smaller planet. This would be easier to adjust to than lunar gravity (0.16g), but still present advantages regarding lower escape velocity from the planet. Mercury's proximity gives it advantages over the asteroids and outer planets, and its low synodic period means that launch windows from Earth to Mercury are more frequent than those from Earth to Venus or Mars. On the downside, a Mercury colony would require significant shielding from radiation and solar flares, and since Mercury is airless, decompression and temperature extremes would be constant risks. Venus Surface conditions on Venus are extremely hostile to human life: average surface temperature is 464 °C (hot enough to melt lead), and average surface pressure is 92 times Earth's atmospheric pressure – roughly equivalent to a depth of one kilometre under Earth's oceans. (There is some variation; due to its altitude, the peak of Maxwell Montes is at only 380 °C and 45 bar, making it the coolest and least pressurised location on Venus' surface. There are also some hot spots at about 700 °C.) Solar energy is not available at the surface due to the constant cloud cover, and the carbon dioxide atmosphere is poisonous. However, the upper atmosphere of Venus has much more Earthlike conditions and has been suggested as a plausible colonization location since at least 1971 by Soviet scientists. At just over 50 km altitude (the cloud tops), atmospheric pressure is roughly equal to that on Earth's surface, and temperatures range from 0–50 °C. The volatile elements necessary for life are present (hydrogen, carbon, nitrogen, oxygen, and sulfur), and above the clouds, solar energy is abundant. Pressurization would not be required; humans could even go outside the habitats safely with oxygen provision and clothing to protect against the sulfuric acid droplets. Geoffrey Landis has pointed out that breathable air is a lifting gas in Venus' atmosphere: a cubic meter of air would lift half a kilogram, and an oxygen- and nitrogen-filled aerostat the size of a city on Venus would be able to lift the mass of a city. This suggests floating aerostat cities as a colonization method for Venus. The lack of pressure differences between the outside and inside means that there is ample time to repair habitat breaches. With just over three times the land area of Earth, there would be space even for a billion such cities. The atmosphere provides enough radiation shielding at this