At approximately 50 Kilometres altitude in the Venusian atmosphere, the temperature and pressure are similar to those at sea-level on Earth. Together with a gravity field that is approximately 90 percent the strength of Earth's gravity, these are the conditions most favourable to human life to be found anywhere in the Solar System apart from on Earth itself. Although the atmosphere is about 96.5 percent carbon dioxide and 3.5 percent nitrogen and hence not breathable, and although it contains sulphuric acid droplets which necessitates some form of protective clothing, personnel can still operate in such conditions without being encumbered by pressure suits, which will prove useful for the construction and maintenance of the type of floating habitats that are planned for Venus as part of its colonization program.
These structures will float at about 50 Km altitude where an internal Earth-like atmosphere of oxygen and nitrogen will give massive lifting power. The overall structure and general parameters of the Venusian atmosphere are known to a reasonable extent, however comprehensive data sets detailing conditions at 50 Km and other altitudes are not yet available. The amount and type of turbulence at different altitudes and latitudes is not precisely known, although all evidence to date suggests that at 50 Km altitude, it should normally be well within the limits of what a reasonably strong floating habitat can cope with. Other issues such as the frequency and strength of lightning in the upper atmosphere and the possibility of volcanic eruption on the surface causing violent atmospheric disturbances will have to be understood before the requirements of a safe design of floating Venusian habitat can be specified.
IST takes the view that despite the limited amount of detailed information about the Venusian atmosphere, it is highly likely that such habitats can be safely operated close to the 50 Km altitude zone. In general the Venusian climate model is simpler than that of Earth which has a complex interaction between atmosphere, land and sea, and a high rate of planetary rotation and inclination. Venus on the other hand only rotates once every 243 days and has an inclination of less than 4 degrees. Without any oceans, and having complete cloud cover at all times over the entire planet, the weather patterns on Venus are much more homogeneous and stable than on Earth. In particular the temperature gradient or lapse rate of the atmosphere is more consistent than on Earth. This prevents the type of convective turbulence that characterises much of the weather on Earth. Turbulence in the Venusian atmosphere around 50 Km altitude is probably more the result of wind shear. However, there are significant fluctuations in observed wind speeds and temperatures at various points in the Venusian atmosphere. Also, Venus lacks a significant magnetosphere and together with the size of its atmosphere and its closer proximity to the Sun, it has a complex relationship with the solar wind, which may be the source of the energy that drives the super-rotation of the Venusian atmosphere.
The upper Venusian atmosphere as a whole rotates around the planet approximately once every four Earth days, and gradually spirals into large vortices at the poles. This super-rotation is characterised at latitudes from the equator to about 45 degrees north or south by wind speeds of about 100 m/s at the cloud tops and somewhat slower speeds of about 50 m/s in the 50 Km altitude region. Floating habitats would generally drift with this flow. Possible gravity wave effects and some cyclic variation of wind speeds in these regions have been observed, but the exact nature of turbulence is not known. From an engineering point of view, an accurate estimate of the most violent turbulent effects than can be expected every hundred years or so is important to determine a safe level of strength in the structure. Unfortunately this cannot currently be estimated to any significant level of accuracy.
Meridional wind strength, i.e. the strength of the wind component that points in a poleward direction, will be crucial to the design of floating Venusian habitats. At the altitude of the cloud tops, this is though to be almost zero at the equator, gradually increasing to about 10 m/s at 50 degrees North or South, then gradually decreasing to zero again at the poles. The corresponding velocities at about 50 Km altitude are thought to be less, but have not been measured accurately. A gradual change from zero to 5 m/s and back to zero again is a reasonably conservative estimate of meridional wind speeds at that altitude. On that basis it would appear that an equatorial location would be ideal for floating habitats. However, it is not known if other factors such as turbulence and lightning are more or less of a hazard just on the equator. In any case a habitat which drifts out of position will have to be able to return to the correct latitude. This would probably required a sustainable speed capability of about 10 knots. This may not sound like much but will require a substantial propulsion system for a craft weighing thousands of tons, or powerful tug airships could pull any habitat craft requiring a major trajectory change.
A required preliminary phase of Venusian colonization is then a detailed exploration of the Venusian atmosphere, in particular, the nature of wind patterns and turbulence at the 50 Km level and above. This could be carried out by autonomous airships, constructed entirely on the Moon and using lunar oxygen as most of their fuel to launch towards Venus. Once in Venusian orbit they would aerobrake down to a slow velocity before inflating their lifting envelope. It is possible that using hydrogen as a lifting gas on Venus would allow the use of semi-rigid or even lightweight rigid airship designs which could operate for several months and return highly detailed knowledge of Venusian wind patterns over time. These vessels could operate at various altitudes but it is not intended that any such craft go deep into the Venusian atmosphere. The temperature at 25 Km altitude is about 200 Celsius, which would already require heavy, robust craft that would be too bulky to send via interplanetary transport. Remote measurements can still be made to estimate conditions in the Venusian troposphere, and on the surface of Venus, and more detailed radar mapping of the surface could be carried out, but this would not be the priority of this phase of Venusian exploration.
