The planet Venus was first observed in antiquity, and continued with telescopic observations, and then by visiting spacecraft. Spacecraft have performed multiple flybys, orbits, and landings on the planet, including balloon probes that floated in its atmosphere. Study of the planet is aided by its relatively close proximity to the Earth, but the surface of Venus is obscured by an atmosphere opaque to visible light.
Ground-based observations
Transits of Venus directly between the Earth and the Sun's visible disc are rare astronomical events. The first such transit to be predicted and observed was the 1639 transit of Venus, seen and recorded by English astronomers Jeremiah Horrocks and William Crabtree. The observation by Mikhail Lomonosov of the transit of 1761 provided the first evidence that Venus had an atmosphere, and the 19th-century observations of parallax during Venus transits allowed the distance between the Earth and Sun to be accurately calculated for the first time. Transits can only occur either in early June or early December, these being the points at which Venus crosses the ecliptic (the orbital plane of the Earth), and occur in pairs at eight-year intervals, with each such pair more than a century apart. The most recent pair of transits of Venus occurred in 2004 and 2012, while the prior pair occurred in 1874 and 1882.
In the 19th century, many observers stated that Venus had a period of rotation of roughly 24 hours. Italian astronomer Giovanni Schiaparelli was the first to predict a significantly slower rotation, proposing that Venus was tidally locked with the Sun (as he had also proposed for Mercury). While not actually true for either body, this was still a reasonably accurate estimate. The near-resonance between its rotation and its closest approach to Earth helped to create this impression, as Venus always seemed to be facing the same direction when it was in the best location for observations to be made. The rotation rate of Venus was first measured during the 1961 conjunction, observed by radar from a 26 m antenna at Goldstone, California, the Jodrell Bank Radio Observatory in the UK, and the Soviet deep space facility in Yevpatoria, Crimea. Accuracy was refined at each subsequent conjunction, primarily from measurements made from Goldstone and Eupatoria. The fact that rotation was retrograde was not confirmed until 1964.
Before radio observations in the 1960s, many believed that Venus contained a lush, Earth-like environment. This was due to the planet's size and orbital radius, which suggested a fairly Earth-like situation as well as to the thick layer of clouds which prevented the surface from being seen. Among the speculations on Venus were that it had a jungle-like environment or that it had oceans of either petroleum or carbonated water. However, microwave observations by C. Mayer et al. indicated a high-temperature source (600 K). Strangely, millimetre-band observations made by A. D. Kuzmin indicated much lower temperatures. Two competing theories explained the unusual radio spectrum, one suggesting the high temperatures originated in the ionosphere, and another suggesting a hot planetary surface.
In September 2020, a team at Cardiff University announced that observations of Venus using the James Clerk Maxwell Telescope and Atacama Large Millimeter Array in 2017 and 2019 indicated that the Venusian atmosphere contained phosphine (PH3) in concentrations 10,000 times higher than those that could be ascribed to any known non-biological source on Venus. The phosphine was detected at heights of at least above the surface of Venus, and was detected primarily at mid-latitudes with none detected at the poles of Venus. This could have indicated the potential presence of biological organisms on Venus, however, this measurement was later shown to be in error.
Terrestrial radar mapping
After the Moon, Venus was the second object in the Solar System to be explored by radar from the Earth. The first studies were carried out in 1961 at NASA's Goldstone Observatory, part of the Deep Space Network. At successive inferior conjunctions, Venus was observed both by Goldstone and the National Astronomy and Ionosphere Center in Arecibo. The studies carried out were similar to the earlier measurement of transits of the meridian, which had revealed in 1963 that the rotation of Venus was retrograde (it rotates in the opposite direction to that in which it orbits the Sun). The radar observations also allowed astronomers to determine that the rotation period of Venus was 243.1 days, and that its axis of rotation was almost perpendicular to its orbital plane. It was also established that the radius of the planet was , some less than the best previous figure obtained with terrestrial telescopes.
Interest in the geological characteristics of Venus was stimulated by the refinement of imaging techniques between 1970 and 1985. Early radar observations suggested merely that the surface of Venus was more compacted than the dusty surface of the Moon. The first radar images taken from the Earth showed very bright (radar-reflective) highlands christened Alpha Regio, Beta Regio, and Maxwell Montes; improvements in radar techniques later achieved an image resolution of 1–2 kilometres.
Observation by spacecraft
There have been numerous uncrewed missions to Venus. Ten Soviet Venera probes achieved a soft landing on the surface, with up to 110 minutes of communication from the surface, all without return. Launch windows occur every 19 months.
Early flybys
On February 12, 1961, the Soviet spacecraft Venera 1 was the first flyby probe launched to another planet. An overheated orientation sensor caused it to malfunction, losing contact with Earth before its closest approach to Venus of 100,000 km. However, the probe was first to combine all the necessary features of an interplanetary spacecraft: solar panels, parabolic telemetry antenna, 3-axis stabilization, course-correction engine, and the first launch from parking orbit.
