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, an unexpected discovery during a 1970s spacecraft flyby. It used to be presumed to be tidally locked, rotating once per orbit, but it was determined in the 1960s to rotate 1.5 times per orbit, a 3:2 resonance such that the same side of the planet faces 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 November 2026. Reaching Mercury from Earth is a challenge, and entering Mercury orbit is an additional challenge: the space probe's orbital speed around the Sun must be reduced from Earth's until the probe falls all the way down (toward the Sun) to Mercury's orbit, and remaining there requires it then be further slowed so that it doesn't coast back toward Earth orbit (in a highly-elliptical, comet-like orbit).