Briefing Day: When to Land
1Adaeze Lindqvist, an eighth grader, spent a summer day at a space agency's mission center for students. She sat in on a planning briefing for a small robotic lander that will touch down on the Moon's near side, the side that always faces Earth. The landing site is near the middle of the near side, close to the Moon's equator. The lander runs on solar panels, with a battery for short dark spells.
2The planner, Ms. Ruth Nakashima, explained that the Moon has day and night just as Earth does, but a lunar day is much longer. In one cycle of phases, about 29.5 Earth days, each place on the Moon goes from one sunrise to the next. Daylight lasts for about half of the cycle and night for the other half. At the landing site, the ground heats to over 100 °C around noon and cools below −170 °C at night. The lander is not built to survive the night, so its work will end at sunset.
3To plan the landing day, Ms. Nakashima used a model. A lamp stood for the Sun, Adaeze's head for Earth, and a foam ball on a stick for the Moon. A sticker on the ball marked the landing site, and Adaeze kept the sticker facing her head as she moved the ball in a circle around herself. The lamp always lit the half of the ball that faced it. Adaeze watched the ball go through its phases and checked whether the sticker sat in the lit half or the dark half.
4The team wants to land soon after sunrise at the site. A low Sun casts long shadows, which make rocks and holes easy to spot during landing, and an early landing leaves the most daylight for science work. The team also checked the forecast for eclipses during the mission. Without sunlight, the lander uses 100 W of power, and its battery stores 600 Wh. During the lunar eclipse in Table 1, Earth's shadow will cover the landing site for about 3.5 hours.
5A second team is planning a lander for a site directly opposite this one, in the middle of the far side, which never faces Earth. That lander will send its radio signals through a satellite that circles the Moon. Ms. Nakashima also compared the Moon with Earth. The Moon travels around the Sun along with Earth, so both stay about the same distance from the Sun. Earth's axis is tilted about 23.5°, while the Moon's axis is tilted only about 1.5°.