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Planet Mercury

Mercury has the shortest and fastest orbit around the Sun and experiences dramatic temperature changes as it rotates. It is a world of extremes.

Explore facts about our solar system's fastest planet.

Mercury facts

  • Equator circumference: 15,329km
  • Radius: 2,440km
  • Average distance from Sun: 58 million km
  • Surface temperature: -180°C to 430°C
  • Day length: 59 Earth days
  • Year length: 88 Earth days
  • Average orbital speed: 170,500km/h (47km/s)
  • Moons: 0
  • Planet type: terrestrial

How big is Mercury?

Mercury is slightly larger than our Moon - 15,329 kilometres around its equator. Its radius, the distance from the core's centre to the surface, is 2,440 kilometres. Mercury is about 2.6 times smaller than Earth.

How hot is Mercury?

Mercury's sun-facing side is scorched by temperatures of around 430°C, hot enough to melt lead.

Without a substantial atmosphere to distribute heat away from the areas facing the Sun, the planet's slow rotation makes for stark differences in temperature between its dark and light sides. The side facing away cools to an approximate -180°C.

But despite the intense heat the planet faces as it rotates, areas that are permanently shaded, such as some polar craters, may hold deposits of ice.

Intense changes in temperature from day to night make it impossible for life as we know it to flourish on Mercury.

Although it is the closest planet to the Sun, Mercury isn't the hottest in the solar system. Venus's dense atmosphere causes a greenhouse effect, resulting in higher temperatures.

Falsely coloured image of Mercury

This artificially coloured image of Mercury shows the chemical, mineralogical and physical differences on Mercury's rocky surface © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Via Flickr (CC BY 2.0)

How far from the Sun is Mercury?

Mercury's orbit is elongated, taking an almost oval- or egg-shaped course around the Sun. This means that its distance from the Sun varies throughout its circuit, between approximately 46 million and 70 million kilometres.

Mercury speeds around the Sun at nearly 47 kilometres per second - almost 60% faster than Earth's orbiting speed. 

How long is a day on Mercury?

Days on Mercury are very long because the planet rotates very slowly. One day-long spin lasts for 59 Earth days. But because of its fast orbit, one Mercury year takes 88 Earth days. This means that two years on Mercury lasts for only three days.

The planet's sunrise would be unusual to us on Earth. Due to its elongated orbit and slow rotation, from some places on Mercury's surface, the Sun appears to rise briefly before setting and rising again. At sunset, the reverse happens, with the Sun appearing to set twice.

The planet spins almost vertically on its axis, so its poles are never fully sunlit. The lack of tilt also means that the planet does not experience yearly seasons like Earth.

This model projects what the northern pole of Mercury may look like. The planet's vertical spin means that the pole is never fully sunlit. © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Via Flickr (CC BY 2.0)

What is Mercury's atmosphere made of?

Mercury has little atmosphere, but what it does have is made up mostly of oxygen, sodium, hydrogen, helium and potassium.

The thin atmosphere, or exosphere, does little to prevent or slow down impacts from extraterrestrial objects, leaving the surface littered with craters. In appearance it is similar to Earth's Moon.

The planet is terrestrial, meaning that it has a solid surface and is mainly made of silicate rocks or metals. The planet Mercury is mainly made up of iron.

Mercury has no moons. The planet's close proximity to the Sun makes having moons impossible, as the star's strong gravitational pull would likely pull them out of the planet's orbit. 

The cratered surface of Mercury © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Via Flickr (CC BY 2.0)

Who discovered Mercury?

Mercury is one of the five classical planets visible with the naked eye and is named after the swift-footed Roman messenger god. It is not known exactly when the planet was first discovered - although it was first observed through telescopes in the seventeenth century by astronomers Galileo Galilei and Thomas Harriot.

Only two spacecraft, both robotic and launched by NASA, have visited Mercury so far, making it the least explored terrestrial planet. Mariner 10 launched in 1973 and imaged around 45% of the planet's surface from 1974 until its end of mission in March 1975.

MESSENGER was launched in 2004 and entered Mercury's orbit in 2011, making it the first spacecraft to do so. This mission lasted for four years before the probe succumbed to the planet's gravity and impacted its surface in 2015. It collected extensive data and images and transmitted them back to Earth to be studied by scientists.

BepiColombo is a joint space mission by the European Space Agency (ESA) and Japan Aerospace Exploration Agency (JAXA), currently proposed to be launched in 2018, with the aim to reach Mercury in December 2025.