Observation by spacecraft
There have been numerous unmanned missions to Venus. Ten Russian probes have included a soft landing on the surface, with up to 110 minutes of communication from the surface, all without return.
On February 12, 1961, the Soviet spacecraft Venera 1 was the first probe launched to another planet. An overheated orientation sensor caused it to malfunction, but Venera-1 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 Venus probe was the American Mariner 2 spacecraft, which flew past Venus in 1962. A modified Ranger Moon probe, it established that Venus has no magnetic field and measured the planet's thermal microwave emissions.
The Soviet Union launched the Zond 1 probe to Venus on April 2, 1964, but it malfunctioned sometime after its May 16 telemetry session.
Sketch of Venera-5
On March 1, 1966 the Venera 3 Soviet space probe crash-landed on Venus, becoming the first spacecraft to reach the planet's surface. Its sister craft Venera 2 had failed from overheating shortly before completing its flyby mission.
The descent capsule of Venera 4 entered the atmosphere of Venus on October 18, 1967. The first probe to return direct measurements from another planet, the capsule measured temperature, pressure, density and performed 11 automatic chemical experiments to analyze the atmosphere. It showed 95% carbon dioxide, and in combination with radio occultation data from the Mariner 5 probe, it 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 missions on May 16 and 17 of 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. It relayed surface temperatures of 455 °C to 475 °C (855 °F to 885 °F). 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 22 miles above the surface. A gamma ray spectrometer analyzed the chemical composition of the crust.
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 bistatic radar measurements of the surface.
The 660 kg (1,455 lb) 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.
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 Venus 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 successes
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.
Venera 13 and Venera 14 carried out essentially the same mission, arriving at Venus on March 1 and March 5, 1982. This time, color camera and soil-drilling/analysis experiments were successful. X-ray fluorescence analysis of soil samples showed results similar to potassium-rich basalt rock.
On October 10 and October 11, 1983, Venera 15 and Venera 16 entered polar orbits around Venus. Venera 15 analyzed and mapped the upper atmosphere with an infrared Fourier spectrometer. From November 11 to July 10, both satellites mapped the northern third of the planet with synthetic aperture radar. These results provided the first detailed understanding of the surface geology of Venus, including the discovery of unusual massive shield volcanoes such as coronae and arachnoids. Venus had no evidence of plate tectonics, unless the northern third of the planet happened to be a single plate.
Image:Vega lander.jpg The Soviet Vega 1 and Vega 2 probes encountered Venus on June 11 and June 15 of 1985. Landing vehicles carried experiments focusing on cloud aerosol composition and structure. Each carried an ultraviolet absorption spectrometer, aerosol particle-size analyzers, and devices for collecting aerosol material and analyzing it with a mass spectrometer, a gas