What is a launch window?

A: On August 24, China called off the launch of its Chang’e 7 mission to the moon’s south pole for the sake of “prudence, reliability and absolute mission success”. It will now have to wait until next year for another chance.

All space missions have specific launch windows — periods when the positions and motions of the satellite (on the rocket), the earth, and the target body are properly aligned. The moon orbits the earth once every 27.3 days, so launch windows to the moon open once a month. But if a mission has to go to the moon’s south pole region, the window changes.

A spacecraft approaching a point near the moon’s rotational pole will have to be orbiting the moon close to a north-south plane. The plane of the moon’s orbit is tilted by 5.1° relative to that of the earth’s orbit. And the moon’s equator is tilted by just around 1.5° relative to the earth. As a result, the desirable trajectory a spacecraft should achieve around the moon will change depending on the moon’s position around the earth.

The end goal is to launch the mission such that it achieves this trajectory after expending the least amount of fuel possible. Since the Chang’e 7 mission spacecraft will be powered by sunlight, it also faces a second constraint. The sun doesn’t rise high above the horizon at the moon’s south pole, so Chang’e 7 will have to land at its designated spot when it is sufficiently sunlit.

This is why missing one launch window can mean waiting for several days, weeks or even months for the next one.