Venus, ‘the morning/evening star’ is an extraordinary planet with many similar characteristics to Earth such that it is often called Earth’s twin. Located next to Earth, despite the similar mass and composition, the two planets are also very different. Examples of this include, Venus being the only planet in the solar system that spins clockwise rather than anti-clockwise, its day being longer than its orbital period and is covered entirely in toxic, perpetual clouds preventing us from viewing the surface directly. With an extremely high surface temperature over 450 degrees Celsius, and crushing atmospheric pressure approximately 92 times of Earth, it makes Venus very hard to explore. 
History of missions to Venus and their achievements
Since the beginning of the space age (1960s), there many space missions with Venus as the primary target. Till date, there is a total of 41 missions launched between the Soviets and Americans with 23 of them being classified as successful.  The Soviet Union through its Venera program composing of different mission types including – fly-bys, landers and orbiters helped with the early exploration of Venus. Space missions by the United States also contributed to many discoveries about Venus through its Mariner and Pioneer programs and the Magellan program, all composed of fly-bys and orbiters.
Mariner 1 & 2
Launched on the 27th of august 1962, Mariner 2 was the second satellite of a series of spacecrafts consisting of orbiters and fly-bys started by the United States. The primary goal of the Mariner series was to launch space probes to the vicinity of Venus to gather information about the planet’s temperature, atmospheric, magnetic field and rotational data using an array of sensors.  The first probe of the series Mariner 1 was launched a month earlier but ultimately a failure as it was quickly destroyed after take-off by the range safety officer since one of the rockets malfunctions causing it to veer off course.  Using thermal radiometers, Mariner 2 was able to gather information of the planet’s very high surface temperature through the thick clouds that covered Venus. Analysis of the planet’s spin showed that its spin was retrograde, meaning it spun in the opposite direction to the other planets and that it had a slow rotation.  The three-axis magnetometer onboard the satellite uncovered that the planet lacked a global magnetic field that was measurable, meaning it was extremely weak or didn’t have one at all.  The lack of a magnetic field only brought more questions as to why Venus was able to maintain such a thick atmosphere since magnetic fields reduce and remove ionised particles from the constant solar wind slamming the planet. 
The early missions of the Venera series by the USSR were plagued with launch and spacecraft failures until the first breakthrough by the impactor Venera 4. Launched on the 12th of June 1967, Venera 4 became the first probe to ever transmit atmospheric data from the atmosphere of another planet. Equipped with a thick heatshield, it allowed the impactor to survive the extreme heat generated through entry of the very thick atmosphere. With little data on how high the atmospheric pressure really was, the altimeter that was equipped quickly went off-scale as its range was not designed for pressures above 7.3 atmospheres. Whilst the satellites life was short-lived during entry, gas analysers onboard provided data that approximately 90-95 percent of the atmosphere composed of carbon dioxide with no signs of water.  This lack of water and extreme heat suggested that the planet was very unlikely to support any forms of life, especially since the lack of water opposed the idea of ‘follow the water,’ a chemical property essential for life.  Despite ionised hydrogen atoms being observed in the atmosphere, the lack of water particles and intense heat makes it impossible for water to form on the surface which means that if life never existed on the planet, it most likely will remain a lifeless planet.
Venera 7 & 8
From Venera 4 up to 6, there were another three successfully missions to gather information of the atmosphere of Venus. It wasn’t until Venera 7 that came another breakthrough by the USSR. Venera 7 was launched on the 17th August 1970, with a different mission plan than its predecessors. Rather than being an atmospheric satellite, it was designed to be a lander to gather information on Venus’ hot surface. The overall mission was classified as a partial failure since the parachute was destroyed during the descent onto the planet causing it to crash land. However, it was able to return limited data for a small period. The sensors onboard revealed a high surface temperature and high-pressure readings through direct data and extrapolation from other measurements, confirming earlier data readings from satellites in earlier missions. 
