Mercury

Mass: 330,104,000,000,000 billion kg (0.055 x Earth)
Equatorial Diameter: 4,879 km
Polar Diameter: 4,879 km
Equatorial Circumference: 15,329 km
Known Moons:  None
Orbit Distance: 57,909,227 km (0.39 AU)
Orbit Period: 87.97 Earth days
Surface Temperature: -173 to 427°C
First Record: 14th Century BCE
Recorded By: Assyrian astronomers

A year on Mercury is just 88 days long:    One solar day (the time from noon to noon on the planet’s surface) on Mercury lasts the equivalent of 176 Earth days while the sidereal day (the time for 1 rotation in relation to a fixed point) lasts 59 Earth days. Mercury is nearly tidally locked to the Sun and over time this has slowed the rotation of the planet to almost match its orbit around the Sun. Mercury also has the highest orbital eccentricity of all the planets with its distance from the Sun ranging from 46 to 70 million km.
Mercury is the smallest planet in the Solar System:     One of five planets visible with the naked eye a, Mercury is just 4,879 Kilometers across its equator, compared with 12,742 Kilometers for the Earth.
Mercury is the second densest planet:     Even though the planet is small, Mercury is very dense. Each cubic centimeter has a density of 5.4 grams, with only the Earth having a higher density. This is largely due to Mercury being composed mainly of heavy metals and rock.
Mercury has wrinkles:     As the iron core of the planet cooled and contracted, the surface of the planet became wrinkled. Scientist have named these wrinkles, Lobate Scarps. These Scarps can be up to a mile high and hundreds of miles long.
Mercury has a molten core:     In recent years, scientists from NASA have come to believe the solid iron core of Mercury could in fact be molten. Normally the core of smaller planets cools rapidly, but after extensive research, the results were not in line with those expected from a solid core. Scientists now believe the core to contain a lighter element such as sulphur, which would lower the melting temperature of the core material. It is estimated Mercury’s core makes up 42% of its volume, while the Earth’s core makes up 17%.
Mercury is only the second hottest planet:     Despite being further from the Sun, Venus experiences higher temperatures. The surface of Mercury which faces the Sun sees temperatures of up to 427°C, whilst on the alternate side this can be as low as -173°C. This is due to the planet having no atmosphere to help regulate the temperature.
Mercury is the most cratered planet in the Solar System:     Unlike many other planets which “self-heal” through natural geological processes, the surface of Mercury is covered in craters. These are caused by numerous encounters with asteroids and comets. Most Mercurian craters are named after famous writers and artists. Any crater larger than 250 kilometres in diameter is referred to as a Basin. The Caloris Basin is the largest impact crater on Mercury covering approximately 1,550 km in diameter and was discovered in 1974 by the Mariner 10 probe.
Only two spacecraft have ever visited Mercury:     Owing to its proximity to the Sun, Mercury is a difficult planet to visit. During 1974 and 1975 Mariner 10 flew by Mercury three times, during this time they mapped just under half of the planet’s surface. On August 3rd 2004, the Messenger probe was launched from Cape Canaveral Air Force Station, this was the first spacecraft to visit since the mid 1970’s.
Mercury is named for the Roman messenger to the gods:     The exact date of Mercury’s discovery is unknown as it pre-dates its first historical mention, one of the first mentions being by the Sumerians around in 3,000 BC.
Mercury has an atmosphere (sort of):     Mercury has just 38% the gravity of Earth, this is too little to hold on to what atmosphere it has which is blown away by solar winds. However, while gases escape into space they are constantly being replenished at the same time by the same solar winds, radioactive decay and dust caused by micrometeorites.   
© Space Facts

