Mercury Makes History

On the 18th of March, something new will happen.

The MESSENGER probe will make orbit around the planet Mercury. This will be a historic event, as it will be the first man-made object ever to orbit this planet.

Mercury is the solar system’s innermost planet. In appearance, as you can see above, it’s somewhat similar to the Moon. Unile the Moon, however, it doesn’t have any maria. Mercury is also a lot more massive, weighing in at just under 6% the mass of the Earth, rather than the Moon’s 1.2%. Mercury is also the second most dense planet in the solar system, with an average density of 5.4 grams per cubic centimetre. (Mercury is narrowly beaten by Earth, with an average density of 5.5 grams per cubic centimetre.)

I mentioned that Mercury is denser and more massive than the Moon. Unlike the Moon, Mercury is believed to possess a very substantial iron core, perhaps as much as 3,600 km across. That’s right – Mercury’s heavy-metal core is bigger than our Moon! In fact, Mercury as a whole is only about 4,900 km across. One indication of the core is Mercury’s high density. Another is that it possesses a weak magnetic field.

Mercury is also very much a world of extremes. It has polar craters so deep the sunlight never reaches their bottoms – these may have temperatures as low -170 Celsius, and there is some evidence for deposits of ice at their bottoms. But by contrasted, the subsolar plains are blasted by the furnace heat of the Sun – there temperature soar to as high as 430 Celsius. The planet’s surface is marked by deep craters and by mountains, some of them soaring kilometres into its airless sky.

Mercury also occupies an interesting place in the history of physics. It has helped serve as a timely reminder that the Universe is rarely as well-behaved as we’d like to imagine.

Mercury posed a puzzle for 19th Century astronomers. They noticed something odd about the planet – its perihelions didn’t quite line up. Each time it orbited the Sun, the planet was failing to return to the same place! In fact, there was a steady drift in the positions of the perihelions. Each new perihelion was slightly ahead of the preceeding one. The amount was small, granted, but it refused to go away on repeated measurements. This was a puzzle, as the existing theories of Newtonian gravity required the perihelions to behave themselves!

The answer came early in the 20th Century, with Einstein’s publications on relativity. The General Theory of Relativity shows that gravity is actually a bit more complicated than Newton’s formulation. Newton’s formulations work perfectly fine in situations where gravity is reasonably weak, of course, and even for Mercury they work fairly well. However, Mercury is the closest planet to the Sun, and it feels the Sun’s influence the most strongly. According to general relativity, gravity emerges from the warping of space by massive objects. This warping of space causes a deviation from purely Newtonian behaviour; the bigger the mass, the greater the deviation. Mercury is close enough to the Sun that its orbit is measurably non-Keplerian. Quite simply, the space through which Mercury moves is more distorted than the space through which Earth revolves!

Sure enough, a relativistic solution for Mercury’s orbit predicts its behaviour much better than a purely Newtonian one. This was a great vindication for relativity!

Mercury’s closeness to the Sun has other consequences. One of them is that it’s probably one of the most difficult planets to send probes to. That close in, the Sun has a tendency to snag any passing object and not give it back! The energy cost to send an object into the inner system is actually greater than it is to send the same object to the outer planets.

Also, the region of space in which something can be put in orbit around Mercury – the region where the planet’s gravity dominates over the Sun’s – is much smaller than it would be if it were further out. There is a sense of threading a needle about the whole affair. The trajectory for MESSENGER has involved a series of fly-bys with Earth and Venus and later on with Mercury itself, all necessary before the probe could be eased into orbit.

The whole process puts me rather in mind of a heavy bucket on a fraying rope, being lowered gingerly and carefully into a deep well. This is partly why Mercury has only ever been visited once before – a handful of fly-bys by the Mariner 10 probe, before the Sun permanently snatched it away.

Mercury’s closeness to the Sun also makes it difficult to observe from Earth. It’s often lost in the twilight, or even the glare of the Sun itself. Of all the ‘classical’ planets, the ones known from Antiquity, it’s the hardest to see.

However, for UK viewers at least, there may be an appropriate opportunity to do so. Mercury is currently approaching its maximum ‘elongation’ – it’s furthest apparent distance from the Sun. On a clear evening, it will be visible after susnet in the south-eastern sky this month. It will achieve maximum elongation on the 23rd of March. In fact, on the 16 of March, it will have an apparent alignment with Jupiter, being positioned two degrees north of the planet. If you’ve never seen it before, this will be an excellent opportunity to do so. Two degrees is about four times the apparent width of the Full Moon, so find Jupiter – a nice, bright, recognisable object – and look about that far straight up. That should take you straight to Mercury.

You might also want to give it a go on the night of the 18th, when MESSENGER makes orbit.


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