A Rollercoaster Ride Around the Sun

Mercury, named after the super swift Roman messenger god, rarely gets a lot of attention. It’s the tiniest planet in our solar system and closest to the Sun. It’s just a bit bigger than Earth’s Moon.

Mercury in true color (by MESSENGER in 2008)

Mercury in true color (by MESSENGER in 2008)

Standing on Mercury, you would witness a Sun that appears three times larger than its Earthly view, casting sunlight seven times more intense. Nevertheless, the title of the hottest planet in our solar system goes to its neighboring Venus because of its thick atmosphere.

Mercury’s atmosphere is super thin, causing surface temperatures to be extreme. The days can reach a fiery 800°F (430°C). But because there’s no air to hold onto that heat at night, it can get icy cold at -290°F (-180°C).

The terrestrial planets of the Solar System: Mercury, Venus, Earth, and Mars (sized to scale)

The terrestrial planets of the Solar System: Mercury, Venus, Earth, and Mars (sized to scale)

Meanwhile, Mercury is the speediest planet. Its path around the Sun is like a stretched-out egg, and it races at almost 29 miles (47 km) per second. It has the shortest year of all the planets in our solar system—just 88 Earth days.

However, Mercury slowly spins around. It takes 59 Earth days to finish one turn. But when Mercury zooms fastest (and gets really close to the Sun), things get interesting. Unlike most planets, there’s no regular sunrise and sunset. Instead, the Sun seems to pop up, then go down, and then pop up again on some parts of Mercury’s surface. The opposite happens when the Sun sets. A complete day-night cycle (one solar day) takes 176 Earth days—just a bit more than two years on Mercury.

Animation of Mercury’s and Earth’s revolution around the Sun

Animation of Mercury’s and Earth’s revolution around the Sun

Mercury’s surface is all bumpy with craters because it doesn’t have a thick atmosphere to stop things from crashing into it. About 4 billion years ago, a massive asteroid, roughly 60 miles (100 km) wide, slammed into Mercury. That impact was like setting off a trillion 1-megaton bombs! It created a gigantic crater around 960 miles (1,550 km) wide called the Caloris Basin. You could fit the whole state of Texas inside it.

Caloris Basin (by NASA/Johns Hopkins’ Applied Physics Laboratory)

Caloris Basin (by NASA/Johns Hopkins’ Applied Physics Laboratory)

Mercury has no rings or moons, and its magnetic field is pretty weak. But it holds the record as the second densest planet, after Earth. It’s got this big metal core that’s about 85% of the planet’s size. This is interesting because Earth’s core is only 15% of its size, and scientists are still unsure why Mercury’s core is so much bigger.

However, studying Mercury is challenging because it takes more energy to send a spacecraft there than even to Pluto. Back in the 1970s, NASA’s Mariner 10 flew past Mercury three times, looking at the same side each time. It showed us the planet is full of craters and has a magnetic field. Later, in 2011 NASA’s MESSENGER became the first spacecraft to go into orbit around Mercury. It told us way more about the planet. MESSENGER finished its journey with a planned crash landing on Mercury in 2015.

The only Mercury mission happening now is BepiColombo, a team effort between Japan and Europe. It started in 2018 and will reach Mercury in 2025. BepiColombo wants to figure out why Mercury has such a giant core and how it gets a magnetic field.

Words of wisdom

“It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn’t feel like a giant. I felt very, very small.” —Neil Armstrong

“It is not true that people stop pursuing dreams because they grow old, they grow old because they stop pursuing dreams.” —Gabriel García Márquez

“Man is always prey to his truths. Once he has admitted them, he cannot free himself from them.” —Albert Camus

“To write well, express yourself like the common people, but think like a wise man.” —Aristotle


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