User:GabrielVelasquez/Planetary human habitability

Human Habitability Environmental Requirements and Tolerances

or Human Extrasolar Habitability

"HUMAN HABITABILITY"

Few studies have been done on a comprehensive level specifically covering reasonable quantitative limits of environmental conditions tolerable for humans to live in, let alone survive. This task was fully undertaken by Dole and Asimov (1963) and they defined a "habitable planet" as "one on which large numbers of people can live comfortably and enjoyably without needing unreasonable protection from the natural environment and without dependance on materials brought in from other planets." They added that in discussing habitability the planet in question must be at least 10% habitable. In their book they went on to list more specific parameters for said habitability ( see below) Nasa has since started programs to introduce comprehention of human planetary habitability in class rooms.

NASA Habitability and Human Factors Branch
The Habitability and Human Factors Branch is responsible for ensuring that space human factors, including human physical parameters and performance capabilities and limitations, are defined, documented.

Temperature
In their scientific book "Planets for Man" (1963), Stephen H. Dole and Isaac Asimov defined a habitable temperature region as one where the mean annual temperature is between 0°C (32°F) and 30°C (86°F), where the highest mean daily temperature in the warmest season is lower than 40°C (104°F) and the lowest mean daily temperatue of the coldest season is higher than &minus;10°C (14°F). As an example of habitability the planet Earth's temperature extreme records are:
 * The hottest temperature measured was 71 °C (159 °F) in Lut desert of Iran in 2004 and 2005.


 * The coldest temperature measured was &minus;89.2°C (&minus;128.5°F), recorded on 21 July 1983 in Vostok, a Russian Antarctic research station located at the center of the East Antarctic Ice Sheet.

Extreme temperatures can have detrimental effects on people: If a location has extremely cold temperatures, people can die of hypothermia. In addition, people can die of dehydration in extremely hot temperatures.

Factors affecting Temperature
The main factors that affect a planet's surface temperature are: Direct Irradiance which is in turn dependant on Star type, orbit mechanics such as Axial tilt (which affects Seasons), Orbital eccentricity, Aphelion distance, Perihelion distance, and also Atmosphere composition and size.

Greenhouse gases combined with perihelion distance can have a very notable effect on even local temperatures, as is evidenced by the plus 15 degree Celsius temperature in Toronto on January 3rd 2008.

Sustenance
Important food crops require temperatures within the ranges of 10°C (50°F) and 30°C (86°F) during their growing season. Most fish don't survive temperatures below 0°C (32°F) or above 30°C (86°F)

Light
Human vision as well as photosynthesis in food plants relies on specific lightwave frequency and intensity ranges. Humans can cope visually in 10-9 to 50 Lumens. Plants bend toward light (reflected not direct)in the range of 380 nanometre to 760 nanometre, which happens also to be the frequency visible to humans, the visible spectrum.


 * Vision limits set by the human eye are 10E-13 lu/cm2 to 50 lu/cm^2


 * since the illumination needs of plants fall within the range of illumination limits for the human eye, and crops sustainability is a habitability factor the illumination requirements for habitability are 0.02 lu/cm2 to 30 lu/cm2


 * Period of rotation is directly linked to illumination requirements and temperature requirements; with the exception of smaller stars and planets with heating factors other than direct insolation, if a planet meets the habitable temperature requirements it will meet the habitable illumination requirements.


 * The very existence of free oxygen in the atmosphere depends on photosynthesis in plants or their indigenous equivalents.

Planet Atmosphere
For a planet to be considered habitiable the pressure of the atmosphere must be between specific ranges and people have to be able to breathe it. This means that the Partial pressure of breathable gases must not be too low or too high, and the partial pressure of toxic gases must also not be more than trace amounts.

