How Mercury Was Discovered

01 The Morning and Evening Star

Mercury is the innermost planet of the Solar System, orbiting the Sun at an average distance of 57.9 million kilometers — less than 40 percent of Earth's distance. Because it never strays far from the Sun in the sky, appearing at most about 28 degrees away from our star, Mercury is difficult to observe and was known to ancient civilizations as two separate objects: the morning star, visible just before dawn in the eastern sky, and the evening star, visible shortly after sunset in the west. The ancient Greeks called it Apollo in the morning and Hermes in the evening, though eventually they recognized these were the same planet. The Romans, who inherited much of Greek astronomical knowledge, gave it the name Mercury, after the swift-footed messenger of the gods — fitting for a planet that moves faster across the sky than any other.

The earliest known records of Mercury come from Babylonian astronomical tablets dating to the 14th century BCE, where it was called Nabu, after the Mesopotamian god of writing. These cuneiform tablets tracked the planet's appearances and disappearances with remarkable precision, noting the dates when it first became visible in the morning or evening and when it vanished into the Sun's glare. The Babylonians may not have understood that Mercury was a world orbiting the Sun, but they had established that its appearances followed a regular and predictable cycle — a cycle they recorded for centuries.

02 Copernicus and the Heliocentric Revolution

The understanding of Mercury as a planet orbiting the Sun, rather than a wandering star in Earth's sky, required the Copernican revolution. When Nicolaus Copernicus published his heliocentric model in 1543, he placed Mercury in its correct position as the innermost planet, closest to the Sun. This model explained why Mercury never appears far from the Sun and why it alternates between morning and evening visibility — it is simply on one side of the Sun or the other in its rapid 88-day orbit. Copernicus himself never observed Mercury well; the fog of the Vistula River near his Polish observatory made sightings of the elusive planet nearly impossible. He reportedly saw it only once or twice in his lifetime, relying on Babylonian and Greek records for his orbital calculations.

The first detailed telescopic observations of Mercury were made by Galileo Galilei around 1610, using his newly invented telescope. Galileo observed the planet's phases — the changing illumination pattern as Mercury moves around the Sun, similar to the Moon's phases — which provided direct evidence for the Copernican model. However, Mercury's small apparent size, rarely exceeding 10 arcseconds, meant that even Galileo's best telescope could resolve little surface detail. The planet remained largely a point of light for another three centuries.

03 The Mystery of Mercury's Orbit

Mercury's orbit presented one of the most enduring puzzles in astronomy: its perihelion — the point closest to the Sun — was advancing slightly with each orbit, drifting by about 43 arcseconds per century beyond what Newtonian gravity could account for. This anomaly, first measured precisely by the French astronomer Urbain Le Verrier in 1859, was one of the most significant unexplained phenomena in 19th-century physics. Le Verrier, who had successfully predicted the existence of Neptune from orbital irregularities of Uranus, proposed that an undiscovered planet called Vulcan, orbiting even closer to the Sun than Mercury, was causing the perturbation. Astronomers searched for Vulcan for decades, but it did not exist.

The puzzle was not resolved until 1915, when Albert Einstein's general theory of relativity provided the explanation. In Einstein's formulation, the Sun's mass warps spacetime itself, causing planetary orbits to precess. For most planets, this effect is negligible, but Mercury, deep in the Sun's gravitational well and moving at 47.9 kilometers per second, experiences it measurably. Einstein's equations predicted a perihelion advance of exactly 43 arcseconds per century — matching the observed anomaly to within the measurement error. This was one of the first triumphs of general relativity and cemented Einstein's reputation. Mercury, the elusive planet that had resisted observation for millennia, had become a precision test of the most revolutionary theory in physics.

04 Mariner 10: The First Visit

The first spacecraft to visit Mercury was NASA's Mariner 10, launched in November 1973. Mariner 10 used an innovative gravity-assist trajectory, flying past Venus first to adjust its orbit so it could make three close passes of Mercury in 1974 and 1975. During these flybys, the spacecraft came within 327 kilometers of the planet's surface and transmitted about 2,700 photographs back to Earth. These images revealed a heavily cratered, Moon-like surface with vast plains of solidified lava and enormous cliffs called rupes, some stretching hundreds of kilometers and rising up to 3 kilometers high. These cliffs, scientists concluded, were thrust faults formed as Mercury's interior cooled and the planet's surface contracted, wrinkling like the skin of a drying apple.

Mariner 10 mapped only about 45 percent of Mercury's surface, leaving half the planet completely unknown. It also detected a trace magnetic field, which came as a surprise — planetary scientists had expected Mercury, with its small size and presumably cooled interior, to be geologically dead. The presence of a magnetic field suggested that the planet still had a partially molten iron core generating a dynamo, a finding that would not be confirmed until decades later. The spacecraft also measured surface temperatures ranging from about minus 180 degrees Celsius on the night side to over 430 degrees Celsius on the day side — one of the most extreme temperature ranges in the Solar System.

05 MESSENGER: Orbital Science

The second mission to Mercury, MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging), was launched in August 2004 and, after a circuitous seven-year journey involving one Earth flyby, two Venus flybys, and three Mercury flybys, became the first spacecraft to enter orbit around Mercury in March 2011. MESSENGER orbited the planet for over four years, completing more than 4,000 orbits before intentionally crashing into the surface in April 2015. The mission transformed our understanding of Mercury, mapping the entire surface at high resolution, measuring the magnetic field in detail, and probing the planet's composition with X-ray and gamma-ray spectrometers.

Among MESSENGER's most significant discoveries was the confirmation that Mercury's polar craters, which never receive sunlight, contain deposits of water ice and organic compounds — preserved for billions of years in permanently shadowed regions near the poles. The mission also revealed that Mercury's surface is unusually rich in volatile elements like sulfur, chlorine, and potassium, challenging earlier assumptions that the innermost planet would be depleted in volatiles by the heat of the early Sun. MESSENGER's gravity data confirmed that Mercury's iron core occupies an extraordinary 55 percent of the planet's volume — proportionally the largest core of any planet in the Solar System, and a clue to Mercury's violent origin, possibly involving a giant impact that stripped away much of its outer layers.

06 BepiColombo and the Future

The third mission to Mercury, BepiColombo, is a joint endeavor by the European Space Agency and the Japan Aerospace Exploration Agency, launched in October 2018. After a complex seven-year cruise involving nine planetary flybys, BepiColombo is scheduled to enter Mercury orbit in 2025. The mission consists of two orbiters: the Mercury Planetary Orbiter, which will study the planet's surface and composition, and the Mercury Magnetospheric Orbiter, which will investigate the planet's magnetic field and its interaction with the solar wind. Together, they will provide the most comprehensive study of Mercury ever attempted.

BepiColombo's arrival marks a new era in Mercurian science. Key questions remain: Why is Mercury's core so large? Was the planet once much bigger, stripped of its outer layers by a collision in the early Solar System? What do the polar ice deposits tell us about the delivery of water and organics to the inner Solar System? How does Mercury's magnetic field, the weakest of any planet with one, interact with the ferocious solar wind at such close range? Each question connects back to the long arc of discovery that began with Babylonian sky-watchers tracking a swift-moving point of light — a journey of more than three thousand years from naked-eye observation to robotic orbiters, and one that is far from over.