Quote of the Day
General, would you rather command an army of slaves?
— Milton Friedman responding to General William Westmoreland, who had said that commanding a volunteer army would be like commanding an army of mercenaries.
I have been keeping a close eye on the discussions occurring about sending people to Mars on both one-way and two-way trips. You do not hear similar discussions about Venus because its surface temperature (467 °C) and pressure (93 bar) are too extreme to imagine people surviving there.
Interestingly, 50 km above the Venusian surface, the temperature and pressure are comparable to that of the Earth's surface. In fact, one Venus researcher (Geoffrey A. Landis) commented that "At cloud-top level, Venus is the paradise planet.” But how could people live in the clouds? Balloons.
While balloon-based cities may seem like pure science fiction, there are credible proposals for sending explorers to bases borne by balloons in Venus' atmosphere. In this post, I will look at what how a balloon filled with Earth air could be used as a floating residence and laboratory. I will also look at the characteristics of two balloons that the Russians actually have floated in atmosphere of Venus.
- Archimedes principles
- Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object (Archimedes).
- Lifting gas
- Lifting gases are gases that can be used in lighter-than-air aircraft because they have average densities lower than that of air; thus, they are buoyant in air.
- Superpressure Balloon
- A superpressure balloon is a style of aerostatic balloon where the volume of the balloon is kept relatively constant in the face of changes in the temperature of the contained lifting gas. This allows the balloon to keep a stable altitude for long periods. This is in contrast with the much more common variable-volume balloons, which are either only partially filled with lifting gas, or made with more elastic materials (Source).
Atmosphere of Venus
Figure 2 shows how atmospheric pressure and temperature vary with altitude on Venus. At an altitude of ~50 km, the air pressure is ~1 bar and the temperature is ~0°C. These conditions are reasonable for human survival – we just need a surface to live on.
The following quote does the best job I have seen of explaining how one of these balloon living quarters would work.
Landis has proposed aerostat[airship] habitats followed by floating cities, based on the concept that breathable air (21:79 oxygen/nitrogen mixture) is a lifting gas in the dense carbon dioxide atmosphere, with over 60% of the lifting power that helium has on Earth. In effect, a balloon full of human-breathable air would sustain itself and extra weight (such as a colony) in midair. At an altitude of 50 kilometres (31 mi) above Venerian surface, the environment is the most Earth-like in the solar system – a pressure of approximately 1 bar and temperatures in the 0 °C–50 °C range. Protection against cosmic radiation would be provided by the atmosphere above, with shielding mass equivalent to Earth's.
Here is a good NASA video on an advanced Venus balloon concept.
In 1985, the Russians successfully deployed two balloons, at 54 km altitude in the atmosphere of Venus. The balloon-based probes returned data for more than 46 hours. So we know that a balloon can be successfully deployed and floated in the Venusian atmosphere.
Why an Air-Filled Balloon?
I have read about concepts for both air and hydrogen-filled balloons. The advantage of an air balloon is that people could actually live inside of a 1 atmosphere-pressure balloon. The disadvantage of the air-filled balloon is that it must be much larger than a hydrogen-filled balloon with the same lift.
Equation 1 shows the formula for the buoyant force per unit volume of balloon's lifting gas. In this calculation, I assume that the atmosphere of Venus is pure CO2 (close to true) and the balloon contains Earth air.
- FB is the buoyant force per unit volume.
- ρAtmosphere is density of the air surrounding the balloon.
- ρBalloon is density of the gas in the balloon.
- gPlanet is the acceleration due to gravity on the planet with the balloon.
Air Balloon Lift on Venus
Figure 3 shows how to compute the lift force per unit volume of an air balloon on Venus.
Russian Balloon Characteristics
The Russians have deployed two Venus balloons for which I have located the following information on the Wikipedia:
- Balloon diameter: 3.54 m
- Total mass:21 kg
- Lifting gas: He
- Atmospheric pressure when floating: 535 mbar
- Temperature when floating: 300K
Given this information, we can approximately calculate the lifting force generated by these balloons (Figure 4). I should mention that this exercise would be a good one for a high-school chemistry or physics class that is learning about the ideal gas law.
The most surprising part of this exercise was seeing that there is a place in our solar system besides Earth that has some physical characteristics similar to here – 1 atmosphere pressure, ~20 °C temperature, and protection from space radiation. On the downside, the Venusian atmosphere is filled with corrosive chemicals.
I am not sure of the value in having people floating around Venus, but I could see the concept being very interesting for a robot explorer. I should mention that the Europeans have discussed attempting a Venus balloon mission (see Figure 5).
I just saw on Quora an artist's concept for a cloud city on Venus (Figure 6). The figure dates from 1971.