Venus

If one does not insist to go down to the solid surface (500°C 90bar)
Venus is actually a nice place for humans to colonize (52.5km 37°C 660mbar).
And it will be rather easy with nanofactories since Venus' atmosphere is is essentially an ocean of building material bathed in intense sunlight.

Breathable air and nitrogen are effective lifting gasses in the dense carbon dioxide atmosphere. Comparison of molecular weights: nitrogen 28, oxygen 32, carbon dioxide 44

Atmosphere

The atmosphere is not your foe its your friend. She ..

• .. provides building material in optimal standardized form
• .. makes the scarce hydrogen better available (sulfuric acid rain is a natural hydrogen concentrator process)
• .. provides radiation protection (except UV)
• .. provides protection against micrometeorites
• .. makes street infrastructure unnecessary
• .. provides an environment with nearly constant temperature
• .. to a degree protects from volcanism on the ground

Colonisation - (conceptual)

The objection is to create a nice place for humans to live.

Basic housing

First a nanofactory (e.g. of the size of a sugar cube) is sent to Venus. There a durable balloon is created with a semitransparent diamond solar foil on top that leaves through enough light for plants to grow. The balloon further needs an "atmospheric converter unit" (air using micro ships) that has a number of functions. It creates among other thing breathable air. The balloon must be inflated wile being built to keep afloat.

Creation of soil for plants

Creating earth like soil with humic substances such that plants can grow in a natural way takes a lot longer then the employment of such a balloon. One could start with hydroponic cultures and compose the dead plants. At that time humans may be present or may not. A small piece of earth soil may be usable to introduce a rich set of microorganisms.

It should be rather easy to design small balloons but to create an earth like landscape a bigger free area and some soil depth is probably desired. For an average soil depth of half a meter a balloon with around one kilometer height is needed to compensate for the weight. (Put that in relation to the floating height of ~ 53km for visualization)

At this size one needs to consider the wind speed gradient in the atmosphere which is around 10m/s per 1km. One doesn't want the balloon to start rolling like a barrel. This may be a difficult problem.

Atmospheric converter unit

• filters nitrogen from the atmosphere
• captures sulfuric acid rain which concentrates the rare hydrogen [Todo: at which heights is sulfuric acid rain present]
• sulfuric acid → hydrogen + sulfur dioxide
• carbon dioxide + hydrogen -> ethyne + oxygen

Because of the reproduction hexagon it may make sense to keep it separate from the nanofactory.

Possible threats

Lightning

Some kind of lightning arrester system needs to be devised.

Wing gusts (danger of toppling over)

Since there are no obstacles high up in the atmosphere on a small scale differences in relative airspeed should be negligible. On a bigger scale this might become an issue [data needed].

Fires

Building a thin walled carbon balloon filled with oxygen is basically asking for fire. To mend this problem one can compartmentalize bigger balloons. Only the bottom few meters get filled with breathable air. A transparent ceiling foil material separates off the majority of the balloons volume. This part gets filled with nitrogen and is uninhabited "empty" space.

An other approach is to use silicon carbide as a building material which may self protect against fire by building glass. For silicon one would need to mine the surface though. Releasing excess oxygen to the atmosphere might get dangerous after a very long period colonization activity (more than centuries). A global firestorm could start making Venus rivaling/exceeding? the sun in brightness for a brief moment (this is some phantastic dystopic SciFi just for entertainment). To get rid of the excess oxygen from the silicates one can use iron as reducing agent. The place where one can get unoxidized iron for sure is the planets core. (See: deep drilling)

A closed material "cycle" can be conceived that protects against fire even if atmosphere gets really crowded.

• carbon dioxide + silicate stone → silicon carbide + oxygen
• oxygen + iron → iron oxides
• sulfur dioxide + iron → pyrite + iron oxides
• sulfuric acid + oxygen → hydrogen + sulfur dioxide