Pressure

 * Pulmonary gas pressures
 * Hyperoxia
 * Hypoxia
 * The inhabitants of the mining settlement of Aucanquilcha, Chile work at the altitude of 19,000 feet where the inspired partial pressure of oxygen is 68 mm Hg.
 * Mountain climbers suggest that life can be carried out normally for indefinite periods at an inspired partial pressure of Oxygen at 53 mm Hg. This is also the lower limit of total pressure as well.
 * The upper limit of inspired partial pressure for Oxygen has been found experimentally to be 400 mm Hg

Composition

 * Human respiration requires oxygen and minor amounts of water vapor.
 * (See page 34 for gas ranges )


 * Greenhouse gases
 * Wind velocities
 * Dust suspended in air.

Planet mass

 * Gravity of a planet's surface is directly related to the planet's mass. This in turn can have ill effects on human beings. Astronauts have been known to suffer bone density loss due to extended stays in zero gravity. Dole and Asimov listed the maximum limit of habitable gravity is 1.5g
 * Magnetosphere

Planet Anomalies
In general for a planet to be considered habitable it must not be hostile to people and so it must not have excessively high rates/levels of Radioactivity, Vulcanism, electrical activity (ie. Lightening), frequency of Earthquakes, or frequency of Meteorite infall.

Uninhabited by inimical intelligent life
It was Asimov and Dole's idea not to discount the possibility of non-terrestrial life on other planets and the possibility that such life may be sentient yet not be willing to share their planet. Such a situation would cause a planet to be unsuitable for human habitation as unwanted residency would make for potentially serious conflict with the locals.

Radioactivity
The average natural background radiation on the Earth's surface is about 0.003 (REM) retrogen-equivalent man per week. Somewhat more has been tolerated by atomic energy workers; the Atomic Energy Commission specifies a tolerance level of 0.3 REM per week. This is too high for a global average, and a limit closer 0.02 REM per week or less for a planet to be considered habitable is more reasonable.

Star Attributes

 * Insolation is the amount of heat received by a planet from its parent star and is the main factor in judging the habitability of a planet. Liquid "water" depends on heat and may freeze to ice or boil away from a planet and destroy all life dependant on water.
 * Radiation from certain types of stars can render all other habitability factors meaningless as human beings can only survive certain doses.
 * Illumination was listed by Dole and Asimov as an essential factor because planets would not have breathable atmospheres without photosynthesizing plants.

Planet Orbit and Axial tilt
The Eccentricity of a planet will have affects on aphelion and perihelion distance, which in turn affect...
 * affects Seasonality
 * affects Cryosphere size
 * affects Desert size and location.
 * affects Hydrosphere size

Habitability classification scale
Based on parameters described above, an "habitability scale" can be set up, to classify new planets as they will be discovered.

A first distinction can be made among "habitable" and "liveable" planet. A planet can be defined as "liveable" if you can stay there for minutes or hour, with the aid of a few technology. An habitable planet, as specified above, is a planet where you can live indefinitely.

Liveability (?) parameters: Temperature, Gravity, Atmosphere

Habitability parameters: Seasons, Liquid water, magnetic field, dryland, animals and vegetables, night/day,...

We can try ordering these parameters by "dependency": if a planet has suitable atmosphere but 10 g gravity, it is not liveable, so atmosphere "depends" on gravity, and so on; so, the order could be:

Liveability (?) parameters:
 * L1 Temperature
 * L2 Atmosphere
 * L3 Gravity

Habitability parameters:
 * H1 Liquid water
 * H2 Magnetic field
 * H3 Dryland
 * H4 Animals and vegetables
 * H5 night/day
 * H6 Seasons

We could also collaps all parameters into a single scale, which starts from class A (=Earth), and goes down to less liveable/habitable planets by "subtracting" features to Earth:

Classes
 * A: Earth
 * B: Earth with no seasons
 * C: Earth with no seasons and no night/day cycle
 * D: B+C and no animals/vegetables
 * E: no dryland
 * F: no magnetic field
 * G: no liquid water
 * habitability limit
 * H: bad atmosphere (no oxygen; toxic gas; high/low pressure,...)
 * I: bad temperature (<-20, > 40 °C)
 * J: bad gravity (>1.5 g)

Currently better earth-like known planet is Gliese 581g, which should be Class H (until we discover more data).