Published Apr 06, 2013 07:00am

Mercury, the fastest messenger

Planet Mercury, the very first planet out from the sun, has been a source of mystery and awe, for a long time. Even the invention of telescope by Lippershey, a Dutchman in 1609 (and then the astute observation of many astronomical objects by Galileo Galilee in 1610), and later on, the invention of camera in 1840, did not help resolve the mystery held out by this planet as would be expected today. It is because of the elusive nature of the wily planet which does not lend itself generously to observation. Curiously, it is believed that Copernicus (1473-1543), the Polish monk who first demonstrated that planets go around the Sun and not the Earth, never observed the planet once throughout his astronomical career, for the difficulties it presented in observation. Although I find it hard to believe. I have seen it many times and enjoyed it each time I had the occasion to do so. I even photographed it once passing in front of the Sun (called transit; more about it later). Staying close to the brilliantly incandescent star, its outreach is a mere 46 million kilometres at its closest to the Sun (called Perihelion in astronomy). Then it is outrageously within a flirtatious handshaking distance from the explosive star. At its farthest in its orbit around the Sun (called Aphelion) it is 69.8 million km from the sheppard star. This makes its orbit highly eccentric, ie highly elliptical, or, far from being round or circular. Here it is worthwhile to remember that nothing in space moves in perfect circles but in an elliptical or elongated orbit. The fastest planet, Mercury needs just 88 days to go around its giant neighbour once (or make a complete one orbit). That is why the Romans named it as the fleet-footed messenger of gods. By being the closest to the Sun, Mercury suffers the widest extremes of temperature: at noon when the Sun is directly overhead, the temperature may soar to 470-480C; while during the long Mercurian night it can plunge to below -175C. The diameter of the planet is 4,878km, which means that after Pluto it is the smallest of all planets. But since Pluto has been relegated to the planetoid category, Mercury now is the smallest of them all. Although its existence has been known since the dawn of history — and it can be brighter than the brightest star (ie Sirius, the lead star in the constellation, Canis Major, or the Big Dog lying just close to the lovely constellation, Orion of the winter skies), Mercury is rather difficult to observe. It is because it always stays in the close proximity of the Sun, in near total brightness and searing heat, setting in the west soon after sunset, and rising in the east just before sunrise. A little delay in observation, or some haze and wispy clouds to obstruct a clear view of horizon and it’s gone! However, with some effort — and perseverance — you can still observe it shining like a beacon close to the horizon either before the sunrise or after the sunset. One interesting thing about Mercury is its rotational period, or the length of its day; only in the 1960s it was discovered from radar measurements that it takes 58.65 earth days to make one complete rotation on its axis, so that its ‘day’ is two thirds of its orbital period, or one year! Mercury’s magnetic field is very weak — only about one per cent of the Earth’s. It has a large iron core which may have cooled and perhaps solidified by now because of the planet’s small size. The presence of a magnetic field suggests that at least a portion of the deep interior must still be liquid, and moving along gently, in placid circles in keeping with the speed of the planet itself. In March 1974, the first spacecraft to venture near Mercury, to within 703km of the planet, and later on in three more encounters during 1974-75 sent much of the information we have about the planet. Some of the remaining information is the result of deductions and sound scientific logic about basic rules applicable in space when we come to deal with the interior (cores) of planets. For instance, a planet can sustain a magnetic field if it has an electrically conductive liquid interior, and rotates rapidly on its axis which is not in keeping with reality in the case of Mercury. The surface of Mercury is heavily cratered from the impacts of meteorites, asteroids and comets, with many over 200km wide. The largest feature is the Caloris Basin, measuring 1,300km across. Stunning, isn’t it? All craters on Mercury are old, very old. They have lain undisturbed on its surface for billions of years. That is because of total absence of erosion of any kind, although curiously some craters are surrounded by lighter coloured ejecta — material splashed out by impacts. In my opinion, the only semblance of erosion on Mercury is the dust kicked up by asteroids, etc, as they land on its surface with a bang. Eventually the dust and small rocks spread all over and settle on the surface nearby. The desolate Mercury has a diameter of 4,878km, as you know. A comparison with some others would make an interesting study: Earth’s diameter is 12,756km, Moon’s diameter is 3,475km, and that of Pluto is around 3,000km. Another very interesting thing about Mercury is that every time it completes an orbit around the Sun, it is not at the same place as the last year (or 88 days before)! It moves away a little, and changes its position, which can be determined from the background stars. It will be found to have moved away a little. Although the average distance may not change, the position does. Which means that its elliptical orbit is gradually swinging around the Sun as it moves at the fantastic speed of 200,000km per hour. There is much more to the planet Mercury than just stated herein. In order to complete our discussion on this, the ‘messenger of gods’ let us wait until the next issue!

The writer is a professional astronomer and a former head of PIA Planetaria. He can be reached at

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