The planet Venus: A Brief Overvall View of our Accumulation of Knowledge

Studies
Early studies
The Venus tablet of Ammisaduqa, dated 1581 BC, records the observations of Babylonian astrologers. It refers to Venus as Nin-dar-an-na, or "bright queen of the sky".Venus was known to ancient civilizations both as the "morning star" and as the "evening star", names that reflect the early understanding that these were two separate objects. The Venus tablet of Ammisaduqa, dated 1581 BC, shows that the Babylonians understood that the two were a single object, referred to in the tablet as the "bright queen of the sky," and could support this view with detailed observations.

The Greeks thought of the two as separate stars, Phosphorus and Hesperus, until the time of Pythagoras in the sixth century BC. The Romans designated the morning aspect of Venus as Lucifer, literally "Light-Bringer", and the evening aspect as Vesper.

The transit of Venus was first observed in 1032 by the Persian astronomer Avicenna, who concluded that Venus is closer to the Earth than the Sun, and established that Venus was, at least sometimes, below the Sun. In the 12th century, the Andalusian astronomer Ibn Bajjah observed "two planets as black spots on the face of the Sun," which was later identified as the transit of Venus and Mercury by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.

17th Century
When the Italian physicist Galileo Galilei first observed the planet in the early 17th century, he found that it showed phases like the Moon, varying from crescent to gibbous to full and vice versa. When Venus is furthest from the Sun in the sky it shows a half-lit phase and when it is closest to the Sun in the sky it shows as a crescent or full phase. This could be possible only if Venus orbited the Sun, and this was among the first observations to clearly contradict the Ptolemaic geocentric model that the Solar System was concentric and centered on the Earth.

The atmosphere of Venus was discovered in 1761 by Russian polymath Mikhail Lomonosov. Venus' atmosphere was observed in 1790 by German astronomer Johann Schröter. Schröter found that when the planet was a thin crescent, the cusps extended through more than 180°. He correctly surmised that this was due to scattering of sunlight in a dense atmosphere. Later, American astronomer Chester Smith Lyman observed a complete ring around the dark side of the planet when it was at inferior conjunction, providing further evidence for an atmosphere.

The atmosphere complicated efforts to determine a rotation period for the planet, and observers such as Italian-born astronomer Giovanni Cassini and Schröter incorrectly estimated periods of about 24 hours from the motions of markings on the planet's apparent surface.

Ground-based research
Little more was discovered about Venus until the 20th century. Its almost featureless disc gave no hint what its surface might be like, and it was only with the development of spectroscopic, radar and ultraviolet observations that more of its secrets were revealed. The first UV observations were carried out in the 1920s, when Frank E. Ross found that UV photographs revealed considerable detail that was absent in visible and infrared radiation. He suggested that this was due to a very dense yellow lower atmosphere with high cirrus clouds above it.

Spectroscopic observations in the 1900s gave the first clues about the Venusian rotation. Vesto Slipher tried to measure the Doppler shift of light from Venus, but found that he could not detect any rotation. He surmised that the planet must have a much longer rotation period than had previously been thought. Later work in the 1950s showed that the rotation was retrograde. Radar observations of Venus were first carried out in the 1960s, and provided the first measurements of the rotation period which were close to the modern value.

Radar observations in the 1970s revealed details of the Venusian surface for the first time. Pulses of radio waves were beamed at the planet using the 300 m radio telescope at Arecibo Observatory, and the echoes revealed two highly reflective regions, designated the Alpha and Beta regions. The observations also revealed a bright region attributed to mountains, which was called Maxwell Montes. These three features are now the only ones on Venus which do not have female names.

Exploration
Early efforts
Mariner 2, launched in 1962The first robotic space probe mission to Venus, and the first to any planet, began on February 12, 1961 with the launch of the Venera 1 probe. The first craft of the otherwise highly successful Soviet Venera program, Venera 1 was launched on a direct impact trajectory, but contact was lost seven days into the mission, when the probe was about 2 million km from Earth. It was estimated to have passed within 100,000 km from Venus in mid-May.

