Source:
10-20-2007 11:14
Chief Commander for Chang'e Program, Luan Enjie
Orbit selection is a very important part of technical content in Chang’e Program. This is the first time for us to make a voyage of 400,000 kilometers.
Orbit selection should meet the following requirements:
To meet the requirements of the scientific objectives: For observing the global lunar surface to the full extent, the lunar polar orbit is selected.
To meet the requirement of acquiring data with the same resolution: The circular low lunar orbit is adopted.
To meet the requirement of high image resolution: The orbit is kept within 100-200km.
To meet the requirement of one-year lifetime: Considering the abnormality of the lunar gravity field, Chang’e-1 may fall on the lunar surface within half a year if a 100km orbit is selected, so a 200km orbit is selected for Chang’e-1 if it does not conduct special probe for lunar gravity field.
To meet the requirement of lunar coverage: This is easy to be realized, because the Moon has a long rotation period, and the adjacent orbital spacing at the equator
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To meet the conditions for illumination: During the flight orbiting the Moon for one year, it can not meet the requirements by only relying on the single-axis rotation of the solar panels, so the satellite is designed to be capable of 90º rotation by adopting two orthogonal flight attitudes.
The included angle between the lunar equator and ecliptic is about 1.5º, so they are nearly parallel.
In the sun, the shadow angle for the satellite is:
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Therefore, during one orbital period, the maximum orbital shadow time is 45min, and the illuminating time is 127-45=82min.
(45min, 82min) occurs when the included angle between the sun and the orbital plane is 0º or 180º.
The short period lunar orbit has a little change, but the long period lunar orbit has a large change because of the complicated lunar gravity field. A circular orbit with an initial altitude of 200km may have an orbit decay of about 100km in a year, so it is very important to keep the orbit altitude of the lunar satellite. The orbit of Chang’e-1 will be adjusted once every two months or so.
A highly elliptical orbit with an apogee of 380000 km is selected to realize the translunar flight. Chang’e-1 will have a perigee of 600km and gradually increase its perigee velocity so as to raise its apogee altitude.
In the current concept, three accelerations will make Chang’e-1’s apogee reach about 51,000km, 71,000km and 120,000km respectively with their corresponding orbital periods 16h, 24h and 48h, which are beneficial to the operation of the ground TT&C system and can realize the conditional orbital waiting.
Lunar acquisition is the most essential point in the orbit design in Chang’e Program. If lunar acquisition is not designed perfectly, the satellite will fly by or impact the Moon, which may lead to grave consequence. With the support of the TT&C system, Chang’e-1 satellite will conduct three decelerations to make its orbital period change from 12h and 3.5h to 127min of a circular polar orbit around the Moon (See the following Figure).
Editor:Liu Fang
