Mercury

Mercury is the smallest planet in our solar system and the closest one to the Sun. It's a small, rocky, heavily cratered world that looks a bit like our Moon. With almost no atmosphere to hold heat, Mercury is the planet with the biggest temperature swings in the whole solar system. Its daytime side bakes at 430°C while its night side plunges to −180°C.

• Position 1st planet closest to the Sun
• Distance from Sun 58 million km 0.39 times Earth's distance
• Diameter 4,879 km 38% of Earth's diameter
• Day length 59 Earth days one of the slowest spins
• Year length 88 Earth days shortest year of any planet
• Moons 0 no moons or rings

Where Mercury sits

Mercury orbits the Sun at an average distance of about 58 million km. That's roughly a third of the distance from the Sun to Earth. Mercury whips round the Sun faster than any other planet, completing one orbit in just 88 Earth days. That's where its name comes from. Mercury was the Roman messenger god, famous for being quick.

The weirdest day in the solar system

One of Mercury's strangest features is the relationship between its day and its year.

Mercury rotates on its axis once every 59 Earth days. But it goes around the Sun once every 88 Earth days. So a Mercurian year is only one and a half Mercurian days long. If you lived on Mercury, the Sun would rise once, take nearly six months to crawl across the sky, and set once, in just two Mercurian years.

Mercury's cratered surface

Mercury's surface looks a lot like the Moon. It's grey, dusty, and covered in millions of impact craters from billions of years of asteroid bombardment. Because Mercury has almost no atmosphere and no weather, these craters never erode. Once made, they stay almost forever.

The Caloris Basin

The biggest crater on Mercury is the Caloris Basin, which is roughly 1,550 km wide. That's about the size of Greenland. It was made by an ancient asteroid impact so huge that shock waves travelled right through the planet and shattered the crust on the opposite side, creating an area now called the "Weird Terrain".

Ice at the poles

Despite Mercury's scorching daytime temperatures, there is actually water ice hiding at its poles. The bottoms of some deep craters at the north and south poles never see sunlight because Mercury barely tilts on its axis. Inside those permanent shadows, temperatures stay below −170°C, cold enough for ice to survive.

Mercury's tiny atmosphere

Mercury barely has an atmosphere at all. What little there is gets called an "exosphere" and is made of atoms blasted off the surface by the solar wind (a stream of charged particles from the Sun) or by tiny meteorite impacts. The exosphere is so thin that you could call Mercury a vacuum.

Without an atmosphere to spread heat around or hold it in, the day side cooks and the night side freezes. There's no wind, no rain, no weather of any kind.

Missions to Mercury

Only two spacecraft have ever visited Mercury, mostly because getting there is really hard. The Sun's gravity pulls so strongly that to reach Mercury, a spacecraft has to slow down a lot. That actually takes more fuel than going to far-away planets like Jupiter.

• Mariner 10 (NASA, 1974-1975) was the first to fly past Mercury, photographing approx. 45% of its surface.
• MESSENGER (NASA, 2011-2015) was the first to actually orbit Mercury, mapping the entire planet and confirming the ice at its poles. It finally crashed into Mercury when it ran out of fuel.
• BepiColombo (ESA and JAXA) is on its way now. It launched in 2018 and is due to enter Mercury's orbit in late 2026.

Could humans visit Mercury?

Mercury would be one of the hardest places for humans to visit. The challenges are huge:

• The temperature extremes would destroy any normal spacesuit.
• The Sun is much closer, so its radiation is 11 times stronger than on Earth.
• There's no atmosphere to slow down a landing spacecraft.
• Getting there takes years, even with modern rockets.

Realistically, robots will continue to explore Mercury for the foreseeable future. Crewed missions are not currently being planned by any space agency.