An opposition occurs when the Sun, Earth and Mars line up with the Earth in the middle. This phenomenon, which happens every two years, brings Earth and Mars relatively close together. This diagram shows four recent oppositions and two future ones. The 2003 opposition was significant because Mars was very near its perihelion - the point n its orbit where it is closest to the Sun. At that time, Mars came within 35 million miles of Earth. Mars will be almost that close again during the opposition in July of 2018.