The surface and the crust of Venus is potentially a very rich source of silicate minerals which will be mined one day using specially designed heavy industrial drag-line type equipment to scoop up rocky material. From the earliest stages of Venusian colonization, the atmosphere itself will be a rich source of materials. It is rich in volatiles like carbon, oxygen, nitrogen and many other substances which can be extracted by fractional distillation and other fairly easy chemical processes. Many elements which are not found in significant amounts in the Venusian atmosphere are found in the Venusian crust, but these will have to be imported from Earth initially, or in many cases from the Moon where they are also plentiful in the Lunar crust.
The preliminary phase of Venusian colonization will also require autonomous craft in orbit both to provide local and interplanetary communications, and to carry out global surveying missions. If this phase of Venusian exploration can be summed up in one word it is "data". This highly detailed data about Venus and its specific weather processes in certain regions of its atmosphere, will allow the first phase of Venusian colonization to be planned with confidence.
There is also the possibility of colonising Mercury which is in may respects very similar to the Moon. It may appear that Mercury could act as Venus's "Moon" trading metals in return for hydrogen and volatiles, however, not only are conditions very difficult on Mercury due to its proximity to the Sun, it also has a very high orbital velocity, which means a direct transfer between Mercury and Venus would be very expensive. This could be reduced somewhat by using solar sails and other low thrust technology. The polar regions on the surface of Mercury are permanently at low temperature and are thought to contain large amounts of ice. Bases and mining operations could be started there and if enough infrastructure is put in place, mining operations could even be safely carried out underground at equatorial regions during the daylight phase. There would still be many dangers in the case of system failure, for example an underground fire, personnel could not evacuate even temporarily to the surface which could be as hot as 700 Kelvin. Mercury is probably even richer than the Moon in terms of the amount of valuable minerals in its crust, but accessing it is currently too difficult to be included even in a phase 1 of Venusian colonization. It will be included in phase 2 where experiments with flying solar sails close to the sun and generating high powered propulsion beams will be carried out along with mining operations.
In summary, a preliminary surveying phase of Venus will be carried out with craft flying in the Venusian atmosphere and in orbit around Venus. This will take place after the establishment of a basic industrial base in LEO, and during phase 1 of Lunar Colonization. Phase 1 of Venusian Colonization will begin once phase 1 of Lunar Colonization is complete. This will require the construction in Venusian orbit of 3 rotating space stations with a total capacity of at least 1000 personnel. There will also be constructed in the Venusian atmosphere 3 floating habitats with the same capacity. These floating structures will supply the Venusian population with most of its basic materials by extracting them from the Venusian atmosphere. The development of technology to facilitate the collection and extraction of silicate metallic minerals from the planetary surface and crust will be a priority in phase 2 of Venusian colonization, however during phase 1 these materials will have to be brought from Earth or the Moon, which will be a major but manageable expense.
During construction of the floating atmospheric habitats, most personnel will be based in the orbiting space stations around Venus. As the atmospheric habitats are developed, the personnel will gradually become based on them, with the space stations being operated by maintenance crews to provide a vital place of refuge should it be necessary to evacuate the atmospheric habitats for any reason, and to provide a hub for transfer flights between Venus and Earth.
It will be necessary to have in place during phase 1 of Venus colonization, a pipeline into space similar to that developed on Earth using High Altitude Platform airships and suborbital rendezvous and payload transfer. This will operate at about 100 Km altitude in the Venusian atmosphere and transfer payloads of between 1 and 2 tons to and from Low Venetian Orbit. Transfer of goods to and from the High Altitude Platforms and the floating habitats at 50 Km altitude, will be accomplished using airships designed to travel through 20 Km thick sulphuric acid rain cloud deck of the Venusian atmosphere, which begins at about 60 Km altitude. This may be a stormy region with frequent lighting discharges, but to what extent is not yet known with any great confidence.
It will be possible to transfer the floating habitat personnel to and from space using the High Altitude Platform and rendezvous method. However, the majority of such personnel transfers will be made using a two stage space plane that is lifted by airship from the 50 Km zone, through the cloud deck to about 80 Km before initiating the first stage launch. The first stage will be strong enough to use aerobrake from about 2 to 3 Km/s orbital velocity back to normal subsonic flight and to descend through the cloud deck. At lower levels it could either deploy its own dirigible flotation structure or land on a suitable floating carrier structure. The second stage will fly to orbit and for the return journey it would make use of a deorbit stage supplied with fuel from the pipeline infrastructure. This would reduce the second stage orbital velocity to virtually zero prior to reentry, eliminating the high fatigue levels associated with reentry at full orbital velocity. Similarly to the first stage the second stage would have to be able to descend through the cloud levels and either deploy its own buoyancy system or effect a landing on a carrier structure.
Safety of personnel will have to take into account the fact that there is no safe ground level onto which an emergency landing can be made in the event of a space plane or other craft either losing buoyancy or failing to land on a carrier craft. Any craft or personnel who fall from the habitable 50 Km zone will surely perish as they descend into the intense heat and pressure of the Venusian troposphere. To prevent such occurrences, all craft involved in personnel transfer will have a jettisonable personnel pod with its own buoyancy system. Should that fail personnel will be able to egress into buoyant survival capsules, and should they fail each person will have their own personal buoyancy system, i.e. a large inflatable helium or hydrogen balloon to which they are suspended by a harness. This maybe a somewhat daunting prospect to be floating in such a precarious fashion in the Venusian atmosphere, but rescue craft will be available in plentiful numbers to retrieve such unfortunate personnel in a short period of time, hopefully within minutes.