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The Moon

The Face of the Moon

The visible face of the moon has light and dark patches, which people interpreted in different ways, depending on their culture. Europeans see a face and talk of "the man in the moon" while children in China and Thailand recognize "the rabbit in the moon." All agree, however, that the moon does not change, that it always presents the same face to Earth. Does that mean the moon doesn't rotate? No, it does rotate--one rotation for each revolution around Earth! We need to go aboard a spaceship and fly halfway around the Moon before we get a view of its other side--as did the Apollo astronauts who took the picture below.

The Craters

What had created those strange round "craters"? They reminded some observers of volcanic craters on Earth, or better, of the large "calderas" (cauldrons) formed by the internal collapse of volcanos, e.g. Crater Lake in Oregon. Others suggested that they were formed by the impact of large meteorites, but this was countered by the argument that most meteorites probably arrived at a slanting angle, and were expected to leave not a round ring but an elongated gouge.

The Airless Moon

In the centuries after Galileo's discoveries, the Moon was extensively studied by astronomers using telescopes. One thing soon became clear: it had no atmosphere. When a star was eclipsed by the Moon, it vanished suddenly and its light showed no refraction or absorption by an atmosphere. The location of that point allows astronomers to deduce the mass of the Moon, and from that, the pull of the Moon's gravity. At the surface of the Moon, it turned out, gravity is only 1/6 as strong as at the surface of the Earth. Gravity is important for the retention of an atmosphere. It holds an atmosphere down, while heat is what can make it escape.

In the Space Age

From the beginning of spaceflight, the Moon was a prime target, but this chapter in space exploration is too long to be covered here in any detail. The first spacecraft to reach the Moon were Luna 1, 2 and 3 of the Soviet Union, in 1959. Of these, Luna 3 rounded the Moon, took photographs of the far side which is not seen from Earth, and later scanned and transmitted those images (on the right); unfortunately, their quality was poor. In the decade that followed, 19 other Soviet missions were aimed at the Moon. In 1970 a Soviet spacecraft landed and returned a rock sample, and later that year a remotely controlled "Lunokhod" vehicle was landed, exploring its surroundings for nearly a year. Other sample returns and Lunokhods followed, the series ending in 1976. However, failures marked tests of a large rocket developed for human Moon flights, ending any plans of manned lunar exploration by the Soviet Union. Early attempts by the US to send unmanned spacecraft to the Moon (1958-64) either failed or returned scanty data. In July 1964, however, Ranger 7 returned clear TV pictures of its impact on the Moon, as did Rangers 8 and 9. Of the 7 "soft landers" in the "Surveyor" series (1966-8), 5 performed well and sent back data and pictures. In November 1969, after Apollo 12 landed 500 feet (160 meters) from the "Surveyor 3" lander, astronauts retrieved its camera and brought if back to Earth. In addition to the Surveyor project, 5 lunar orbiters photographed the Moon and helped produce accurate maps of its surface. On May 25, 1961, about one month after Russia's Yuri Gagarin became the first human to orbit the globe, US president John F. Kennedy proposed to the US Congress "that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to Earth. " The Apollo missions followed, with Apollo 8 rounding the Moon in 1968 and Apollo 11 finally landing there, on July 20, 1969. Five other lunar landings followed, the last of them in December 1972. Only Apollo 13 failed to land, its crew members narrowly escaping with their lives after an explosion aboard their craft on the way to the Moon.

Achievements of "Project Apollo."

Among the activities of Apollo astronauts on the Moon were: Bringing back to Earth extensive samples of lunar rock and soil. The rocks turned out to be ancient, suggesting no significant change since the surface of the Moon formed, about 4.5 billion years ago. The "soil" (regolith) had probably been pulverized by impacts, but as the "Surveyor" missions showed, it was firm enough to provide support. Crews of Apollo 15, 16 and 17 explored the Moon aboard an electrically driven "moon buggy. " (picture on the right). Extensive video pictures from the Moon were beamed to Earth--even one (by a remotely controlled camera) of the take-off from the Moon by the Apollo 17 crew. Also, the Earth and its "geocorona" of glowing hydrogen were photographed by a special camera using ultraviolet light. A seismometer was placed on the Moon, showing that the Moon was seismically much quieter than Earth. Metal foils were hung out (like a flag) to receive the solar wind. They were then returned to Earth where the composition of the ions caught in them was analyzed.

Corner reflectors were placed on the Moon, so that laser beams reflected from them could accurately measure the distance. No humans have visited the Moon from 1972 until now, but some orbital missions have studied the Moon's magnetic field as well as X-ray and gamma-ray emissions, from which some variations of the surface composition could be inferred. The Moon was found to have no global magnetic field like the Earth, but its surface was weakly magnetized in some patches. Molten rock can become permanently magnetized if it solidifies in the presence of an external magnetic field, suggesting that in some ancient era the Moon, like Earth now, had a molten metallic core in which electric currents generated a magnetic field. Somewhat similar observations were made near Mars in 1998-2000. Some excitement was caused by indications from the Lunar Prospector spacecraft, which suggested that ice may exist on the moon, inside a deep crater near the Moon's south pole. A possible explanation was that some time in the past (perhaps long ago) a comet had crashed into the Moon, and comets contain considerable amounts of water ice. The energy of the impact, turned into heat, would of course evaporate the ice. However, some of the water vapor would form a temporary atmosphere around the Moon, and might condense again to ice in very cold locations, like craters near the pole, which are permanently shaded from sunlight. At the end of its mission, on July 31, 1999, Lunar Prospector was therefore steered to deliberately crash inside the crater. It was hoped the impact might create (briefly) a cloud of water vapor, which could be observed from Earth, but none was detected.

There is little doubt that the future will see further lunar exploration, though a "lunar base" is probably far off. Astronomical and other observations can readily be made from Earth orbit, and providing life support on the Moon is not easy. Such a base will probably become attractive only after ways are developed for utilizing local lunar materials for construction and for fuel.