3.3 Mission Analysis
55
(3) Telecommunication condition. Because the rotation period was almost same
as the period of revolution of the Moon around the Earth, the near side of the
Moon is always in front of the Earth while the far side of the Moon could not
be seen on the Earth. If the landing site was on the near side of the Moon, the
telecommunication condition to the Earth was good. All lunar landers were
landed on the near side of the Moon up to now (before 2017).
The telecommunication condition during powered descent should be considered besides the condition on lunar surface. During powered descent, the height
and velocity would be change greatly, and there would be a lot of operation
of sensors and thrusters of lunar lander. The risk of powered descent was the
highest one of the flight profile of lunar lander. Just before powered descent
process of lunar lander was initiated, a lot of command and data should be sent
to lunar lander by ground control, which meant that the ground station should
be able to access lunar lander at that time. For the safety of powered descent, it
would be better if the ground station could access lunar lander during powered
descent in case of possible malfunction for the ground operators to try possible
operation to save lunar lander.
(4) Sun light condition. There would be adequate sun light during lunar daytime on
lunar surface, so the solar cells were able to provide energy source for operation.
The inclination of the Moon equator to the ecliptic is only about 1.5°, so the
sun light condition is the best on the Moon equator. The sun elevation is getting
smaller when the latitude is getting higher on lunar surface. For the landing
site selection, the variation pattern of the sun elevation and azimuth should
be studied for the design of the size and the orientation of the solar panel.
Generally the landing time should be on the lunar day to get enough energy
supply for the landing site. At the same time, the effects of the sun elevation
on the optical sensors should be considered, because it was hard for the optical
sensor to obtain good images when the sun elevation was too small or too big.
(5) Thermal environment. There is nearly no atmosphere around the Moon. So
the temperature on lunar surface is up to the solar radiation. When it is lunar
daytime, the difference of the temperature on the equator and polar region of
the Moon is relative big. The highest temperature on higher latitude is low
and it is higher when the latitude is getting to zero. When it is lunar night,
the difference of the temperature on the equator and polar region of the Moon
is relative small. The heat flux on different location of the Moon should be
considered when the thermal control of lunar lander was designed.
The effects of all above constraints on the system design of lunar lander should
be analyzed and weighed. The optimum selection of the landing site should be
determined after comprehensive evaluation.
2. Scientific requirements
The principles for landing site selection could be summarized as three aspects for
scientific exploration.
55
(3) Telecommunication condition. Because the rotation period was almost same
as the period of revolution of the Moon around the Earth, the near side of the
Moon is always in front of the Earth while the far side of the Moon could not
be seen on the Earth. If the landing site was on the near side of the Moon, the
telecommunication condition to the Earth was good. All lunar landers were
landed on the near side of the Moon up to now (before 2017).
The telecommunication condition during powered descent should be considered besides the condition on lunar surface. During powered descent, the height
and velocity would be change greatly, and there would be a lot of operation
of sensors and thrusters of lunar lander. The risk of powered descent was the
highest one of the flight profile of lunar lander. Just before powered descent
process of lunar lander was initiated, a lot of command and data should be sent
to lunar lander by ground control, which meant that the ground station should
be able to access lunar lander at that time. For the safety of powered descent, it
would be better if the ground station could access lunar lander during powered
descent in case of possible malfunction for the ground operators to try possible
operation to save lunar lander.
(4) Sun light condition. There would be adequate sun light during lunar daytime on
lunar surface, so the solar cells were able to provide energy source for operation.
The inclination of the Moon equator to the ecliptic is only about 1.5°, so the
sun light condition is the best on the Moon equator. The sun elevation is getting
smaller when the latitude is getting higher on lunar surface. For the landing
site selection, the variation pattern of the sun elevation and azimuth should
be studied for the design of the size and the orientation of the solar panel.
Generally the landing time should be on the lunar day to get enough energy
supply for the landing site. At the same time, the effects of the sun elevation
on the optical sensors should be considered, because it was hard for the optical
sensor to obtain good images when the sun elevation was too small or too big.
(5) Thermal environment. There is nearly no atmosphere around the Moon. So
the temperature on lunar surface is up to the solar radiation. When it is lunar
daytime, the difference of the temperature on the equator and polar region of
the Moon is relative big. The highest temperature on higher latitude is low
and it is higher when the latitude is getting to zero. When it is lunar night,
the difference of the temperature on the equator and polar region of the Moon
is relative small. The heat flux on different location of the Moon should be
considered when the thermal control of lunar lander was designed.
The effects of all above constraints on the system design of lunar lander should
be analyzed and weighed. The optimum selection of the landing site should be
determined after comprehensive evaluation.
2. Scientific requirements
The principles for landing site selection could be summarized as three aspects for
scientific exploration.