Mercury

Mercury is the nearest solar system planet to the Sun and the smallest. Characteristics:

• 0.055 Earth masses (about 4.5 × mass of the Moon).
• 0.38 Earth radii (about 1.4 Moon radii).
• 88 Earth days - orbital period.
• 176 Earth days - solar day.
• 0.39 AU - orbital radius (semi-major axis).
• 0.206 - orbital eccentricity.

It has no moons. It is a rocky planet with virtually no atmosphere, but has a magnetic field, which was not expected. It was also thought to be tidally locked to rotate once per orbit, but in fact rotates 1.5 times per orbit, a 3:2 resonance which has the same side of the planet facing the Sun every two perihelia. Its orbit is considerably more eccentric than those of the other seven solar system planets (though Pluto's is a bit more eccentric). Two NASA spacecraft have visited Mercury: Mariner 10 flew by and MESSENGER orbited it for 4 years. A current ESA/JAXA mission, BepiColombo was launched in 2018, aiming to reach Mercury in 2025.