The United States exploration of Venus also started badly with the loss of the Mariner 1 probe on launch. The subsequent Mariner 2 mission enjoyed greater success, and after a 109-day transfer orbit on December 14, 1962 it became the world's first successful interplanetary mission, passing 34,833 km above the surface of Venus. Its microwave and infrared radiometers revealed that while the Venusian cloud tops were cool, the surface was extremely hot—at least 425 °C, finally ending any hopes that the planet might harbor ground-based life. Mariner 2 also obtained improved estimates of its mass and of the astronomical unit, but was unable to detect either a magnetic field or radiation belts.

Atmospheric entry
The Soviet Venera 3 probe crash-landed on Venus on March 1, 1966. It was the first man-made object to enter the atmosphere and strike the surface of another planet, though its communication system failed before it was able to return any planetary data.

Venus's next encounter with an unmanned probe came on October 18, 1967 when Venera 4 successfully entered the atmosphere and deployed a number of science experiments. Venera 4 showed that the surface temperature was even hotter than Mariner 2 had measured at almost 500 °C, and that the atmosphere was about 90 to 95% carbon dioxide. The Venusian atmosphere was considerably denser than Venera 4's designers had anticipated, and its slower than intended parachute descent meant that its batteries ran down before the probe reached the surface. After returning descent data for 93 minutes, Venera 4's last pressure reading was 18 bar at an altitude of 24.96 km.[90]

Another probe arrived at Venus one day later on October 19, 1967 when Mariner 5 conducted a flyby at a distance of less than 4000 km above the cloud tops. Mariner 5 was originally built as backup for the Mars-bound Mariner 4, but when that mission was successful, the probe was refitted for a Venus mission. A suite of instruments more sensitive than those on Mariner 2, in particular its radio occultation experiment, returned data on the composition, pressure and density of the Venusian atmosphere.

The joint Venera 4–Mariner 5 data were analyzed by a combined Soviet-American science team in a series of colloquia over the following year, in an early example of space cooperation.[93]

Armed with the lessons and data learned from Venera 4, the Soviet Union launched the twin probes Venera 5 and Venera 6 five days apart in January 1969; they encountered Venus a day apart on May 16 and May 17 that year. The probes were strengthened to improve their crush depth to 25 bar and were equipped with smaller parachutes to achieve a faster descent. Since then-current atmospheric models of Venus suggested a surface pressure of between 75 and 100 bar, neither was expected to survive to the surface. After returning atmospheric data for a little over fifty minutes, they both were crushed at altitudes of approximately 20 km before going on to strike the surface on the night side of Venus.

Surface and atmospheric science
Venera 7 represented an effort to return data from the planet's surface, and was constructed with a reinforced descent module capable of withstanding a pressure of 180 bar. The module was pre-cooled before entry and equipped with a specially reefed parachute for a rapid 35-minute descent.

Entering the atmosphere on December 15, 1970, the parachute is believed to have partially torn during the descent, and the probe struck the surface with a hard, yet not fatal, impact. Probably tilted onto its side, it returned a weak signal supplying temperature data for 23 minutes, the first telemetry received from the surface of another planet.

The Venera program continued with Venera 8 sending data from the surface for 50 minutes, after entering the atmosphere on July 22, 1972. Venera 9, which entered the atmosphere of Venus on October 22, 1975, and Venera 10, which entered the atmosphere three days later on October 25, sent the first images of the Venusian landscape.

The two landing sites presented very different terrain in the immediate vicinities of the landers: Venera 9 had landed on a 20 degree slope scattered with boulders around 30–40 cm across; Venera 10 showed basalt-like rock slabs interspersed with weathered material.

In the meantime, the United States had sent the Mariner 10 probe on a gravitational slingshot trajectory past Venus on its way to Mercury. On February 5, 1974, Mariner 10 passed within 5790 km of Venus, returning over 4000 photographs as it did so. The images, the best then achieved, showed the planet to be almost featureless in visible light, but ultraviolet light revealed details in the clouds that had never been seen in Earth-bound observations.

The American Pioneer Venus project consisted of two separate missions. The Pioneer Venus Orbiter was inserted into an elliptical orbit around Venus on December 4, 1978, and remained there for over thirteen years studying the atmosphere and mapping the surface with radar. The Pioneer Venus Multiprobe released a total of four probes which entered the atmosphere on December 9, 1978, returning data on its composition, winds and heat fluxes.

