Opposition
In positional astronomy, opposition is the state where two celestial bodies are on opposite sides of an observer body. In the case of Earth, this is when a planet or other object is on the opposite side of the Earth from the Sun. This results in an approximately straight line, known as a syzygy, from the Sun to the Earth to the object. Opposition in relation to Earth can only occur with Luna and objects further from the Sun than Earth, so Venus and Mercury are never in opposition. However, Earth can be said to be in opposition from Venus or Mercury's perspective. The opposite of an opposition is known as a Conjunction.
When an object is in opposition, it is usually at the closest distance to Earth for a given orbital period. It will appear its largest and brightest in the night sky, if visible, and will be in full phase. More importantly, the short distance make an Opposition a useful event to plan interplanetary travel around.
The Opposition of Mars occurs approximately every 26 Earth Months. Every 15 to 17 years, the Opposition occurs close to Mars' perihelion, Mars' closest distance to the Sun (and by extension Earth). Approximately every two years, a flurry of activity takes place between Earth and Mars as ships set off for the red planet or begin their return journey to Earth. Travel is possible at other points in the orbital period, but at a much greater resource cost. TASA Marine Corps deployments to Mars are built around the Opposition cycle to ensure the shortest possible transit.
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