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How close can Mars come to Earth?
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Mars
An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.
At its closest approach, Mars comes within less than 56 million km (35 million miles) of Earth.1
Because both planets follow elliptical orbits around the Sun, the distance between them varies significantly. When the two planets are on opposite sides of the solar system, they can be as far as 400 million km (250 million miles) apart.1
The best opportunities to observe Mars occur during opposition, a configuration where Earth passes directly between Mars and the Sun. While oppositions happen roughly every 26 months, particularly close approachesāwhich provide the best viewing conditionsāoccur approximately every 15 years.1