Mercury

Because it has almost no atmosphere to retain heat, Mercury experiences the greatest temperature variation of all the planets, ranging from -280F at night to 800F during the day. Its orbital speed and proximity to the Sun (heat, radiation and gravity) makes it unapproachable by manned spacecraft. Several unmanned probes have circled Mercury but no significant (economic) reason for overcoming the difficulties of a manned or robotic landing mission have been discovered.

Mercury

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