Upon landing on the Moon, Apollo astronauts followed a tradition started by explorers centuries before: They planted a flag representing their country of origin in the newly discovered land. Unlike most flags, however, the ones that the astronauts brought to the Moon required some extra support to make them recognizable in images that were broadcast around the world. Metal wire supported the Apollo missions' flags to make them appear to flap gracefully, despite the complete absence of wind on the Moon's surface.

The lack of wind on the Moon was just one of a host of novel conditions the 12 Apollo astronauts experienced in six missions. For example, silty Moon dust captured every footprint and infiltrated every piece of unprotected equipment. The sky appeared breathtakingly black even in broad "daylight." The Moon's gravity, at one-sixth that of Earth, allowed astronauts to jump farther and higher than they ever could on Earth, but also made controlling one's movement extremely challenging. To top it off, the astronauts faced extreme temperatures -- from -184°C (-300°F) in the shade to 101°C (214°F) in sunlight -- and a lack of breathable air.

Many of the differences between lunar conditions and those on Earth are the result of the difference in size, or more importantly mass, between Earth and the Moon. The Moon is about one-fourth the diameter of Earth, which is relatively large compared to the moons of other planets in our solar system. The Moon's mass, however, is just over 1 percent of Earth's mass.

One of the most important consequences of the Moon's small mass is that its gravity is too weak to maintain an atmosphere. Earth exerts a gravitational force on gas molecules, such as nitrogen, oxygen, water vapor, and carbon dioxide, just as it does on much larger objects on or near its surface. The resulting blanket of gas regulates Earth's surface temperature, produces weather, makes the sky blue, and provides all the elements necessary for life. The Moon has a very weak, nearly non-existent atmosphere and thus lacks many of the conditions we take for granted here on Earth.

Print Background Essay