Although Mars gets the most attention as a potential destination for human exploration, it is actually not the closest planet (most of the time anyway) to Earth. That honour goes to Venus, a planet that is so similar in size and mass to the Earth that is sometimes called Earth’s “sister planet”. So why isn’t Venus considered a target for human exploration or colonisation? The answer is simple – the surface of Venus is a hellish landscape, with an average temperature of 460C, an atmosphere of carbon dioxide and sulfuric acid, and thick clouds that prevent sunlight from ever reaching the ground.
The conditions on the surface of Venus are so severe that even robots stuggle to survive. Between 1967 and 1984 eleven Soviet-made space probes reached the surface of Venus. One of these, Venera 13, reported temperatures of 457C (855F) and an atmospheric pressure 89 times higher than we experience on Earth. Venera 13 survived for just over two hours on the surface, and holds the record for the longest surving lander on Venus.
Although the thick clouds in Venus’s atmosphere mean we can never directly see the surface, in 1990 the Magellan spacecraft used radar to image the surface, and was able to produce a map of the whole planet. Magellan revealed a landscape shaped by volcanoes. The surface of Venus consists of vast plains of solidified lava, huge volcanoes and channels through which rivers of lava once flowed.
Venus is also unusual in another way – it rotates extremely slowly, so slowly in fact, that a single day on Venus is longer than a Venusian year. Venus orbits the Sun once every 225 Earth days, while each Venus day lasts 243 Earth days. It has been suggested that Venus’ slow rotation was caused by a massive impact billions of years ago.
Despite the hardships and difficulties of exploring Venus, several ideas have been raised for future missions and even eventual colonisation of Venus. Previous missions have found that using atmospheric probes (e.g balloons, aircraft) has been more successful than using surface probes. High in the atmosphere of Venus conditions are actually quite Earth-like, with temperatures around 30C, atmospheric pressure about half of that on Earth and a similar gravity to Earth. NASA has proposed that humans could visit Venus in giant airships that would float high in the atmosphere.