Mercury
Closest to the Sun and fastest in our solar system
Overview
Mercury is the smallest and innermost planet in the Solar System. It completes an orbit around the Sun every 88 Earth days, and it's named after the Roman deity Mercury, the messenger of the gods.
Like Venus, Mercury orbits the Sun within Earth's orbit, making it appear in Earth's sky only as a "morning star" or "evening star" near sunrise or sunset. Though it can be seen with the naked eye, it is never easy to observe due to its proximity to the Sun.
Characteristics
Mercury is a rocky planet with a solid surface covered with craters. It has no natural satellites and no substantial atmosphere. However, it does have a thin exosphere containing hydrogen, helium, oxygen, sodium, calcium, and potassium.
Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system—that title belongs to Venus due to its dense atmosphere. However, Mercury does experience the most extreme temperature variations, ranging from 430°C (800°F) during the day to -180°C (-290°F) at night.
Exploration
Mercury has been visited by two spacecraft: Mariner 10 in 1974-1975 and MESSENGER, which flew by Mercury in 2008 and 2009 and orbited the planet from 2011 to 2015.
The BepiColombo mission, a joint project between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), is currently en route to Mercury, with planned orbital insertion in 2025.
Interesting Facts About Mercury
Fastest Planet
Mercury orbits the Sun at about 47 km/s (29 mi/s), making it the fastest planet in our solar system.
Smallest Planet
With a diameter of 4,879 km (3,032 mi), Mercury is the smallest planet in our solar system.
Extreme Temperatures
Mercury has the most extreme temperature fluctuations in the solar system.
Dense Core
Mercury has a massive iron core that makes up about 75% of its radius.
Size Comparison
Mercury is significantly smaller than Earth. Here's how they compare:
Diameter: 4,879 km
Diameter: 12,742 km
Mercury is only about 38% the diameter of Earth, making it only slightly larger than Earth's Moon.