Mercury is the smallest planet in our solar system ever since Pluto got kicked out. Known to humanity since ancient times, it had many names in different languages. Its current name stuck with it because of the roman messenger god Mercury, who could move very fast. This is because the planet Mercury moves across the sky fast compared to other "stars" (they thought back then it was just another star).

Mercury. Bear in mind that it isn't actually blue. The colour enhances the the chemical, mineralogical, and physical differences between different rocks. Credit: NASA

Mercury is not only the smallest planet with radius of 2,439.7 km, but also the closest one to the Sun orbiting at distances between 46 million and almost 70 million kilometers. This proximity makes days on the surface really hot, 430 degrees Celsius. Since it is a rocky planet with no real atmosphere, nothing stops the surface from cooling down to -173 degrees Celsius at night. In these conditions, life as we know it can't be supported. Even so, its second hottest planet behind Venus.

The reason Mercury has such extreme temperatures lies in the fact that its days and nights last long. The planet completes its orbit around the Sun in 88 Earth days, while its day lasts 176 Earth days. This is interesting because rotation around its own axis actually lasts some 59 Earth days, but because of gravitational resonance, if you were on the surface of the planet, you'd see the sunset once every 176 Earth days. You may have noticed that it's exactly 2 Mercurian years for each Mercurian day even though rotation lasts only 2/3 times the year. If you are confused, check the image below. It should make it clearer why is this so.

Mercury has a 3:2 spin–orbit resonance meaning that it spins around its axis three times for every two revolutions around the Sun. Credit:

Oh! One more thing about Mercury's orbit. The planet doesn't just revolve around the Sun – its orbit actually rotates, and since it's not a perfect circle but rather an ellipse, it looks like rosette if looked at from above. You can get the idea by checking on the image below. Have in mind that in reality, the effect is not this extreme and the planet's orbit is not this elliptical. This is something that Newton's gravity theory couldn't explain, so after Einstein introduced his theory of general relativity, he used Mercury's strange orbit to prove the theory is correct.

Mercury's orbit rotates itself, so over time it makes a rosette-like patterns.

Only two satellites visited Mercury – the Mariner 10 in 1975 and MESSENGER in 2008. The third mission, BepiColombo, is scheduled to launch in 2016. Mercury itself has no natural satellites.

Image of Mercury taken by MESSENGER. Credit: NASA

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