Leaving aside the Moon and the Sun, Venus is the brightest object in the sky, and at times the closest. Further, Venus is the only planet that is comparable to Earth; its mass is about 81.5%, its size is about 95%, and its gravity is about 90.5% that of Earth. The orbit of Venus has only 1/3 of Earth’s eccentricity, and while Earth has an axial tilt of 23.5 degrees (which results in right now I am embedded in winter and many of the readers will be enjoying a pleasant summer, or maybe a heat wave) Venus has a tilt of only 2.6 degrees. That means that Venus has a more or less uniform temperature and no seasons. At first sight, that would make it an attractive target for space probes, but while NASA has sent eleven orbiters and eight landers to Mars, it has only sent two orbiters to Venus. Why the lack? Not for lack of interest since from 1990 NASA has considered nearly thirty proposals, but it approved none. The dead hand of the committee strikes again. The reason is that Venus, up close, is strangely unattractive.
The first problem is atmospheric pressure, which is about 90 bar over most of the planet, and it has an average surface temperature of about 460 degrees C but this can vary by +160 degrees C. The second problem is the nature of the atmosphere. Most of it is carbon dioxide. Venus also has about four times the amount of nitrogen than Earth has, and all of that is relatively harmless. What is less harmless is the atmosphere has clouds of sulphuric acid, together with hydrogen chloride and hydrogen fluoride. Hydrogen fluoride is particularly nasty, because it reacts with glass, and while the sulphuric acid will attack all the basic electronics, etc, the hydrogen fluoride will attack lenses. Very shortly, photography, or seeing where a rover is going, will no longer be possible. And, of course, if it can survive, the heat soon kills it. The first lander to return data was Venera 7, a 1970 Soviet lander that survived for 23 minutes. In 1975, Venera 9 sent back the first pictures from the surface, but it too did not last very long. Funding committees do not encourage very expensive rovers with a very short life.
This may change. NASA is designing a “station” that should last at least sixty days, and operate at the ambient temperature. The electronics would be made of silicon carbide, a substance that conducts electricity and melts somewhere above 2,800 degrees C. No danger of that melting, although all the metals in the craft would have to be resistant to the ambient heat and the corrosion. Titanium would probably manage reasonably well. So maybe we shall get to know more about the planet.
There have apparently been proposals to “colonise” Venus through “settlements” floating above the cloud levels, i.e.presumably some ship-like structure supported by gigantic balloons. Personally, I feel this is unreal. The total weight must displace an equal weight of gas, and the idea is to get above the clouds. Up there, the gas is nowhere near as dense (the pressure is only about half that 90 bar at the top of the highest mountain) and to go higher the pressure really drops away. So to support sufficient mass you would need very large balloons, made of what? Any fabric or rubber would be broken down by the solar UV at that height. Metals would corrode. And what would the gas be? The obvious ones would be hydrogen and helium (no danger of fire because there is no air) but these gases leak like crazy. You may think you can hold it, but for centuries? Then there is another minor problem: at the top of the atmosphere winds can reach several hundred kilometres per hour.
So what is “wrong” with Venus, from our point of view? There are two things. The first is the very slow rotation, which happens to be retrograde. The direction is not so much a problem, but the slowness is. However, the main one is, no significant water. If Venus had the amount of water Earth has, it would have fixed all that carbon dioxide as limestone or dolomite, in which case the atmospheric pressure would be about 3 times our atmosphere (because it has four times the amount of nitrogen). If we wanted to have breathable air, we would have to add another atmosphere of oxygen.
So in theory we could terraform Venus. At the expense of much energy what we would have to do is bring in a number of Kuiper Belt objects, or maybe cometary material from around Jupiter would be better because they contain much less additional nitrogen and carbon monoxide, and make them hit Venus, preferably on the side in a way that the angular momentum of the incoming object was added to the current Venusian rotation, in other words, spin it up. Give it water, and chemistry would do the rest, although it would probably also be preferable to cool it by shading it from the sun at least to some extent. Yes, the temperatures would still be high, but as long as it can cool to 300 degrees C, the pressure will ensure there is some liquid, and the fixing of the gas will start, and initiate positive feedback
Suppose we could give Venus as much water as Earth, then the planet would be more like a water world. It is an interesting question whether Venus has any felsic/granitic material. This is the stuff that makes continents. The great bulk of the material on any rocky planet is basaltic, which in turn is because the oxides of silicon, magnesium and iron are the most commonly available rock-forming materials. Aluminium, as an element, is over an order of magnitude less common than silicon, which it replaces in aluminosilicates. Being less dense than basalt, granite floats on the basalt, provided it can separate itself from the basalt. In my ebook “Planetary Formation and Biogenesis”, I propose that the separation essentially has to take place prior to and during planetary formation. Venus does have two minicontinents: Ishtar and Aphrodite Terrae.
The actual differentiation of the planet, when the granite moves from the deep and comes out on the surface occurs slowly (the small amounts of plagioclase on Mars apparently took about two billion years.) and the rate probably depends on the amount actually accreted. The evidence is that on Earth very large amounts erupted in massive pulses. In the absence of such granite, a large planet will be rather flat, apart from some volcanic peaks.
There would still be a problem in that Venus has no plate tectonics. They are needed to provide the recycling of carbon dioxide, as eventually if the lot were fixed, any life would presumably die. We don’t know what starts plate tectonics. One possibility is the presence of granitic continents, another is the forces arising from rotational motion. It is just possible they could start if there were more rotational motion, but we don’t know. All in all, not an attractive planet in detail, so maybe we should look after our own better.