Mercury's oval-shaped orbit is very elliptical, putting it as near to the sun as 29 million miles (47 million km) and as distant as 43 million miles (70 million km). If one could stand on Mercury at its closest approach to the sun, it would look more than three times as enormous as it appears from Earth. It would also be instantly vaporized.
Our planet Earth orbits the sun once every 365 days. Because Mercury rotates around the sun twice for each circuit it makes around our star, the mercury planet is actually orbiting the sun three times for every two orbits it makes about the center of mass formed by the gravitational pull of Earth and Sun. This means that even though Mercury is only a little bit bigger than Earth, it travels around the solar system twice for every time Earth goes around once!
It takes Mercury about 225 days to complete one orbit, but because it takes it so long to rotate around the sun, it gets red hot at one end and white hot at the other. At perihelion (the point in its orbit when it is nearest the sun), the temperature on Mercury can reach 450 degrees Fahrenheit (232 degrees Celsius). But at aphelion (the point in its orbit when it is farthest from the sun), the temperature drops down to -180 degrees F (-292 degrees C).
About 77 million kilometers Mercury is around 77 million kilometers (48 million miles) from Earth at its closest approach and approximately 222 million kilometers (48 million miles) from Earth at its furthest approach (138 million miles).
It takes Mercury about 58 days to orbit the Sun, which is slower than any other planet except for Venus. Because of this slow rotation, most features on Mercury are marked by a large zone of darkness called a syrtis. The only place on Mercury not in syrtes is the relatively small area where they converge. This region is called the maria (singular: mare).
The maria were formed when lava solidified before it had a chance to flow away from the volcano that created it, forming hollow spheres. Over time, more lavas were added, covering some maria and leaving others exposed again. The open space within some of these objects may be filled with gas that was trapped when they were first formed.
Mares appear on all solar systems planets with very high concentrations of silicon dioxide (silica), such as volcanoes and sand dunes. They are common on Mars and Vesta, the two largest asteroids, but rare on Earth because our crust and mantle are too dense to allow much penetration by molten rock.
The diameter of Mercury is 3,030 miles (4,878 km), which is equivalent to the size of the continental United States. It is thus around two-fifths the size of the Earth. The mass of Mercury is 4.9 billion kilograms (10.9 billion lb), about one-quarter that of Earth.
Mercury has a dense atmosphere composed of sulfur dioxide (SO2) with traces of other gases. It is this atmosphere that causes the dark surface and allows radiation from the sun to evaporate water from the planet's surface. The average density of the atmosphere is 100 times that of air, but there are pressure peaks where the density can be as high as 15,000 times that of air at the bottom of the atmosphere.
It is these pressure waves that drive the large tides on Mercury that were first observed by American astronomer Carl Vogel in 1960. He found that the height of the tide on Mercury varies as the square of the period it takes to go around the planet, similar to the motion of Moon over the Earth. This result was later confirmed by data obtained by Mariner 10 during its closest approach to the planet in 1974.
However, despite its similarity to Earth, the interior of Mercury is completely different. It is mostly iron with a thin veneer of silicate rock, just like Earth's core.
And the volume of Mercury is 6.1 x 1010 km3, or 5.4 percent of the volume of the Earth. In other words, you could fit Mercury inside Earth 18 times and still have some space left over. Mercury and Earth's orbits are most likely diametrically opposed. That is, if one planet is in motion, then the other must be in stable orbit around the common center of mass.
Therefore, if Earth were empty except for Mercury, then Mercury would still appear to move in a circle as it orbited the Sun. Because of this apparent contradiction with physics, scientists used to think that Earth must be filled with something else, like air or water, in order to keep Mercury from falling toward the Sun.
In fact, both Earth and Mercury are empty of significant mass apart from their rocky interiors, so they cannot remain in orbit around the Sun for any length of time without being ejected from the Solar System. However, because of their relative sizes, Earth is emptied out faster than Mercury. By the time 100 million years have passed, only 1/10th of Mercury's original mass remains, while Earth has lost only half of its initial mass. Therefore, even if Earth were completely empty, it would still be able to retain stable orbit for quite some time.
The actual figure depends on how much mass you assume Earth to have started with.