A guide to the planets: Mercury

Mercury is the smallest of the planets in our Solar System and the closest to the Sun.

Image of Mercury taken by the MESSENGER spacecraft. Credit: NASA/JPL-Caltech

What is Mercury like?

Mercury, the closest planet to the Sun, is a desolate world. The planet has almost no atmosphere, meaning there’s no protection from the Sun’s intense radiation.


As the planet is almost tidally locked and takes 58 days to rotate once.

It’s 176 days from one sunrise to the next, meaning the Sun bakes the surface of Mercury for months at a time.

With no active volcanism renewing the rock, the planet is now covered in impact craters, leaving no doubt that Mercury is a dead world.

It’s the smallest of the major planets – even moons Ganymede and Titan are larger – but Mercury’s heart hides a heavy secret.

While most planets have a modest iron core at their centre, Mercury’s is thought to be enormous, spanning 85 per cent of its radius.

The planet Mercury
The planet is usually grey, but the colour has been enhanced in this MESSENGER image. The changes in colour come from mineralogical difference across the surface, while the bright spots show impact craters.
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury facts

  • Diameter: 4,879km (0.38 times Earth)
  • Mass: 330 billion trillion kg (0.05 times Earth)
  • Distance from the Sun: 57.9 million km (0.39 AU)
  • Length of day: 176 days
  • Length of year: 88 days
  • Number of moons: 0
  • Average temperature: 167ºC
  • No of spacecraft visitors: 2
  • Planet type: Rocky

How can I observe Mercury?

Mercury is an interior planet, meaning it orbits closer to the Sun than Earth. Though you can see Mercury during the day, it will always appear near the Sun.

Looking directly at the Sun can seriously damage your eyesight so we strongly recommend only looking at Mercury during twilight – either before the sunrises or after it sets depending on when the planet is up in the sky.

Mercury is visible with the naked eye, but looking through binoculars and telescopes should reveal the planet’s phases, depending on where it is in relation to the Sun.

From where does Mercury get its name?

The planet’s name comes from its rapid movement across the sky. The Roman god Mercury, Hermes in Greek, was renowned for his swiftness as he delivered messages between the other gods.

Bepi Columbo is a joint mission between the European and Japanese Space Agency to investigate Mercury.
Bepi Columbo is a joint mission between the European and Japanese Space Agency to investigate Mercury.

How have we explored Mercury?

Mercury’s closeness to the Sun means spacecraft have to be specially built to withstand the heat without melting. So far, only two space probes have visited the planet, both from NASA.Mariner 10 flew past the planet three times in 1974 and 1975, giving a first glimpse of the planet.

It was another 40 years before the planet got another visitor, when MESSENGER orbited Mercury between 2011 and 2015, investigating the world’s geology, magnetic field and chemical composition.

A third mission, ESA’s BepiColumbo probe, is currently making its way to the planet, but won’t arrive until 2025.


Explored by: Mariner 10 (1974-1975, NASA); MESSENGER (2011, NASA); BepiColumbo (2025, ESA)