Four more Venera lander missions took place over the next four years, with Venera 11 and Venera 12 detecting Venusian electrical storms;[98] and Venera 13 and Venera 14, landing four days apart on March 1 and March 5, 1982, returning the first color photographs of the surface. All four missions deployed parachutes for braking in the upper atmosphere, but released them at altitudes of 50 km, the dense lower atmosphere providing enough friction to allow for an unaided soft landing.

Both Venera 13 and 14 analyzed soil samples with an on-board X-ray fluorescence spectrometer, and attempted to measure the compressibility of the soil with an impact probe.[98] Venera 14, though, had the misfortune to strike its own ejected camera lens cap and its probe failed to contact the soil.

The Venera program came to a close in October 1983 when Venera 15 and Venera 16 were placed in orbit to conduct mapping of the Venusian terrain with synthetic aperture radar.

In 1985 the Soviet Union took advantage of the opportunity to combine missions to Venus and Comet Halley, which passed through the inner Solar System that year. En route to Halley, on June 11 and June 15, 1985 the two spacecraft of the Vega program each dropped a Venera-style probe (of which Vega 1's partially failed) and released a balloon-supported aerobot into the upper atmosphere. The balloons achieved an equilibrium altitude of around 53 km, where pressure and temperature are comparable to those at Earth's surface. They remained operational for around 46 hours, and discovered that the Venusian atmosphere was more turbulent than previously believed, and subject to high winds and powerful convection cells.

Radar mapping
The United States' Magellan probe was launched on May 4, 1989 with a mission to map the surface of Venus with radar. The high-resolution images it obtained during its 4½ years of operation far surpassed all prior maps and were comparable to visible-light photographs of other planets. Magellan imaged over 98% of the Venusian surface by radar and mapped 95% of its gravity field. In 1994, at the end of its mission, Magellan was deliberately sent to its destruction into the atmosphere of Venus to quantify its density.[103] Venus was observed by the Galileo and Cassini spacecraft during flybys on their respective missions to the outer planets, but Magellan would otherwise be the last dedicated mission to Venus for over a decade.

Current and future missions
NASA's MESSENGER mission to Mercury performed two flybys of Venus in October 2006 and June 2007, to slow its trajectory for an eventual orbital insertion of Mercury in March 2011. MESSENGER collected scientific data on both those flybys.[106]

The Venus Express probe was designed and built by the European Space Agency. Launched on November 9, 2005 by a Russian Soyuz-Fregat rocket procured through Starsem, it successfully assumed a polar orbit around Venus on April 11, 2006.

The probe is undertaking a detailed study of the Venusian atmosphere and clouds, including mapping of the planet's plasma environment and surface characteristics, particularly temperatures. One of the first results emerging from Venus Express is the discovery that a huge double atmospheric vortex exists at the south pole of the planet.

The Japan Aerospace Exploration Agency (JAXA) devised a Venus orbiter, Akatsuki (formerly "Planet-C"), which was launched on May 20, 2010 but the craft failed to enter orbit in December 2010. Hopes remain that the probe can successfully hibernate and make another insertion attempt in six years. Planned investigations included surface imaging with an infrared camera and experiments designed to confirm the presence of lightning as well as the determination of the existence of current surface volcanism.

The European Space Agency (ESA) hopes to launch a mission to Mercury in 2014, called BepiColombo, which will perform two flybys of Venus before it reaches Mercury orbit in 2020.

Under its New Frontiers Program, NASA has proposed a lander mission called the Venus In-Situ Explorer to land on Venus to study surface conditions and investigate the elemental and mineralogical features of the regolith. The probe would be equipped with a core sampler to drill into the surface and study pristine rock samples not weathered by the very harsh surface conditions. The Venera-D probe is a proposed Russian space probe to Venus, to be launched around 2016 with its goal to make remote-sensing observations around the planet Venus and deploying a lander, based on the Venera design, capable of surviving for a long duration on the planet's surface. Other proposed Venus exploration concepts include rovers, balloons, and airplanes.

NASA has recommended the Surface and Atmosphere Geochemical Explorer (SAGE) candidate mission to land on Venus, with a possible launch in 2016.

Manned flyby
A manned Venus flyby mission, using Apollo program hardware, was proposed in the late 1960s. The mission was planned to launch in late October or early November 1973, and would have used a Saturn V to send three men to fly past Venus in a flight lasting approximately one year. The spacecraft would have passed approximately 5,000 kilometres from the surface of Venus about four months later.