Mercury has been known to humanity since ancient times and although its discovery date is unknown, the first mentions of the planet are believed to be around 3000 BC by the Sumerians.
A year in Mercury is 88 days, yet a Mercury day is 176 Earth days. Mercury is nearly tidally locked to the Sun – also known as a gravitational lock – and over time this has slowed the rotation of the planet to almost match its orbit around the Sun.
Mercury orbits so quickly around the Sun that early civilizations believed it was actually two different stars – one which appeared in the morning and another which appeared in the evening.
Mercury is the smallest planet in the solar system with a diameter of 4,879 km and is one of five planets that is visible to the naked eye.
After the Earth, Mercury is the second densest planet. Despite its small size, Mercury is very dense because it is composed mainly of heavy metals and rock – the main characteristic of terrestrial planets.
Mercury is named after the messenger of the Roman gods, who is also known as Hermes in Greek mythology. This is because of the speed in which Mercury orbits the Sun and the speed with which Mercury the Roman deity was able to deliver messages.
Astronomers didn’t realize that Mercury was a planet until 1543 when Copernicus published his Sun-centered model of the Solar System – putting the Sun as the center of the solar system rather than the previously believed center, the Earth.
The planet has just 38% of the gravity on Earth. This means that Mercury isn’t able to hold the atmosphere it has and it instead gets blown away by solar winds. However, those same solar winds are also bringing in new gases, radioactive decay and dust from micrometeorites – replenishing the atmosphere.
Mercury has no moons or rings because of its low gravity and lack of atmosphere.
It was once believed that a planet called Vulcan existed between the orbit of Mercury and the Sun – however the existence of such a planet was never found.
The orbit of Mercury is an ellipse rather than circular. It has the most eccentric orbit in the solar system and the least circular of all of the planets, according to scientists and astronomers.
Mercury is only the second hottest planet. Venus, though farther from the Sun than Mercury, actually experiences higher temperatures. This is because Mercury has no atmosphere to regulate temperature and results in the most extreme temperature change of all the planets – ranging from -170°C (-280°F) during the night to 430°C (800°F) during the day.
Mercury does not experience any seasons. The axis of Mercury has the smallest tilt of all other planets, and this results in a lack of seasons on its surface.
Mercury is the only planet which doesn’t rotate exactly once every year – instead rotating three times for every two orbits of the Sun. This is because it is nearly tidally locked to the Sun.
The orbit of Mercury was important in proving Albert Einstein’s theory of General Relativity.
Mercury has a large iron core that is around 40% of its volume (compared to a core volume of 17% for Earth) in its center whose radius is 1800 to 1900 kilometers (1100 to 1180 miles). Scientists believe the core of Mercury is probably molten.
The outer shell of Mercury is only 500 to 600 kilometers (310 to 375 miles) thick. Earth’s outer shell (the mantle and crust) is 2930 kilometers (1819 miles) thick.
Mercury has a very thin atmosphere, which is made up of atoms from the surface of the planet that have been blown away by solar winds. As Mercury is so hot, these atoms quickly escape into space and so its atmosphere is constantly being replenished.
Mercury has a weak magnetic field whose strength is about 1% of the magnetic field on Earth.
Only two spacecraft have ever visited Mercury. It is difficult to reach the planet due to its proximity to the Sun and any spacecraft visiting would need to travel 91 million kilometers into the Sun’s gravitational potential well. The Mariner 10 visited during 1974-75, flying by Mercury three times and mapping half its surface. On March 24, 1975 it ran out of fuel and is still believed to be orbiting the Sun. The MESSENGER probe was launched in 2004 to explore Mercury’s high density, its geological history, the nature of its magnetic field and more. Another mission, BepiColombo, by the European Space Agency and Japan is expected to reach Mercury in 2019.
Mercury has more craters and impact marks that any other planet. The surface is similar to that of the Moon, as unlike most planets, Mercury isn’t geologically active and cannot “self-heal” from impacts with asteroids or comets. Most of the Mercurian craters are named after famous writers and artists. If a crater is larger than 250 km in diameter, it is known as a Basin. The largest Basin on Mercury, the Caloris Basin, is around 1,550 km in diameter and was discovered by the Mariner 10. 
© The Planets.org

MERCURY MESSENGER  (2011--2015)

