In the mid-1980s, NASA began talking about a flagship Mars mission, the first since Viking, to be called Mars Observer. The many instruments on the orbiter--radiometers, spectrometers and laser ...
From Mars, Earth transits the Sun four times in a 284-year cycle. The transits occur in either May or November at intervals of 100.5, 79, 25.5, and 79 years. During these events, Earth and the Moon ...
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