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Mars is the focus of much speculation and scientific study about possible human colonization. Its surface conditions and the presence of water on Mars make it arguably the most hospitable of the planets in the Solar System, other than Earth. Mars requires less energy per unit mass (delta-v) to reach from Earth than any planet except Venus. However, at minimum energy use, a trip to Mars requires 6–7 months in space using current chemical spacecraft propulsion methods. Other challenges of human missions to Mars have been described. ==Relative similarity to Earth== Earth is similar to its "sister planet" Venus in bulk composition, size and surface gravity, but Mars's similarities to Earth are more compelling when considering colonization. These include: *The Martian day (or sol) is very close in duration to Earth's. A solar day on Mars is 24 hours, 39 minutes and 35.244 seconds. *Mars has a surface area that is 28.4% of Earth's, only slightly less than the amount of dry land on Earth (which is 29.2% of Earth's surface). Mars has half the radius of Earth and only one-tenth the mass. This means that it has a smaller volume (~15%) and lower average density than Earth. *Mars has an axial tilt of 25.19°, similar to Earth's 23.44°. As a result, Mars has seasons much like Earth, though they last nearly twice as long because the Martian year is about 1.88 Earth years. The Martian north pole currently points at Cygnus, not Ursa Minor like Earth's. *Recent observations by NASA's ''Mars Reconnaissance Orbiter'', ESA's ''Mars Express'' and NASA's ''Phoenix'' Lander confirm the presence of water ice on Mars. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Colonization of Mars」の詳細全文を読む スポンサード リンク
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