Aug. 3, 2004: LaunchMar. 11, 2011: MESSENGER finally entered orbit around Mercury nearly seven years after launchApr. 30, 2015: MESSENGER plunged into Mercury at end of is mission
MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) was the seventh Discovery-class mission, and the first spacecraft to orbit Mercury. Its primary goal was to study the geology, magnetic field, and chemical composition of the planet. It was the first mission to Mercury after Mariner 10, more than 30 years before.
MESSENGER was launched at 06:15:57 UT Aug. 3, 2004, into an initial parking orbit around Earth. After that, its PAM-D solid motor fired to put the spacecraft on an escape trajectory into heliocentric orbit at 0.92 × 1.08 AU and 6.4 degrees inclination to the ecliptic.
The six-and-a-half-year road to Mercury was punctuated by several gravity-assist maneuvers through the inner solar system, including one flyby of Earth (Aug. 2, 2005), two flybys of Venus (Oct. 24, 2006, and June 5, 2007), and three flybys of Mercury (Jan. 14, 2008, Oct. 6, 2008, and Sept. 29, 2009).
The gravity-assist maneuvers allowed the spacecraft to overcome the problem of massive acceleration that accompanies flight toward the Sun: the flybys helped to decelerate MESSENGER’s velocity relative to Mercury and also helped conserve propellant for its orbital mission (although it prolonged the length of the trip).
The Earth flyby allowed mission controllers to properly calibrate all of the spacecraft’s instruments while also returning spectacular images of the Earth-Moon system.
During the second Venusian flyby (at a range of only 210 miles or 338 kilometers), MESSENGER relayed back a vast amount of data, including visible and near-infrared imaging data on the upper atmosphere. Some of the investigations, especially its study of the particle and fields characteristics of the Venus, were coordinated with ESA’s Venus Express mission.
The three Mercury flybys further slowed the spacecraft, although, during the last encounter in September 2009, MESSENGER entered a safe mode and, as a result, collected no data on Mercury. Fortunately, the spacecraft revived seven hours later.
MESSENGER finally entered orbit around Mercury at 00:45 UT March 18, 2011, nearly seven years after launch. It started formal data collection April 4.
The vehicle’s orbit was highly elliptical, approximately 5,800 x 125 miles (9,300 × 200 kilometers) with a 12-hour orbital period.
One of MESSENGER’s most remarkable images was its mosaic of our solar system, obtained Feb. 18, 2011, with all the planets visible except Uranus and Neptune—a visual counterpart to the image of the solar system taken by Voyager 1 on Feb. 14, 1990.
The spacecraft completed its primary yearlong mission March 17, 2012, having taken nearly 100,000 images of the surface of Mercury.
Among its initial discoveries was finding high concentrations of magnesium and calcium on Mercury’s night side, identifying a significant northward offset of Mercury’s magnetic field from the planet’s center, finding large amounts of water in Mercury’s exosphere, and revealing evidence of past volcanic activity on the surface.
In November 2011, NASA announced that MESSENGER’s mission would be extended by a year, allowing the spacecraft to monitor the solar maximum in 2012. The extended mission lasted from March 18, 2012, to March 17, 2013.
By April 20, 2012, with the help of three engine firings, the orbital period was reduced to eight hours. It was also during this period, in early May 2012, that MESSENGER took its 100,000th photograph from orbit. By this time, the imaging instrument had globally mapped both in high-resolution monochrome and in color, the entire surface of the planet.
It was also during this first extended mission that the spacecraft found evidence of water ice at Mercury’s poles, frozen at locations that never see the sunlight (made possible by the fact that the tilt of Mercury’s rotational axis is almost zero.)
A second mission extension was soon granted that took the mission to March 2015.
On Feb. 6, 2014, NASA reported that MESSENGER had taken its 200,000th orbital image, far exceeding the original expectation of at least 1,000 photographs.
During the second extension, MESSENGER photographed two comets: Comet 2P/Encke and Comet C/2012 S1 (also known as Comet ISON).
Beginning the summer of 2014, controllers began gradually moving MESSENGER to a very low orbit for a new research program.
By Sept. 12, 2014, just after the 10th anniversary of its launch, the spacecraft’s orbit was down to a mere 15.5 miles (25 kilometers).
Mission controllers implemented at least two orbital maneuvers (Sept. 12 and Oct. 24) to raise the spacecraft’s orbit to continue its latest extended mission.
By Christmas Day 2014, it was clear that the spacecraft’s propellants were running out and that MESSENGER would impact the planet in late March 2015. On Jan. 21, 2015, mission controllers carried out one last maneuver to raise the spacecraft’s orbit enough to continue science activities into early spring.
On April 16, 2015, NASA announced that the spacecraft would impact the surface of Mercury by April 30, 2015, after it ran out of propellant.
As expected, on April 30, 2015, at 19:26 UT, MESSENGER slammed into the planet’s surface at about 8,750 miles per hour (14,080 kilometers per hour), creating a new crater on Mercury.
Impact coordinates were probably close to 54.4 degrees north latitude and 149.9 degrees west longitude, near the Janácek crater in Suisei Planitia.

(c) SolarSystem.NASA.gov