Credit: Mattias Malmer, from NASA/JPL data
Closest Planet to Earth
Venus is the second closest planet to the Sun and orbits in an almost circular orbit at 108 million km. As it orbits, Venus comes closer to Earth than any other planet in the solar system and can come to within about 40 million km.
Spins Slowly the wrong way!
Venus takes about 225 Earth days to orbit the Sun and rotates at the incredibly slow rate of once every 243 days - and in a clockwise direction (as seen from looking down on the Suns north pole). Only Uranus (which almost spins on its side) also has a clockwise spin. Because of the direction of spin and orbital motion, a day on Venus (mid day to mid day) lasts 117 Earth days. So even though Venus has the slowest rotational rate of any planet, a day on spent Venus is shorter than a day spent on Mercury.
Venus can be said to have either a gentle axial tilt of 2.6 degrees, or, because scientists take the direction of spin into account, a massive tilt of -177.4 degrees.
Closest in size to Earth
Venus, with a diameter of 12100 km, is very nearly the same size as Earth (1000km smaller), and has 80% of Earth’s mass. its gravity on the surface is 90% that of Earth’s.
The Hottest Planet in the Solar System
Venus has a very dense atmosphere with pressures at the surface over 90 times that of Earth’s. The atmosphere is comprised of carbon dioxide with thick clouds of sulphur dioxide. This atmosphere has the strongest greenhouse effect known in the solar system which keeps the planet at a reasonably constant temperature of 460 degrees C. This makes Venus the hottest planet in the solar system, far hotter even then mercury which is twice as close to the Sun.
The surface of Venus, although hidden from view by thick clouds, has been mapped using radar and it is known that is is covered by large flat volcanic plains with two higher areas of land (continents) with mountains and valleys. The surface also shows impact craters and volcano like structures. Venus has a very weak magnetic field.
Venus and Man
Because Venus is so close to the Sun, it is often the first star to appear in the evening and the last to disappear in the morning. Hence it has long been known as the "evening star" and the "morning star".
The early Greeks named these two aspects of Venus "Phosphorus" and "Hesperus" and the Romans "Lucifer", (literally "Light-Bringer"), and "Vesper".
Attempts to send probes to the planet started in the infancy of space flight. In 1961, the Russian probe Venera 1 was sent to impact with the planet, but communications were lost in transit. The USA then attempted to launch mariner 1 in 1962, but it was destroyed by a command from the control centre a few minutes after launch as it veered of course. Mariner 2 was launched a month later and made a fly past of Venus in December of 1962 and was the first successful robotic interplanetary mission. It successfully measured the atmosphere, surface temperature magnetic field and radiation levels.
The Russians sent the probe Venera 3 in 1966 which became the first probe to enter the atmosphere of another planet. Unfortunately it returned no planetary data. It was followed by Venera 4 which did return data which showed among other things, that the atmosphere was much denser than expected. Using the data from Venera 4, improved probes Venera 5 and 6 were also sent to the planet in the 60's - neither surviving long enough to reach the surface. Mariner 5 also performed a fly-by and data was shared between Russian and American scientists.
During the 70's and 80's many more missions were sent to the planet with Venera 7 being the first probe to send scientific data from the surface (rather than just the atmosphere) of another planet. Venera 9 and 10 were the first missions to send back images of the surface of Venus. These were the first images received from the surface of another planet. Venera 11 and 12 both suffered lens cap release failures preventing images from being returned.
Venera 13 followed and analysed soil samples and Venera 15 and 16 entered orbit to map the surface using radar. The Americans sent Mariner 10 to photograph the planet in detail and the Pioneer Venus project sent an Orbiters and 4 atmospheric probes to the surface. In the 90's NASA's Magellan Spacecraft orbited the planet from 1990 to 1994 before deliberately crashing into the planet.
Venus was studied by Europe's Venus Express Spacecraft which orbited the planet from April 2006 until crashing into it some time in January, 2015.
Venus Express made many discoveries including:
1. There are unstable vortices in the atmosphere at the poles.
2. There appears to be ongoing volcanic activity beneath the clouds detected as heat sources and sulphur clouds.
3. Venus seems to be spinning slower (by 6.5 minutes a day) than measured by the Magellan spacecraft giving rise to speculation on how molten its core is.
4. Wind speeds in the upper atmosphere increased from 300kmph to 400kmph throughout the mission.
5. There is a very cold layer (-175 degrees C) 125km up in the atmosphere that has much hotter layers above and below it. This layer might contain frozen carbon dioxide snow.
The Messenger probe also made measurements during two fly-pasts in 2006/7 on its way to mercury.
Venus is currently being studied by the Japanese spacecraft Akatsuki. A rocket motor failure prevented Akatsuki from entering orbit in 2010 causing it to orbit the Sun for 5 years before being inserted into orbit in December 2015 using its thrusters. Since May 2016 it has been carrying out scientific studies of the Venusian atmosphere.
Among other observations, Akatsuki has discovered a bow wave in the Venusian atmosphere which seems to be caused as the winds blow over the Venusian continent called Aphrodite Terra, which reaches an altitude of approximately 5 km. Article.
First image from the surface of another planet
The surface of Venus from Venera 9 October 20th 1975. The first image from the surface of another planet. (Image photo shopped to replace some lines of pixels that were garbled in the original image). By Soviet Union/Roscosmos/Venera 9 - http://www.mentallandscape.com/C_CatalogVenus.htm, Fair use, https://en.wikipedia.org/w/index.php?curid=25484636