Hermes is not a planet, but the name is used in two separate astronomical contexts. The confusion stems from the ancient practice of naming celestial bodies after figures in mythology. In modern astronomy, Hermes is the name given to a specific near-Earth asteroid. It also serves as the Greek mythological source for the name of the innermost planet in our solar system.
Hermes as the Name for Mercury
The planet closest to the Sun, Mercury, owes its name directly to the Greek god Hermes. Ancient Greek astronomers associated the swiftly moving celestial body with Hermes because he was the messenger of the gods, renowned for his speed.
When the Roman Empire adopted Greek mythology, Hermes was syncretized with the Roman god Mercury. The Romans continued the tradition of naming planets after their deities. Since the planet Mercury moves faster across the sky than any other, it was a fitting tribute to the speedy messenger.
This naming convention highlights the historical connection between mythology and early astronomy. The planet’s modern designation, Mercury, is simply the Roman interpretation of the Greek god Hermes. The historical practice of using Roman gods for planetary names, while using Greek gods for many asteroids, is a reason for the dual usage of the name Hermes in space science.
The Asteroid Hermes and Its Characteristics
Beyond the mythological link, Hermes is the formal name of the sub-kilometer-sized asteroid 69230 Hermes. Classified as a Near-Earth Object (NEO) belonging to the Apollo group, its orbit crosses that of Earth. Its history is notable, as it was discovered in 1937 by German astronomer Karl Reinmuth but was observed for only four days before it became too faint to track.
Lacking sufficient data to calculate a precise orbit, Hermes became one of the most famous “lost” asteroids for over six decades. In 2003, a new observation was linked back to the 1937 discovery, allowing scientists to assign it the permanent number 69230. Radar observations soon after its recovery revealed that Hermes is a binary system, composed of two components orbiting a common center of mass.
The combined effective diameter of the two bodies is estimated to be 810 meters, making it a relatively large member of the NEO population. Hermes has a highly eccentric orbit, bringing it as close as 0.62 Astronomical Units (AU) to the Sun and extending out to 2.69 AU, beyond the orbit of Mars. It completes one full orbit around the Sun in about 778 days, or 2.13 years.
Monitoring Near-Earth Objects
The study of celestial bodies like 69230 Hermes is part of planetary defense efforts. Because its orbit crosses Earth’s path, Hermes is classified as a Potentially Hazardous Asteroid (PHA). PHAs are NEOs that come within 0.05 AU (about 7.5 million kilometers) of Earth and are large enough (over 140 meters in diameter) to cause significant regional damage upon impact.
Scientists continually track thousands of NEOs to accurately predict their future trajectories and assess collision risk. This tracking is accomplished through a global network of dedicated sky surveys, such as the Catalina Sky Survey and Pan-STARRS, which use large telescopes. Ground-based radar facilities, like the Goldstone Solar System Radar, bounce radio signals off an asteroid to determine its precise position, velocity, and physical characteristics.
Organizations like NASA’s Planetary Defense Coordination Office rely on data from the Center for Near-Earth Object Studies (CNEOS) to maintain a catalog of known objects. CNEOS runs automated impact monitoring systems, such as Sentry, which constantly assess impact probabilities for the next century. By gathering precise measurements of PHAs like Hermes, scientists refine their orbital models, providing the necessary lead time should any deflection effort ever be needed.