Venera 7 was the first satellite to complete a soft landing on another planet which later, its successor Venera 8 was able to successfully land on Venus. For approximately one hour, Venera 8 was able to continuously transmit data back to earth about the surface temperature, pressure and illumination from the sun. Data from the photometer on Venera 8 illustrated that the surface of Venus had a similar amount of light as Earth’s on a cloudy day permitting surface photography in future missions. Equipped with a gamma-ray spectrometer, the search for naturally occurring radioactive material in the soil could be conducted. The data from the mission also concluded that corrosive sulfuric acid precipitated out from the layers of cloud on Venus. 
Venera 9 & 13
Upon the completion of successfully landing on Venus came the next generation of satellites in the Venera series. This new series of satellites starting from Venera 9 composed of an orbiter and a lander. Launched in June 8, 1975 fitted with an ultraviolet imaging spectrometer and many equipment, the orbiter was able to photograph the planet in ultraviolet light, act as a communications relay and perform many atmospheric investigations. Fitted with a photographic camera, the lander of Venera 9 became the first spacecraft to take a panoramic photograph of another planet’s surface in black and white colour.  These images allowed scientists to visually see the terrain of Venus mostly the flat rocks that covered the surface.
Six years after the launch of Venera 9 on the 30th of October 1981, was the launched of Venera 13. Venera 13 was a landing probe fitted with more equipment, most notably a camera that could take a coloured photograph. The coloured photographs transmitted provided scientists to get a grasp what colours the surroundings were after the filtering of sunlight by the thick atmosphere. 
Pioneer – Venus 1
With advancements in technology, Pioneer Venus 1 was designed and fitted as an orbiter with a surface radar mapper (ORAD) to provide detailed topographic maps for 93 percent of Venus’ surface. Pioneer Venus 1 was the first American spacecraft to successfully orbit the planet for a relatively long period of time providing new data and finding new discoveries. Some of these discoveries through camera was that the planet was covered in continuous lightning activity and that the surface of the planet as an average, was smoother and more circular than Earth.  The orbiter also measured the composition of the upper atmosphere and how the solar winds interacted the ionosphere since there was little to no knowledge from prior spacecrafts. 
Launched in May 1989, Magellan was an orbiter satellite sent with a synthetic aperture radar (SAR) mapper used to map the surface of Venus. Using radio waves which can completely penetrate though the thick cloudy atmosphere, SAR can be used to create detailed three-dimensional reconstructions of objects, in this case the surface of Venus. Within its 4 years of orbiting the planet and mapping the topographic reliefs and surface, it was able to obtain enough data to cover 98% of the surface in might higher resolutions than the previous satellites sent.  Within the 1200 gigabits of data received, it provided evidence that majority of the planet was covered in volcanic flow with the terrain showing signs of surface winds and craters reflecting past impacts of meteorites and volcanos. 
Life on Venus?
It is not certain if Venus once harboured oceans prior to the ‘runaway greenhouse effect’ that allowed life to flourish but what is certain is that the currently conditions and climate of Venus make it essentially impossible for life to exist on the surface of Earth’s ‘sister planet.’ The greenhouse effect is a natural occurring cycle that is responsible in keeping a planet ‘warm’ such that it traps a certain of heat within the atmosphere. Examples of greenhouse gases include water vapour and carbon dioxide. The greenhouse effect becomes ‘runaway’ when the surface temperature from the greenhouse effect reaching the boiling point of water, in doing so, water molecules turn into water vapour. The excess water vapour enhances the greenhouse effect causing the temperature to further rise contributing to greater evaporation called a positive feedback loop.  In other words, the hotter the temperature is, the faster it warms up.
The idea of Venus undergoing a runaway greenhouse effect may help researchers explain why the planet lacks water as most of it evaporated away over a long period of time.
In modern times, there has been a shift away from Venus towards another planet in our solar system, Mars. With a focus of searching for evidence of extra-terrestrial life, Venus drifts slowly away from scientist’s scope. Whilst there could be much to learn about the runaway greenhouse effect and comparing it to Earths greenhouse effect, from its thick, toxic atmosphere and high surface temperatures, we should consider other sites for life like Mars.
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