Mercury

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Ask people what Mercury is and some will say that it's a planet while others will say that it's a shiny liquid metal that you see in thermometers. Both are right, of course, and the reason is that they share some characteristics. Mercury - the metal - is also known as quicksilver, because it flows just like water yet looks like silver, so it never stays put. Mercury the planet is very much the same. It's only visible for a week or two at a time, either in the evening or morning twilight, and then it's gone again. 

Mercury was a Roman god, and one of his jobs was to be the messenger for the other gods. So he wore winged shoes and helmet, and could flit about just like the metal or the planet.

The planet's great speed is due to it being the closest to the Sun. That also means it's hard to reach with spacecraft, or at least it is hard to stop once you've got there, because the Sun's gravity is constantly tugging the spacecraft into it. That's one reason why only three spacecraft have ever been sent there - Mariner 10 in the 1970s, Messenger, launched in 2004, and more recently BepiColombo. 

This is a joint venture between the European and Japanese space agencies. It's named after the Italian mathematician who helped NASA to plan Mariner 10's voyage. BepiColombo carries a camera plus instruments to analyse Mercury's surface and thin atmosphere. Following its launch in 2018 it has had to swing past Venus, Earth and Mercury itself several times in such a way as to reduce its speed before finally going into orbit around Mercury in 2024, taking over six years for a journey that otherwise might have taken only a few months.

We're used to Earth's 24-hour day, and 365-day year, but Mercury's day is quite different. Instead of the planet spinning 365 times in its year, it spins so slowly that one day, as measured from noon to noon, lasts two of its years. To make things more complicated, Mercury has a very elongated orbit around the Sun, so when it is closest to the Sun it moves much faster than when it is farthest away. This means that at its close points, the Sun's slow movement through the sky reverses for about an Earth week. From some places, you would see it rise, then appear to change its mind and set, then finally rise again. 

It wasn't until 1965 that astronomers realised this odd state of affairs, and before that they thought that Mercury's day was the same as its year, because many observations of Mercury showed the same side. It was Bepi Colombo, the man, who realised that the day and the year were locked into  a different cycle, and we only saw the same face every second orbit. 

Mercury is still a planet of mystery, and it will be a long time before we discover more about it.

Image credit: Mercury, as seen by the Messenger spacecraft.
Photo: NASA/JHUAPL/Carnegie Institution of Washington .