The Moon is Earth's nearest neighbor. It orbits the
at an average distance of approximately 240,000 miles (384,000
The Moon completes an orbit of the Earth every 27.3 days (approximately 655 hours).
The Moon also rotates on its axis. Because of tidal forces, it
completes one revolution every 655 hours. So one "side" of the Moon
is always facing the Earth, and the other "side" is always facing
away from the Earth. On the surface of the Moon, you would experience
327.5 hours of sunlight and 327.5 hours of darkness. If you lived on
the side of the Moon facing the Earth, you would always see the Earth in the same
location in the sky, although the stars and Sun would move across the sky.
If you lived on the side of the Moon facing away from the Earth, you would
never see the Earth, although you would see all the stars and the Sun.
At the time of its formation, the Moon had a liquid core and experienced
flows of lava which created the flat basins, or "mare's", which we see
from Earth. The Moon gradually cooled, forming a thick crust. We see
no evidence of plate tectonics, as we have on Earth, so the surface
material on the Moon has remained nearly the same for more than 3 billion years.
Because of the lack of an atmosphere, the Moon's surface has
been bombarded by passing debris, creating many, many impact craters.
The mass of the Moon is approximately 8.1 x 10^19 tons (7.3 x 10^22 kg or
.01 x Earth mass).
The mean diameter of the Moon is 2,159 miles (3,476 km or .27 x Earth
diameter). The mass of the Moon is not evenly distributed; mass concentrations,
called Mascons, lie beneath many of the lunar basins, and the center
of mass of the Moon is displaced several kilometers towards the Earth.
The non-uniform mass distribution creates orbital stability problems for
spacecraft orbiting the Moon.
The average surface
acceleration (gmn) of the Moon is given by:
gmn = 5.3 ft/sec^2 = 1.62 m/sec^2 = .165 Earth gravity
The Moon has no appreciable atmosphere. The sky is always black and the
radiation from the Sun strikes with full force on the surface of the Moon.
The lack of atmosphere produces
temperature extremes on the Moon that range from -250 degrees F in the dark
to +250 degrees F in the light.
Because of the lack of atmosphere, and high radiation environment, there
are no living organisms on the Moon.
The lack of atmosphere means that there is no
on a rocket leaving the surface of the Moon, but it also means
that a landing spacecraft cannot use aerodynamic braking and must use
to land on the surface.
Robotic exploration of the Moon began in the early 1960's by both the
United States and the Soviet Union. Of particular note, the American Ranger spacecraft
returned detailed pictures of the Moon as the spacecraft were crashed into the surface.
The Ranger project was followed by the soft landing Surveyor spacecraft that performed
a number of experiments on the lunar soil as well as photographic investigations
from the surface.
The Lunar Orbiter spacecraft produced photographic maps of the surface of the Moon
that were used to select landing locations for the Apollo program. The Soviets
also soft landed on the Moon as part of the Lunik (Luna) program. A lunar
rover (Lunikhod) and a sample return mission were also successfully executed
by the Soviet Union.
Human exploration of the Moon was begun by the United States during the
Apollo program. Apollo 8 and Apollo 10 orbited the Moon without landing
astronauts on the surface. Six Apollo missions (11, 12, 14, 15, 16, & 17)
each landed two astronauts on the surface of the Moon. The last three missions
included an electric-powered "buggy" which allowed the astronauts to explore
several kilometers around the landing location. All of the missions returned
lunar rocks and soil to the Earth and established research stations that continued
to return data after the astronauts left the surface. Apollo 13 experienced an
explosion on the way to the Moon. The spacecraft swung around the Moon and
safely returned the crew to the Earth.
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