Online Astronomy eText: Satellites (Moons)
Pictures of The Moon: Men on the Moon Link for sharing this page on Facebook
     Magnificent Desolation: A view of the Sea of Tranquillity just before the Apollo 11 landing module "Eagle" touched down on July 20, 1969. A totally barren landscape pockmarked by craters large and small, covered with rocks and boulders blasted out of other craters and overlain by a thick powdery layer of dusty debris created by innumerable meteorite and micrometeorite impacts over a period of billions of years awaited those inside the module. During the next three and a half years twelve men would explore this hostile environment, carry out dozens out experiments and bring half a ton of lunar rocks back to Earth, providing clues to the origin of the Earth and Moon and the early history of the Solar System. For many of the hundreds of millions of people who watched and waited on the Earth, the first Moon landing remains one of the most memorable moments in their lives. For the twelve men who walked on the Moon it was literally the thrill of a lifetime. (Apollo 11, NASA)

      The Apollo 11 landing site photographed from the lunar landing module, the Eagle, a few days later. The airless, waterless, previously barren landscape will bear the footprints of those who walked there for millions of years to come. (Apollo 11, NASA, apod020720)

Apollo 11 astronaut Buzz Aldrin near the seismometer left on the Moon when the mission ended.
     An analysis of twenty-eight moonquakes recorded between 1972 and 1977 shows that lunar earthquakes are surprisingly strong and frequent for a geologically 'dead' body, and thanks to the Moon's stiff structure, shake the surface far longer than similar quakes shake the "softer" Earth. One suggestion for the frequency of the quakes is that perhaps impact-induced landslides on crater walls produce the shaking. Whatever the cause, any future Moon base will need to meet unexpectedly stringent 'moonquake codes'. (Neil Armstrong, Apollo 11 Crew, GRIN, NASA, apod060327)

The Apollo 11 Lunar Module (lander) returning to the orbiting Command and Service Module on July 21, 1969 (The quarter-Earth view of our planet indicates this occurred at quarter-phase for the Moon as seen from the Earth) (Apollo 11, NASA, apod090719)

Apollo 12 astronaut Pete Conrad visiting the Surveyor 3 lander.
(Apollo 12 Crew, NASA, apod010303)

A panorama of the Hadley Rille area taken by Apollo 15 astronaut James Irwin.
Astronaut David Scott and the Lunar Rover are shown near the summit of Hadley Delta, while a portion of Hadley Rille occupies the central portion of the image. (Apollo 15 Crew, USGS, NASA, apod040113) (Click on the image to open a larger panorama in a new window)

Apollo 16 on the Moon, April 1972.
Charles Duke, photographed near Plum crater, at the Descartes landing site.
(John W. Young, Apollo 16 Crew, NASA, apod000319)

Apollo 17 astronaut/geologist Harrison Schmitt on the Moon.
(Apollo 17, NASA, scanned by Kipp Teague, apod001209)

     A panoramic view of the Apollo 17 landing site. Harrison Schmitt prepares to take a soil sample in this photograph taken by Eugene Cernan in December of 1972. The last men to walk on the Moon returned home only a few days later. It has been more than forty years since the last manned Moon mission; how long will it be before men walk on the Moon again? (Click on the image to open a larger panorama in a new window) (Apollo 17 Crew, NASA, apod020128)

A strange-looking harbinger of dangerous conditions
     The bright object in the center of this odd-looking image is the Moon imaged in gamma radiation -- photons of light with more than a million times the energy of visible light photons -- by the Compton Gamma Ray Observatory spacecraft, combining eight exposures made from 1991 - 1994. The strange thing about this image is that if the object in the middle were the Sun it would not be visible in images of this quality, even though all its energy comes from the creation of gamma-ray photons through thermonuclear reactions in its core. Those photons are transformed during their more than one million year journey to the solar surface into visible, infrared and ultraviolet radiation, so that the Sun emits virtually no gamma radiation.
     If even an object as bright and as big as the Sun gives off no significant gamma radiation, why does the Moon glow so brightly in gamma radiation? Just as the Earth's atmosphere protects our surface from small to middling size meteoroids which are vaporized by their passage through our atmosphere, but the Moon's airless surface is constantly struck even by microscopic meteoroids, cosmic rays -- extremely energetic pieces of atoms moving at nearly the speed of light -- are blocked by our atmosphere (see the image of the Earth below), but strike the lunar surface with no impediment, and when they do their energy of motion is transformed into the gamma-ray photons imaged above.
     During the short trips that astronauts took to the Moon in the late 1960's and early 1970's the dangers posed by meteoroids, cosmic rays, toxic particles of lunar dust and other lunar phenomena were relatively minor. But when men return to the Moon (perhaps between 2020 and 2040) for stays of much longer duration, protecting them from the dangers posed by its lack of air will be a major problem, and a great deal of NASA's current research is aimed not at getting men back to the Moon, but making sure that once they get there they can live and work in relative safety. (Dave Thompson (NASA/GSFC) et al., EGRET, Compton Observatory, NASA, apod060527)

     In contrast with the Moon, which glows brightly in gamma radiation across its entire surface because it is the surface itself which produces the gamma rays (as described above), the Earth's gamma radiation (shown above) is brightest at the edges, because cosmic rays do not reach the surface of the Earth, but interact with its upper atmosphere, making the atmosphere the source of Terrestrial gamma rays. Looking at the center of the image we are looking straight down through the atmosphere, but near the edge (or "limb") of the Earth we are looking nearly horizontally through the atmosphere, greatly increasing the amount of air we have to look through, and the amount of gamma radiation we see coming from the atmosphere. (Dirk Petry (GLAST Science Support Center), EUD, EGRET, NASA, apod060603)