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Mercury’s surface temperatures swing from a blistering 430 °C (800 °F) during daylight to a frigid –180 °C (–290 °F) at night. No crewed missions have yet reached the planet, largely because its extreme heat and long travel distance demand expensive, heavy‑load preparations that are impractical for human crews. However, two unmanned spacecraft have studied Mercury in detail, with a 36‑year interval between their visits.
Launched in 1973, Mariner 10 pioneered the exploration of Mercury by combining a Venus flyby with a Mercury encounter, using Venus’s gravity to slingshot toward the innermost planet. Equipped with cameras, spectrometers, and magnetometers, it flew past Mercury three times, reaching a closest approach of 327 km (203 mi) on March 16, 1975. Seven days later, NASA lost contact as the craft drifted beyond the reach of its power source.
NASA launched MESSENGER in 2004, a spacecraft built from lighter materials and armed with advanced, miniaturized instruments. The name MErcury Surface, Space ENvironment, GEochemistry and Ranging reflects its mission to map the planet’s surface, magnetic field, and composition. Designed to survive extreme solar heat, MESSENGER orbited Mercury in a highly elliptical path, ranging from 200 km (124 mi) to 15,193 km (9,420 mi) from the surface. By 2013 it had completed roughly 2,600 orbits, providing unprecedented data.
Mercury’s thin exosphere, composed of oxygen, sodium, hydrogen, helium, and potassium, is constantly buffeted by solar wind due to its proximity to the Sun (≈58 million km, 36 million mi). The planet’s surface is heavily cratered, mirroring the Moon’s appearance, and no signs of life—past or present—have been detected by either mission.
Mariner 10’s imagery revealed a chaotic terrain of rugged ridges and craters, and it detected a surprisingly weak intrinsic magnetic field. MESSENGER’s high‑resolution imaging uncovered evidence of a large, partially liquid core and exposed volcanic vents that once spewed molten rock across the surface, reshaping Mercury’s geology over time.
