Imagine a planet without seasons, where two years pass in three days and light never reaches the poles. This strange planet is right next door and it’s called Mercury.

Mercury travels around the Sun faster than any other object in the solar system, completing a full orbit every 88 days. It’s very bright in the sky, but because of its proximity to the Sun it’s very difficult to study, so we don’t know too much about it.

As it moves quickly through the sky and is small, the planet was associated in ancient Greece with the role of an intermediary, bringing one god into contact with another. Hermes is named after the Roman god of shopkeepers and merchants. It is related to the Egyptian god Thoth and the Norse god Odin, as “El Pais” writes, while it also held a prominent place in the Mayan culture as a celestial object. All this mythological relevance in various cultures highlights something very simple: it is a prominent object in the night sky.

The planet rotates very slowly, which is why its days and nights are so long – one day on Mercury equals 58.6 Earth days. Its year, which lasts 88 Earth days, is very short as it is the closest planet to the Sun.

Its rotation period is not synchronized with its orbital period, as is the case with the Moon, but both periods are similar, in what we know as a 3:2 resonance orbit. This means that for each revolution around the Sun – the orbital period – Mercury rotates one and a half times around its axis – and that in two complete orbits around the Sun the planet rotates three times around its axis. One side of the planet is not always facing the Sun and the other in total darkness, but the alternating periods of darkness and light are very long.

The planet Mercury

According to Kepler’s second law, in such an elliptical orbit, the speed of the planet changes significantly at its most extreme points. Therefore, when Mercury is at the perihelion—the closest point of its orbit to the Sun—it is moving at a speed of 59 kilometers per second (note that the Earth is moving at a speed of 30 kilometers per second).

On the surface of Mercury, the Sun rises in the east and sets in the west, just as it does on Earth. But once a year, when it passes through the perihelion, the orbital motion overtakes the slow rotation of the planet, and on that day the course of the Sun is interrupted. At this point in its orbit one can observe the strange sunsets. The star “stops” completely in Mercury’s sky and moves backwards to return to its normal course as the planet slows down and moves into its orbit.

Because Mercury is closer to the Sun, the light reaching its surface is seven times more intense than on Earth. And that light has about three months to heat its surface. The planet rotates very slowly and consequently the temperatures on its surface are high enough – no less than 420ºC – to melt lead. Likewise, the time from sunset to sunrise is about three Earth months. Three months in total darkness to cool the planet’s surface to nighttime temperatures that drop below -170ºC, low enough to freeze methane and carbon dioxide.

Mercury has no seasons because it spins on its axis almost perpendicular to its orbit, which also means that in its polar regions the interior of its large craters is permanently in shadow. One of the big mysteries that the BepiColombo craft will try to solve is whether these craters contain sulfur or ice.

The planet Mercury

Mercury has a magnetic field similar to ours, only 1% as strong, and is unique among the rocky planets in our solar system because, like Earth, it has a self-sustaining magnetic field. Why Earth and Mercury maintain a magnetic field, while Venus, Mars and the Moon do not, is something we do not yet understand.

Mercury still hides mysteries, which the joint BepiColombo mission of the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) is trying to solve. It has been visited only twice, by the probes Mariner 10 and Messenger.

BepiColombo will be the third probe on this unique, small, mysterious and hard-to-explore planet that has helped us lay the foundations of today’s physics and will surely provide us with fundamental clues about the evolutionary history of our solar system and the formation of the Earth .