The chang ‘e-4 lunar probe was launched with a long March 3b carrier rocket at the xichang satellite launch center at 2:23 am Beijing time on December 8, marking a new journey for China’s lunar exploration. The chang ‘e-4 mission will see China achieve two firsts: the first soft landing of a human probe on the far side of the moon; It is the first time to realize the relay communication of human spacecraft in earth-moon L2 point. At the same time, chang ‘e-4 is expected to achieve a number of major original scientific research results, and will accumulate important experience for civil-military integration and innovative development in the field of deep space exploration.
However, to explore the far side of the moon, it is necessary to have the magpie relay in orbit. When chang ‘e-4 reaches the far side of the moon, the relay satellite will be used to send instructions, relay signals, download data and other functions to link the earth and the moon. The relay satellite also carries a laser pointer reflector to verify China’s ultra-long range laser ranging technology.
On January 27, 2018, China realized the laser ranging of the distance between the earth and the moon. Laser measurement principle is the distance to the moon through a telescope from the ground station will highly ecotropic pulse laser beam into artificial placed on the surface of the moon’s corner reflectors, using special optical path properties of corner reflector, received from the surface of the moon reflected laser echo, through to send and receive time interval, calculate the distance to the moon. Most of the lasers used in the lunar laser ranging system are pulsed ruby lasers with pulse power up to 1000 mw and pulse width of 2 ~ 4 nanoseconds. At present, the measurement accuracy of earth-moon distance can reach millimetre.
The lunar laser ranging principle is completely different from the classical celestial azimuth measurement principle. The influence of atmosphere on the distance measurement is very small, which can be modified according to the meteorological data of the station. Above 10° above the horizon, the error of atmospheric correction is less than 1 cm, so atmospheric refraction is no longer a serious obstacle to the accuracy of observation. But because the echoes are weak, good transparency is required.
The chang ‘e-4 mission, which will also test China’s ultra-long range laser ranging technology, will add about 80,000 kilometers to the distance (the moon is at its farthest point from earth, at 400,000 kilometers).