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Mercury, the Closest Planet to the Sun

The Smallest Planet in the Solar System

Mercury is the smallest planet in the solar system, measuring only about 3,032 miles across, barely larger than Earth's own moon. It also orbits closer to the sun than any other planet, sitting an average of about 36 million miles away, close enough that a year on Mercury, one full orbit, takes just 88 Earth days.

Despite its speedy orbit, Mercury spins very slowly on its axis, completing one full rotation in about 59 Earth days, which means the sun rises and sets much more slowly on Mercury than the length of its own year would suggest.

A Surface Covered in Craters

The round craters covering Mercury in this scene reflect a real surface shaped by billions of years of asteroid and comet impacts, largely unchanged because Mercury has almost no atmosphere to erode them away. The largest known crater, called the Caloris Basin, stretches about 960 miles across, big enough to hold the entire state of Texas.

Without a thick atmosphere to trap heat, Mercury swings between extreme temperatures, reaching around 800 degrees Fahrenheit on its sunlit side during the day and dropping to about minus 290 degrees Fahrenheit at night, one of the widest temperature swings of any planet.

Ice Hiding in Permanent Shadow

Even though Mercury sits closest to the sun, patches of water ice have been found frozen inside deep craters near its poles, protected from sunlight because those crater floors never tilt enough to catch direct sunlight. NASA's MESSENGER spacecraft confirmed this ice in 2012 using radar images and neutron detectors, surprising scientists who once assumed the closest planet to the sun would be far too hot for any ice to survive.

Mercury has no moons at all, making it one of only two planets in the solar system, along with Venus, that orbit the sun without a single natural satellite of its own.

Missions to the Sun's Nearest Neighbor

NASA's Mariner 10 became the first spacecraft to visit Mercury, flying past it three times between 1974 and 1975 and mapping about 45 percent of its surface. It took more than three decades before another mission arrived: NASA's MESSENGER spacecraft entered orbit around Mercury in 2011 and spent four years mapping the entire planet in detail before intentionally crashing into its surface in 2015.

The European and Japanese BepiColombo mission launched in 2018 and is expected to reach Mercury in the coming years, using a complex path of flybys past Earth, Venus, and Mercury itself to slow down enough for the sun's powerful gravity to allow a safe orbit insertion.

Mercury's thin, wispy atmosphere is called an exosphere rather than a true atmosphere, made of atoms knocked loose from the surface by solar wind and meteor impacts rather than gases held in place by gravity. That exosphere is so sparse that its total mass would fit inside a modest cargo truck, far too thin to trap any of the daytime heat that quickly radiates away once the sun sets. Future missions may study whether Mercury's iron-rich core, unusually large for its size, still generates enough heat to keep part of that core molten today.