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Mariner
Magnetosphere
A giant invisible shell beyond the Earth’s atmosphere. It reaches about ten Earth diameters forward of the Earth and several thousand Earth diameters back into space, forming a tail.
The magnetosphere does not contain any air from the Earth, but it is where the magnetism from the Earth reaches out into the solar wind. The power produced in this region is about 100 billion watts, and part of that power produces the
nighttime displays called auroras. The magnetotail is the portion
of a planetary magnetosphere that is pushed in the direction of the solar wind.
 Magnetosphere—This cross section shows the way the magnetosphere is buffeted by the solar wind. The result is to squash the magnetosphere in the direction of the solar wind and to produce a long tail in the “lee” of the Earth.
Solar wind
Magnitude
The degree of brightness of a body in space. Magnitude was first used by the Ancient Greeks. They named the bright stars that were first seen in the evening sky as magnitude 1. They then defined less bright stars in steps as the sky darkened further, until magnitude 6 was allocated to stars that could only be seen with the naked eye in a totally dark sky.
Since that time the scale has been redefined. The brightest star has a magnitude of –1.4, and the faintest visible star has a magnitude of 6. The scale is organized so that a decrease of one unit represents an increase in apparent brightness by a factor of about 2.5.
Mare
Latin word for sea. Galileo thought the dark areas on the Moon were seas. We now know the Moon is completely dry, but the name is still used for the flat, featureless areas of the Moon.
 Mare—Mare Tranquillitatis (Sea of Tranquillity) is the large, dark area near the center of the photograph. Apollo 11 landed here on July 20, 1969.
Mariner
A series of early NASA space probes. Mariner 2 (1962) was the first probe to reach another planet (Venus). Mariner 4 (1964) gave the first close up views of Mars, and Mariner 9 (1971) was the
first probe to go into orbit around another planet (Mars). Mariner 10 (1973) was the first probe to visit more than one planet (Venus and Mercury).
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