny patches of desert lands. The green landscapes that harbour most human life would not be easily seen from space. Not only do they constitute a modest fraction of the land area, which itself is less than one-third of Earth's surface, but they are often obscured by clouds. Over the course of the seasons, some changes in the storm patterns and cloud belts on Earth would be observed. Also prominent would be the growth and recession of the winter snowcap across land areas of the Northern Hemisphere.
Scientists have applied the full battery of modern instrumentation to studying Earth in ways that have not yet been possible for the other planets; thus, much more is known about its structure and composition. This detailed knowledge, in turn, provides deeper insight into the mechanisms by which planets in general cool down, by which their magnetic fields are generated, and by which the separation of lighter elements from heavier ones as planets develop their internal structure releases additional energy for geologic processes and alters crustal compositions.
It is convenient to consider separate parts of Earth in terms of concentric, roughly spherical layers. Extending from the interior outward, these are the core, the mantle, the crust (including the rocky surface), the hydrosphere (predominantly the oceans, which fill in low places in the crust), the atmosphere (itself divided into spherical zones such as the troposphere, where weather occurs, and the stratosphere, where lies the ozone layer that shields Earth's surface and its organisms against the Sun's ultraviolet rays), and the magnetosphere (an enormous region in space where Earth's magnetic field dominates the behaviour of electrically charged particles coming from the Sun).
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