The first successful flyby Venus probe was the American Mariner 2 spacecraft, which flew past Venus in 1962, coming within 35,000 km. A modified Ranger Moon probe, it established that Venus has practically no intrinsic magnetic field and measured the temperature of the planet's atmosphere to be approximately .
The Soviet Union launched the Venera 2 probe to Venus in 1966, but it malfunctioned sometime after its May 16 telemetry session. The probe completed a flyby of Venus, but failed to transmit any data.
During another American flyby in 1967, Mariner 5 measured the strength of Venus's magnetic field. In 1974, Mariner 10 swung by Venus on its way to Mercury and took ultraviolet photographs of the clouds, revealing the extraordinarily high wind speeds in the Venusian atmosphere. Mariner-10 provided the best images of Venus taken so far, the series of images clearly demonstrated the high speeds of the planet's atmosphere, first seen in the Doppler-effect velocity measurements of Venera-4 through Venera-8.
Early landings
On March 1, 1966, the Venera 3 Soviet space probe crash-landed on Venus, becoming the first spacecraft to reach the surface of another planet.
The descent capsule of Venera 4 entered the atmosphere of Venus on October 18, 1967, making it the first probe to return direct measurements from another planet's atmosphere. The capsule measured temperature, pressure, density and performed 11 automatic chemical experiments to analyze the atmosphere. It discovered that the atmosphere of Venus was 95% carbon dioxide (), and in combination with radio occultation data from the Mariner 5 probe, showed that surface pressures were far greater than expected (75 to 100 atmospheres).
These results were verified and refined by the Venera 5 and Venera 6 in May 1969. But thus far, none of these missions had reached the surface while still transmitting. Venera 4'''s battery ran out while still slowly floating through the massive atmosphere, and Venera 5 and 6 were crushed by high pressure 18 km (60,000 ft) above the surface.
The first successful landing on Venus was by Venera 7 on December 15, 1970 — the first successful soft landing on another planet, as well as the first successful transmission of data from another planet's surface to Earth. Venera 7 remained in contact with Earth for 23 minutes, relaying surface temperatures of , and an atmospheric pressure of 92 bar. Venera 8 landed on July 22, 1972. In addition to pressure and temperature profiles, a photometer showed that the clouds of Venus formed a layer ending over above the surface. A gamma ray spectrometer analyzed the chemical composition of the crust. Venera 8 measured the light level as being suitable for surface photography, finding it to be similar to the amount of light on Earth on an overcast day with roughly 1 km visibility.
Lander/orbiter pairs
Venera 9 and 10
The Soviet probe Venera 9 entered orbit on October 22, 1975, becoming the first artificial satellite of Venus. A battery of cameras and spectrometers returned information about the planet's clouds, ionosphere and magnetosphere, as well as performing bi-static radar measurements of the surface. The descent vehicle separated from Venera 9 and landed, taking the first pictures of the surface and analyzing the crust with a gamma ray spectrometer and a densitometer. During descent, pressure, temperature and photometric measurements were made, as well as backscattering and multi-angle scattering (nephelometer) measurements of cloud density. It was discovered that the clouds of Venus are formed in three distinct layers.
On October 25, Venera 10 arrived and carried out a similar program of study.
Pioneer Venus
In 1978, NASA sent two Pioneer spacecraft to Venus. The Pioneer mission consisted of two components, launched separately: an orbiter and a multiprobe. The Pioneer Venus Multiprobe carried one large and three small atmospheric probes. The large probe was released on November 16, 1978, and the three small probes on November 20. All four probes entered the Venusian atmosphere on December 9, followed by the delivery vehicle. Although not expected to survive the descent through the atmosphere, one probe continued to operate for 45 minutes after reaching the surface. The Pioneer Venus Orbiter was inserted into an elliptical orbit around Venus on December 4, 1978. It carried 17 experiments and operated until the fuel used to maintain its orbit was exhausted and atmospheric entry destroyed the spacecraft in August 1992.
Further Soviet missions Also in 1978, Venera 11 and Venera 12 flew past Venus, dropping descent vehicles on December 21 and December 25 respectively. The landers carried colour cameras and a soil drill and analyzer, which unfortunately malfunctioned. Each lander made measurements with a nephelometer, mass spectrometer, gas chromatograph, and a cloud-droplet chemical analyzer using X-ray fluorescence that unexpectedly discovered a large proportion of chlorine in the clouds, in addition to sulfur. Strong lightning activity was also detected.
In 1982, the Soviet Venera 13 sent the first colour image of Venus's surface, revealing an orange-brown flat bedrock surface covered with loose regolith and small flat thin angular rocks, and analysed the X-ray fluorescence of an excavated soil sample. The probe operated for a record 127 minutes on the planet's hostile surface. Also in 1982, the Venera 14 lander detected possible seismic activity in the planet's crust.