2. Unlike Uranus, Neptune, and Pluto, which were discovered in the last few hundred years, ancient peoples have known about Mercury for thousands of years.
3. Mercury is so small that it has too little gravity to hold an atmosphere like Earth or Venus.
4. Only one spacecraft has ever visited Mercury: Mariner 10 in 1974-75. Mariner 10's discovery that Mercury has a very weak magnetic field, similar to but weaker than Earth's, was a major surprise.
5. Mercury speeds around the Sun in a wildly elliptical (non-circular) orbit that takes it as close as 47 million km and as far as 70 million km from the Sun
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