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2 Environment Analysis of Lunar Soft Landing Exploration
2.3 Lunar Thermal Environment
The natural thermal environments that the lunar lander encounters in lunar orbit or
on lunar surface are the solar radiation, the lunar reflection and the lunar radiation
[8].
2.3.1 Solar Radiation
Solar radiation is the primary heat source of the lander in operation. It is mainly
comprised of the visible light, the infrared light and the X ray of the electromagnetic
radiation of the Sun. The relative distance and angle between the Sun and the Earth
vary according to different position of Earth on the ecliptic plane. Therefore, the
solar radiation energy that reaches Earth varies with the distance from Earth to the
Sun when Earth is at different position on the ecliptic plane.
The direct solar radiation is generally expressed in solar constant S. The solar
constant is a flux density in the unit of W/m
2 measuring mean solar electromagnetic
radiation (solar irradiance) per unit area. It is measured on a surface perpendicular
to the rays, one astronomical unit (average distance from the Sun to Earth is 1AU,
which is about 1.5 × 10
8 km) from the Sun.
For the lander on lunar surface, since the Moon may be at the closest or furthest
point of the Earth-Sun line, the distance between the Sun and the Moon is ranging
between 1AU − 3.8 × 10
5 km and 1AU + 3.8 × 10
5 km. Therefore, the average
distance between the Moon and the Sun is approximately 1AU in analyzing the
solar radiation energy. Meanwhile, since the lunar atmosphere is extremely thin, the
attenuation of solar radiation energy by lunar atmosphere can be ignored, similar as
Earth spacecraft beyond Earth atmosphere.
Therefore, the value of solar radiation energy for the lunar lander is the same as it
for the Earth spacecraft that are beyond Earth atmosphere. The solar radiation energy
at 1AU is denoted by 1S and the value is 1367 W/m
2 . S varies in different seasons.
It reaches its maximum of 1414 W/m
2 in winter solstice (perihelion) and reaches its
minimum of 1322 W/m
2 in summer solstice (aphelion).
2.3.2 Lunar Albedo
For the solar radiation that reaches the Moon, part of it is absorbed and the other part
is reflected. The reflected part is called lunar albedo. The albedo of solar radiation
from the lunar surface is another important thermal environment for the lunar lander
in lunar orbit or on lunar surface. The albedo of Moon is closely related to the thermal
property (absorption and reflection) of lunar substance (such as lunar soil and lunar
rock). The authoritative lunar albedo is documented in the design criterion SP-8023
2 Environment Analysis of Lunar Soft Landing Exploration
2.3 Lunar Thermal Environment
The natural thermal environments that the lunar lander encounters in lunar orbit or
on lunar surface are the solar radiation, the lunar reflection and the lunar radiation
[8].
2.3.1 Solar Radiation
Solar radiation is the primary heat source of the lander in operation. It is mainly
comprised of the visible light, the infrared light and the X ray of the electromagnetic
radiation of the Sun. The relative distance and angle between the Sun and the Earth
vary according to different position of Earth on the ecliptic plane. Therefore, the
solar radiation energy that reaches Earth varies with the distance from Earth to the
Sun when Earth is at different position on the ecliptic plane.
The direct solar radiation is generally expressed in solar constant S. The solar
constant is a flux density in the unit of W/m
2 measuring mean solar electromagnetic
radiation (solar irradiance) per unit area. It is measured on a surface perpendicular
to the rays, one astronomical unit (average distance from the Sun to Earth is 1AU,
which is about 1.5 × 10
8 km) from the Sun.
For the lander on lunar surface, since the Moon may be at the closest or furthest
point of the Earth-Sun line, the distance between the Sun and the Moon is ranging
between 1AU − 3.8 × 10
5 km and 1AU + 3.8 × 10
5 km. Therefore, the average
distance between the Moon and the Sun is approximately 1AU in analyzing the
solar radiation energy. Meanwhile, since the lunar atmosphere is extremely thin, the
attenuation of solar radiation energy by lunar atmosphere can be ignored, similar as
Earth spacecraft beyond Earth atmosphere.
Therefore, the value of solar radiation energy for the lunar lander is the same as it
for the Earth spacecraft that are beyond Earth atmosphere. The solar radiation energy
at 1AU is denoted by 1S and the value is 1367 W/m
2 . S varies in different seasons.
It reaches its maximum of 1414 W/m
2 in winter solstice (perihelion) and reaches its
minimum of 1322 W/m
2 in summer solstice (aphelion).
2.3.2 Lunar Albedo
For the solar radiation that reaches the Moon, part of it is absorbed and the other part
is reflected. The reflected part is called lunar albedo. The albedo of solar radiation
from the lunar surface is another important thermal environment for the lunar lander
in lunar orbit or on lunar surface. The albedo of Moon is closely related to the thermal
property (absorption and reflection) of lunar substance (such as lunar soil and lunar
rock). The authoritative lunar albedo is documented in the design criterion SP-8023