9. Mercury
Mercury is the Sun’s nearest neighbor and the solar system’s smallest planet.
Mercury’s surface has an odd wrinkled appearance due to the planet’s iron core cooling and contracting, and it also has volcanic characteristics.
Because of its proximity to the sun, Mercury is very difficult to visit. A spacecraft launched from Earth to Mercury has to travel over 57 million miles into the Sun’s gravitational potential well. Additionally, the trip would require more rocket fuel than that required to escape the Solar System completely.
Only 2 probes have made the journey so far. The first one, Mariner 10, ran out of fuel during its endeavor and is now thought to be orbiting the Sun.
source: Wikipedia
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