Mercury

**Object**
Gallery

Enhanced color image taken by MESSENGER (2008)

**Mercury** is the smallest planet in the Solar System, and is also the innermost. It has the shortest orbital period in the Solar System, only being 87.97 Earth days.

## Description

Mercury's surface appears to be loaded with craters, and is very similar in appearance to the Moon. Having no atmosphere to retain heat, Mercury's temperatures vary wildly from −280 °F at night (−173 °C) to 800 °F during the day at the equator (427 °C). The temperatures at the polar region are constantly below −136 °F (-93 °C). Mercury has no known natural satellites.

## Orbital and rotational characteristics

Mercury is tidally locked with the Sun in a 3:2 spin-orbit resonance, and rotates in a very unique fashion compared to the rest of the Solar System. Every two revolutions it makes around the Sun, it rotates exactly three times.

Mercury's axis has the smallest tilt of any other planet in the Solar System (approx. 1⁄30 degree). Out of all the planets in the Solar system, Mercury's orbit is the most eccentric. At perihelion, Mercury's distance from the Sun is only about 66% of its distance at aphelion.

## Observation history

### Ancient astronomers

The earliest known recorded observations of Mercury date back to the 14th century BC, from the Mul.Apin tablets. These observations were most likely made by an Assyrian astronomer. Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians named the planet Nabu after the messenger to the gods in their mythology.

The ancient Greeks knew the planet as Στίλβων (Stilbon), meaning "the gleaming," Ἑρμάων (Hermaon) and Ἑρμής (Hermes). The Romans named the planet after the swift-footed god, Mercury, because it moves across the sky faster than any other planet.

In ancient China, Mercury was known as "the Hour Star." It was commonly associated with the direction north, as well as the phase of water in the Wu Xing system of metaphysics.

In India, the astronomer Nilakantha Somayaji developed in the 15th century a partially heliocentric system in which Mercury orbits the Sun, which in turn orbits Earth.

### Telescopic research

The first telescopic observations of Mercury were made by Galileo early in the 17th century. With his telescope, he was able to observe the transit of Venus, however it was not powerful enough to observe the transit of Mercury. In 1631, Pierre Gassendi was able to observe the transit of Mercury via the use of a telescope. In 1639, Giovanni Zupi used a telescope to discover that Mercury had orbital phases similar to Venus and the Moon. Zupi's observations were able to conclude that Mercury orbited close to the Sun.

In 1800, Johann Schröter made observations of the surface of Mercury, claiming to have seen a 12-mile-high (20 km) mountain. Friedrich Bessel used Schröter's estimates to erroneously conclude that Mercury had a rotational period of 24 hours. In the 1880s, Giovanni Schiaparelli was able to map the planet more accurately, and conclude that Mercury had a rotation period of 88 days, the same as its orbital period due to tidal locking.

In June 1962, Soviet scientists led by Vladimir Kotelnikov became the first to bounce a radar signal off Mercury, and receive it. This was the beginning of radar-based observations of planet Mercury. Three years later in 1965, the Americans Gordon Pettengill and R. Dyce used a radar telescope to conclusively show that Mercury has a rotational period of around 59 days. Italian astronomer Giuseppe Colombo noted that Mercury's rotational period was about two thirds of its orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance. Data from *Mariner 10* confirmed this to be true.

### Research with space probes

The first spacecraft to visit Mercury was NASA's *Mariner 10* probe (1974-1975). The spacecraft used the gravity of Venus to "slingshot" itself towards Mercury, being the first spacecraft to use this method. *Mariner 10* provided the first close-up images of the planet, immediately showing the planet's heavily cratered nature. Special geological features discovered on Mercury's surface, such as scarps, were later attributed to the planet shrinking slightly as its iron core cools. Unfortunately, the same face of the planet was lit too brightly in order for *Mariner 10* to get closer. This resulted in only around 45% of the planet's surface being mapped.

The spacecraft made three close approaches to Mercury, the closest of which brought it within 203 miles (327 km) of the planet's surface. On the first approach, a magnetic field was discovered to the surprise of scientists. It was originally thought that Mercury's rotation was far too slow to generate a significant magnetic force. The second approach was mainly used for imagery, but on the third approach extensive magnetic data was gathered. The data revealed that the planet has a magnetic field just like Earth's, which deflects solar wind from around the planet.

On March 24, 1975, only eight days after making its final close approach, *Mariner 10* ran out of fuel. Because its orbit could no longer be controlled, missing controllers instructed the probe to shut down. *Mariner 10* is thought to still be orbiting the Sun, passing near Mercury every few months.

A second NASA mission to Mercury, named *MESSENGER*, was launched on August 3, 2004. It made a fly-by of Earth in August 2005, and of Venus in October 2006 and June 2007 to place it onto the right trajectory to orbit Mercury. The first fly-by of Mercury occurred on January 14, 2008, the second on October 6, 2008, and the third on September 29, 2009. Most of the surface that was not imaged by *Mariner 10* was mapped during these fly-bys. On March 18, 2011, the probe successfully entered an elliptical orbit around Mercury. The first orbital image of Mercury was obtained on March 29, 2011. The probe finished a one-year mapping mission in 2012, and afterwards the mission was extended into 2013. In addition to Mercury, *MESSENGER* observed the 2012 solar maximum. The probe's final maneuver around Mercury occurred on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015. The impact left a crater estimated to be 52 feet in diameter (16 meters).

The European Space Agency and the Japanese Space Agency launched a mission to Mercury called *BepiColombo* with two probes: one to map the planet and the other to study its magnetosphere. The probe launched on October 20, 2018, and is scheduled to reach Mercury in 2025.

## Trivia

* Mercury orbits so quickly around the Sun, that some ancient civilizations believed that Mercury was a star.
* In the 19th century, it was believed that a planet named Vulcan existed between Mercury and the Sun. Despite multiple claimed observations, no definitive evidence of the planet was ever found.
* The orbit of Mercury played in important role in proving Albert Einstein's theory of General Relativity.
* Mercury has a rather large core, taking up around 40% of the planet's mass.
* Mercury's magnetic field is very weak, only about 1% as strong as Earth's.
* Despite the fact that Mercury is the closest planet to the Sun, it is only the second hottest planet. The planet with the hottest temperatures is Venus.