After an eight-year journey through space, the British-built spacecraft BepiColombo is entering its arrival phase at Mercury, where it will begin orbiting the planet in November. The mission, a joint effort between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), aims to shed light on the longstanding scientific question of Mercury’s origin and its possible role in the formation of the Moon.

Mercury, the innermost planet in the Solar System, has a dense iron core constituting about 65 percent of its mass—nearly double the proportion found in Earth’s core. This unusual composition has led researchers to propose that Mercury may have formed further from the Sun, perhaps near Mars, before experiencing a massive collision that stripped away much of its lighter outer layers. Some scientists suggest that this impact could have knocked Mercury closer to the Sun and, intriguingly, that Mercury itself may have been involved in the ancient collision with early Earth approximately 4.5 billion years ago that created the Moon.

ESA’s BepiColombo spacecraft is equipped with a suite of scientific instruments designed to analyze Mercury’s surface composition and interior structure in unprecedented detail. The mission’s lead project scientist, Professor Geraint Jones of ESA, explained that studying Mercury’s interior density and geological properties will help test the theory that a planetesimal impact removed the planet’s outer crust, leaving behind a dense core.

Supporting this hypothesis, findings from NASA’s Messenger mission, which orbited Mercury between 2011 and 2015, revealed potassium and thorium levels on Mercury’s surface that resemble those found on Mars, rather than those typical of planets formed closer to the Sun. Potassium tends to evaporate rapidly at high temperatures, whereas thorium remains, so the relative abundance of these elements can indicate a planet’s formation zone. Scientists on the BepiColombo team, including project scientist Johannes Benkhoff, note that Mercury’s unexpectedly high potassium levels suggest it originated in a cooler part of the Solar System before being displaced.

Further complicating the picture, some researchers, such as Professor Dirk Schulze-Makuch of the Technical University of Berlin, have posited that the Moon could be composed of material lost from Mercury's crust during these early collisions. Others at the University of Arizona have theorized that Mercury underwent multiple glancing blows involving proto-Earth and proto-Venus before arriving at its current orbit and composition.

BepiColombo consists of two orbiters: ESA’s Mercury Planetary Orbiter (MPO), built by Airbus in Stevenage, UK, and JAXA’s Mercury Magnetospheric Orbiter. The mission will provide high-resolution mapping of Mercury’s surface and investigate whether it harbors water and maintains geological activity.

Named after Giuseppe “Bepi” Colombo, the Italian scientist who contributed significantly to NASA’s Mariner 10 mission, BepiColombo promises to offer new insights into Mercury’s complex history and its potential connection to one of the Solar System’s greatest mysteries: the origin of the Moon.