Facts About Mercury

Mercury compared to Earth

Above: The size of Mercury compared to Earth (Credit: NASA).

Quick Fact File


4,876 km - the diameter (3,030 miles) of the planet.

2,440 km - the equatorial radius (1,516 miles).

38% - comparison in size to Earth.

15,329 km - the equatorial circumference (9,525 miles).

74,797,000 km² - the surface area (28,879,000 mi²).


70 million km - maximum distance from the Sun (43 million miles).

46 million km - minimum distance from the Sun (29 million miles).

57 million km - average distance from the Sun (35 million miles).

77.3 million km - closest distance to Earth (48 million miles).


88 days - the orbital period of the planet Mercury - the time taken to orbit the Sun in Earth days (87.969 days).

59 days - the rotation period - the time taken for the planet to rotate on its own axis (58.785 days).

38% - the gravity of Mercury expressed as a percentage of the gravity on Earth.

88 days - the length of a year on Mercury, in equivalent Earth days. It is the fastest orbiting of all the planets.

172,364 km/h - the speed that Mercury travels through space, relative to the sun (107,102 mph); it is the fastest travelling of the planets.

47.87 km/s - that speed expressed in terms of distance covered each second (29.75 mi/s).

480°C - the temperature (800°F) that the surface of Mercury can reach.

-180°C - the temperature (-290°F) that the surface can plummet to (Mercury has the most extreme temperature range of all the planets).

176 days - the period from one sunrise to the next.

40% - the approximate amount of Mercury's surface area covered by smooth plains, caused by lava flows.

4,000,000 km² - the area (1,500,000 ml²) of the North Pole of Mercury covered by plains, an area equivalent in size to around half of the USA.

650 km - the length (400 mi) of the longest cliff on Mercury, called Discovery Rupes. Rupes are cliffs, most likely formed by the cooling and contracting of iron ore as the planet formed, which then caused areas of the outer crust to push upwards.

2 km - the height (1.2 mi) of Discovery Rupes and the highest point.

The cratered surface of Mercury, recorded by Messenger in 2008

Above: The cratered surface of Mercury, recorded by Messenger in 2008 (Credit: NASA, JHU APL, CIW).

1,300 km - the diameter of the Caloris Basin, the largest basin on Mercury, likely formed by a major asteroid Impact early in the planet's existence.

1,000 km - the distance (600 mi) that some of the Caloris Basin debris ('ejecta') spread after the impact.

3,600 km - the width (2,237 mi) of the liquid iron core of Mercury.

Did You Know?

Scientists have discovered the core to be liquid rather than solid, by using radio waves bounced off the surface. These revealed a wobbly, rather than rigid, rotation, which indicated the existence of liquid at the centre of the planet.

61% - the amount of Mercury's total volume represented by the core (by comparison, the Earth's core represents 17%).

600 km - the depth (400 mi) of the semi-molten mantle that surrounds the core. It consists of silica rocks, similar to the mantle of Earth.

20% - the approximate amount of the planet's radius made up by the mantle.

100-300 km - the width (70-190 mi) of the crust, which consists of basalt and other silica rocks.

Did You Know?

Mercury has an atmosphere, but it is exceedingly thin, and called an exosphere. The planet may once have had an atmosphere as dense as that of our earth, eventually blown away by solar winds that were stronger than the small planet's gravity could resist.

1000 BCE - the approximate time of the first known sighting of Mercury, recorded in the ancient Babylonian Mul.Apin tablets (Mercury is visible to the naked eye around sunrise and sunset, and in the times before artificial light pollution it would have been clearer still). The Babylonians called the planet Nabu, named after their messenger god.

Did You Know?

With remarkable accuracy, and via a method unknown to modern historians, an Indian astronomer working almost 2500 years ago guessed the size of Mercury to within 99% accuracy. Recorded in Surya Siddhanta, he guessed the diameter to be 4,841 km (3,008 mi). It actually measures 4,879 km (3,032 mi).

Back to top

Missions to Mercury

2 - the number of missions that have been sent to Mercury; Mariner 10 between 1974-75 and Messenger between 2011-15. Mercury is the least explored of the rocky terrestrial planets in our solar system, although a third expedition, the joint European-Japanese Bepi Columbo mission, is due to launch in early 2017, and should arrive at Mercury sometime in January 2024.

Mariner 10 (1974-75)

3 - the number of times Mariner 10 passed the planet during its mission.

45% - the approximate surface of Mercury that was mapped by Mariner 10.

16 March 1975 - the date of Mariner 10's final flyby of the planet.

Messenger (2011-15)

03 August 2004 - the date Messenger was launched from Cape Canaveral, aboard a Delta II 7925 rocket.

7.9 billion km - the length of the trajectory that was required to slow Messenger enough to enter orbit around Mercury.

6 years, 7 months and 16 days - the time it took Messenger to reach orbital entry with Mercury.

14 January 2008 - the date of Messenger's first flyby of Mercury (this was followed by two further flybys on 06 October 2008 and 29 September 2009).

18 March 2011 - the date Messenger first entered orbit around Mercury, beginning data collection early the following month, on 04 April.

100,000 - the approximate number of images captured by Messenger during its primary mission, which ended on 17 March 2012 (an extension was then made to complete 2 further missions).

30 April 2015 - the date Messenger spacecraft sent its final image before crashing into the surface of Mercury, having finally run out of maneuvering propellant and succumbed to the pull of solar gravity.

$450 million - approximate cost of the Messenger mission.

Final image sent back from the Messenger probe, 30 April 2015

Above: The final image sent back from the Messenger probe. This image shows an section of the surface approximately 1 km (0.6 miles) across (Credit: NASA, JHU APL, CIW).

Back to top