An Eco-sustainable World
AstronomySolar system



Mercury is the innermost planet of the solar system and the closest to the Sun; furthermore it is the smallest and its orbit is also the most eccentric, i.e. the least circular, of the eight planets. Mercury orbits direct (counterclockwise, like all the other planets in the solar system) at an average distance of 0.3871 au from the Sun with a sidereal period of 87.969 Earth days. Mercury is also in orbital-rotational resonance: it completes three rotations around its axis every two orbits around the Sun.

Astronomy –
Mercury is characterized by a fairly high orbital eccentricity which is 0.205, 15 times that of the Earth. From the surface of Mercury, the Sun has an average apparent diameter of 1.4°, about 2.8 times that visible from Earth, and reaches 1.8° during the passage to perihelion. The ratio between solar radiation at the perihelion and that at the aphelion is 2.3. For Earth this ratio is 1.07. Mercury’s surface experiences the greatest thermal excursion among all the planets, with temperatures in the equatorial regions ranging from 100 K (−173 °C) at night to 700 K (427 °C) during the day; the polar regions, on the other hand, are constantly lower than 180 K (−93 °C). This is due to the absence of the atmosphere which, if present, would play a role in heat redistribution. The heavily cratered surface indicates that Mercury has been geologically inactive for billions of years.
This planet, known since the time of the Sumerians, owes its name to Roman mythology. The planet was associated with Mercury, messenger of the gods, probably due to the rapidity of its movement in the sky. His astronomical symbol is a stylized version of the god’s caduceus.
Mercury orbits very close to the Sun and takes about 88 Earth days to complete one revolution around the Sun. Due to its proximity to the Sun, Mercury is often difficult to observe from Earth, as it appears near the horizon at sunrise or sunset.
Mercury has a diameter of approximately 4,880 kilometers, making it the smallest planet in the solar system. Its surface is covered by craters caused by meteorite impacts and lava plains.
Mercury has an extremely thin atmosphere composed mainly of helium and traces of other gases. Due to its low gravity and lack of a significant atmosphere, Mercury cannot retain a real atmosphere.
This planet exhibits a phenomenon known as “perihelion precession.” This means that Mercury’s orbit rotates slowly over time, due to gravitational perturbations from other planets, particularly Venus. This precession is one of the proofs of the accuracy of Albert Einstein’s theory of general relativity.
Mercury is a fascinating planet for astronomers and space scientists, and has been studied to better understand the formation processes and evolution of rocky planets within our solar system.

Physical characteristics –
With its approximately 4,880 kilometers it has a diameter of approximately 38% of the Earth’s diameter. Its mass is about 11% that of Earth.
Mercury’s surface is mostly rocky and features numerous unique geological formations, including craters, cliffs, plains, and mountain peaks. The lack of a thick atmosphere means that the surface has not undergone significant atmospheric erosion.
Mercury has an extremely thin atmosphere, with an atmospheric pressure only a fraction of Earth’s. This atmosphere is composed mainly of helium, with traces of other gases such as hydrogen, oxygen and sodium. Because of its thinness, Mercury cannot retain significant atmospheric heat, making its temperatures extremely variable.
Mercury rotates very slowly, with a solar day (the time it takes to complete one rotation) of about 176 Earth days. However, its period of revolution around the Sun is just under 88 Earth days.
This planet has a relatively weak magnetic field compared to Earth’s. This magnetic field is generated by a liquid iron core in the planet’s core, which has been an object of interest to scientists studying the evolution of planetary magnetic fields.
Despite extreme temperatures, Mercury hosts some permanently shadowed areas at its poles, where water ice has been discovered. This phenomenon has been confirmed by space missions such as NASA’s MESSENGER.
Mercury’s surface is dotted with craters caused by meteorite impacts. These craters testify to the ancient history of collisions that shaped the planet.

Space missions –
Space missions to Mercury have been relatively rare due to the unique technical challenges associated with exploring this planet. Mercury is the closest planet to the Sun in our solar system and is subject to extreme temperatures and intense solar radiation. However, in recent decades some missions have been launched to study this planet more closely.
Here are some of the most important space missions dedicated to the exploration of Mercury:
1. Mariner 10 (1973-1975): The Mariner 10 mission, launched by NASA, was the first to provide close-up observations of Mercury. The probe performed three close flybys of the planet, capturing images and scientific data. He helped discover that Mercury has a thin atmosphere composed mostly of helium, along with information about the planet’s surface features.
2. MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) (2004-2015): This was one of the most ambitious missions to Mercury. The MESSENGER probe, also launched by NASA, orbited the planet for over four years, collecting detailed data on its geology, atmosphere, composition and magnetic field. MESSENGER has also identified water ice in Mercury’s polar regions.
3. BepiColombo (Launched in 2018): BepiColombo is a joint mission between the European Space Agency (ESA) and the Japanese Space Agency (JAXA). The mission consists of two orbiters, one European and one Japanese, which will study Mercury from different perspectives. BepiColombo will provide important data on the planet’s geology, atmosphere and magnetosphere.
These missions have greatly increased our understanding of Mercury, but many open questions still remain about this mysterious planet. Exploration of Mercury is challenging due to extreme environmental conditions, but the information collected from these missions is fundamental to our understanding of the solar system and the evolution of the planets.

Guido Bissanti

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