Mercury (planet)
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Mercury is the innermost and smallest of the nine eight planets of the Solar System after Pluto got demoted back in 2006. It receives its name from the messenger of the gods in the Roman mythos, borrowed from Hermes who had the same role in the Greek mythos.
The planet received that name because it moves relatively fast through the sky, which together to be the closest one to the Sun means it is pretty hard to catch as it does not go very far away from it -around 25 degrees at most- unless you know where to look just for one or two hours either after sunset or before dawn, which explains why even some astronomers have been unable to spot it being certainly much less known in such regard than Venus, appearing as a conspicuous star but much less bright than the former. As noted below this explains why so little was known of it until the Space Age.
Mercury has no morning of evening star to speak of as it has no planet closer to the Sun. However its nights are adorned by a Venus that appears much brighter than as seen from Earth, as it appears at its closest as a full disk unlike the crescent that offers to Earthlings, plus the dazzling blue star that is Earth, that while less bright than Venus it is still comparable to our view of the latter in such regard and comes with the Moon as a quite bright star[1]
Physical characteristics
Mercury's diameter is small, having a size of 4.880 kilometers (40% of Earth's one) and being smaller than the two largest moons of the solar system: Ganymede
and Titan. However it is much more dense than both, thus much more massive having a gravity comparable to that of Mars. Such density is thought to be caused by a very large iron-rich core, much larger in proportion to the size of the planet than Earth's one and what causes it to be a Heavy metal world, Mercury having 70% of its mass in the form of metal and everything else as rock (ie, silicates).
Mercury is your standard airless planet, with no atmosphere to speak of, given its closeness to the Sun, the effects of the solar wind, and its weak gravity that does not allow it to hold one for extensive periods. This translates to severe variations in temperature between day and night, with extremes going from 700K (427°C, 800°K) to 80K (-193°C, -315,4°F) but at least spacecrafts have found water ice at its poles, which at least makes it somewhat more friendly than Venus.
Like the Moon, the planet is also heavily cratered, with craters of all sizes overlapping older craters of all sizes and the largest one -the Caloris basin- reaching more than 1.500 kilometers, them mixed in with plains with less cratering probably of volcanic origin, plus large rips and scarps thought to be the result of Mercury contracting after formation. The planet seems to be geologically dead in these times.
Mercury's orbit has the greatest eccentricity (0.2056, cf. Earth 0.017) of all the planets in the Solar System. Most notably, it cannot be accurately predicted using the laws of mechanics derived by Newton and it wasn't until Einstein's general relativity, which explains gravity as distortions in spacetime, that the orbit could be predicted accurately. Scientists had previously attributed a partially-molten core to be the source of the anomaly in Mercury's orbit.
Observation
Mercury's closeness to the Sun means that very little of the planet was known before it was possible to send spacecrafts there, as it was difficult to see with a telescope little more than a blob due to atmospheric turbulence being so close to the horizon, and having so few time to observ it before either the planet set or was drowned in daylight. Despite that and with a lot of patience maps as this one were made ("solitudo" is Latin for "desert", appropiated given its presumed hot climate). One surprise brought by radar observations back in 1965 was that the planet was not tidally locked to the Sun as was thought, having one side permanently in daylight and other in darkness, but instead completed three orbits per two revolutions around the Sun.
Just two spacecrafts have reached Mercury (Mariner 10 and MESSENGER, both from NASA), given that it is very difficult to go there as you cannot simply hurl the probe towards the Sun and let Mercury catch it when the former is close to the latter. One must first cancel a whole lot of kinetic energy to reach its orbit, then spent quite a lot of fuel to brake and enter into orbit as it lacks an atmosphere so aerobraking is not available (it is said, in fact, that it is easier to abandon the Solar System than going there), and have allowed us to know way more of it than little more than its diameter and orbit. A third one, the European-Japanese BepiColombo, is on the way and expected to enter in Mercurial orbit in 2025.