China has postponed the launch of Chang’e-7, its lunar south pole mission, by about a month. The National Astronomical Research Institute of Thailand, which built one of the mission’s instruments, announced on Aug. 23 that the Long March 5 Y14 launch, set for Aug. 25, “will be postponed by approximately 1 month.”

Xinhua carried a one-sentence statement the same day from the China Manned Space Agency: applying “the principle of prudence, reliability and absolute mission success,” the mission “does not meet launch conditions” and cannot be carried out in the scheduled window.

Tropical Depression Narra, which formed in the Gulf of Tonkin on Aug. 19 and is tracking toward Hainan, is blamed for closing the launch site.

The specialist newsletter China in Space, which had the postponement right on the day, explains why the schedule is so tight. Every previous Chang’e landing targeted a non-polar site, where launch opportunities recur readily. Chang’e-7 is aimed at a permanently illuminated ridge beside Shackleton crater, at 123.4° East and 88.8° South, and the trajectories that reach it from Wenchang while allowing an efficient braking burn into lunar orbit are available only about a week at a time.

The binding constraint is sunlight rather than weather. The spacecraft is to spend some ninety days in lunar orbit surveying the site before it descends, and the landing itself requires the Sun on the site during descent. A Chinese Academy of Sciences study places the best-illuminated landing opportunities in November and December of this year.