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    Planet

    Mercury

    The smallest planet and the closest to the Sun, with a giant iron core.

    • Surface swings from about 430 °C by day to −180 °C at night
    • Rotates exactly three times for every two orbits (3:2 resonance)
    • Water ice survives in permanently shadowed polar craters

    Key facts

    Mean diameter
    4,879.4 km
    Mass
    3.3 × 10^23 kg (0.0553 Earths)
    Surface gravity
    3.7 m/s² (0.377 g)
    Escape velocity
    4.3 km/s
    Mean temperature
    167 °C
    Rotation period
    1,407.6 hours
    Orbital period
    88 days
    Mean distance from the Sun
    57.9 million km
    Sunlight travel time
    3 min 13.1 s
    Axial tilt
    0.034°
    Confirmed moons
    0

    Atmosphere

    A very thin exosphere of oxygen, sodium, hydrogen, helium and potassium atoms.

    Composition and structure

    Rocky planet with a metallic core making up roughly 85% of its radius.

    Discovery and exploration

    Mariner 10 flew past three times in 1974–75. MESSENGER orbited from 2011 to 2015 and mapped the whole surface. ESA–JAXA BepiColombo, launched in 2018, is on its way to orbit.

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