An elusive planet

Of all the planets of the solar system visible to the naked eye, Mercury is perhaps the most difficult to observe, especially in temperate latitudes. The fact is that the planet can be seen in a relatively short period of time immediately after sunset or before sunrise, and quite low above the horizon. Therefore, there is even a popular astronomical legend, as if at the end of the years, the brilliant Nicholas Copernicus once complained that in his life he had never seen the elusive planet.

Another rather interesting fact is that the 5th century Indian astronomical treatise "Surya-siddhanta" estimates the radius of Mercury at 2,420 km, which differs from the true value by only 1%. But there is an important point. This estimate was based on fundamentally erroneous ideas about the angular size of the planet in the sky. That the authors of the treatise turned out to be so close to the real number is, rather, an example of an amazing coincidence than some lost knowledge of the ancients.

Planet Mercury in space. Image courtesy of NASA

The invention of the telescope helped increase the body of knowledge about Mercury - although not very significantly. In 1631, Pierre Gassendi first observed this planet passing through the disk of the Sun. He was struck by how small she turned out to be. Astronomers soon discovered the phase changes of Mercury, and finally proved that it revolves around the Sun.

As more powerful telescopes were built, astronomers began the first attempts to map the surface of Mercury. Of course, they were not particularly successful. Already in our time, it turned out that all the maps of the pre-space age had little to do with reality.

In 1859, the famous astronomer Urbain Leverrier (it was he who, 13 years before, made the calculations that allowed the discovery of Neptune) announced the discovery of an anomalous removal of the perihelion of Mercury's orbit, which could not be explained within the limits of classical Newtonian mechanics. He himself considered this to be the existence of another, undiscovered planet between the Sun and Mercury. This hypothetical object was named Vulcan. Leverrier even calculated its orbit. But, despite numerous attempts, astronomers failed to find a new planet. The mystery of the elimination of Mercury's orbit was solved only after the publication of the General Theory of Relativity (GRT) in 1915. It turned out that everything is explained by relativistic effects associated with the proximity of the planet to the Sun.

Before the beginning of the space age, most scientists were sure that Mercury always faces the Sun on the same side, because when observing through telescopes, astronomers always saw the same light and dark spots on its surface. Fantasists quickly picked up on this idea and began to paint fascinating pictures of a world with rivers of molten lead flowing on one side, and darkness and bitter cold reigning on the other.

However, in 1965, a sensation broke out. The measurements of the Arecibo radio telescope showed that the planet still rotates around its axis with a period of 58 days, while its revolution period around the Sun is 88 days. That is, these indicators are in resonance 2:3.

Mercury has a magnetic field, the intensity of which is about 100 times less than Earth's. Image: NASA

So why were astronomers so wrong? Observations of Mercury were usually made when it was in inferior conjunction with Earth—when the planets were at their shortest distance from each other. But the fact is that, by a rather interesting coincidence, the synodic period of Mercury (the time interval between two consecutive conjunctions) is 116 days. That is, during the time between the two lower conjunctions, Mercury manages to make exactly two revolutions around its axis and again turns to the Earth on the same side. Observing always the same picture through their telescopes, astronomers logically concluded that the planet is in tidal capture relative to the Sun.

Problems of traveling to Mercury

One of the little-known facts about Mercury is that it is actually the closest planet to us, not Venus. Yes, if we take the minimum possible distance between the orbits, then Venus, of course, is closer. But if we consider the average distance between the planets, then for most of the year Mercury will be the closest planet to Earth.

It is all the more surprising that Mercury is also the planet that is perhaps the most difficult to reach. And the reason for this is the Sun. The problem is not even its powerful radiation, which requires the installation of a protective screen on the probe. It's all about gravity. Due to the proximity of this planet to the Sun, its gravity greatly affects any spacecraft launched to Mercury. In order to go into orbit around the planet, this speed must be extinguished, which requires significant fuel consumption.

Unfortunately, the capabilities of modern rocket technology do not allow simply sending an automatic station with a sufficient amount of fuel to perform such a braking maneuver. Even the launch of a run mission to Mercury is associated with the need for very expensive course corrections. Also, jumping ahead a bit, Mercury has no atmosphere, which precludes the possibility of using that