For a low cost mission Mercury is probably your best bet. There's no atmosphere to speak of, and external propulsion won't have to be terribly elaborate, because the sun's gravity will do most of the work. Also, as the two posters above me mentioned, very little has been studied about Mercury. On a more scientific note, mercury is the perfect place to study the effects of extreme heat and gravity on a planet and the inorganic substances that exist there. Moreover, it can provide more information about the level of radiation bombardment that could be expected for objects passing close to the sun.
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For a low cost mission Mercury is probably your best bet. There's no atmosphere to speak of, and external propulsion won't have to be terribly elaborate, because the sun's gravity will do most of the work. Also, as the two posters above me mentioned, very little has been studied about Mercury. On a more scientific note, mercury is the perfect place to study the effects of extreme heat and gravity on a planet and the inorganic substances that exist there. Moreover, it can provide more information about the level of radiation bombardment that could be expected for objects passing close to the sun.