The duration of a single day on the Moon, from sunrise to sunrise, is approximately 29.5 Earth days. However, this is broken down into roughly 14.75 days of daylight followed by 14.75 days of nighttime, due to the Moon's synchronous rotation with Earth.
When we refer to the "life of one lunar day" in the context of a moon mission, it usually pertains to the duration of sunlight (daytime) that the mission equipment can operate within. This is because most moon missions, including rovers and landers, rely on solar panels for power. During the lunar night, temperatures can drop drastically (to below minus 200 degrees Celsius), and without sunlight, there's no power generation from solar panels. This extreme cold can also be damaging to equipment.
So, when a mission states its operational life as "one lunar day," it typically means it is designed to function during the approximately 14 Earth days of sunlight on the Moon, after which it may go into a dormant state during the lunar night, or it might cease operations altogether due to the harsh conditions.
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