Expedition tasked with number of firsts for China

Reasons for predawn launch time explained

Many people have asked why the China National Space Administration chose to launch the Chang'e 5 in the early morning, which forced those wanting to witness the spectacular event to get up well before sunrise.

The robotic lunar probe was sent from the Wenchang Space Launch Center in the island province of Hainan, South China, at 4:30 am.

The launch time was decided by mission planners and rocket designers based on several considerations, said Qian Hang from the China Academy of Launch Vehicle Technology, which built the Long March 5 carrier rocket used to lift the probe.

"The first element we considered when choosing the launch time was the preferred trajectories and the relative position between Earth and the moon," said Qian, from the academy's general design department. "The selected time must fit with several requirements - from the trajectories of the rocket, the orbiter as well as the re-entry capsule."

The second factor taken into account was the sun.

"Launching the rocket before sunrise helped to avoid the impact of solar radiation on the rocket, especially a possible burst of solar wind," he said.

The third reason behind the early morning launch was the amount of cloud cover. At night, the cover is thinner, creating favorable conditions for signals communications between ground control and the Chang'e 5.

The last consideration was monitoring and tracking Chang'e 5 from the ground, Qian said. If the mission started in the daytime, the strong sunlight could "dazzle" the telescopes. Instead, the night sky allowed ground-based monitors to better track and observe the trajectory of the lunar probe after it separated from the rocket, he said.