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The return to Mercury

Mercury has long been neglected. The closest to the Sun, its orbital distance is about 58 million kilometres, about one third of the Earth’s distance. Named after the Roman messenger-god, probably due to its fleeting appearances in the twilight skies, its brief appearances are partly due to its proximity to the Sun, confining visibility to morning or evening twilight. Also, its fast solar orbit (about 88 days) means it is never longer than a week in a good viewing position. (Warning – never use a telescope or binoculars while the Sun is above the horizon – instant blindness can occur). Visibility problems increase at higher latitudes like ours, where Mercury’s proximity to an often cloud-strewn horizon makes naked eye viewing a real challenge. The spring (evening) and autumn (morning) appearances of Mercury are the best times to see it, when the geometry of the Earth, Sun and Mercury ensures the planet is at its highest above the horizon, improving chances of spotting it. Knowing the times and dates of its appearance is essential to have a chance of seeing this elusive planet. This information is found in most astronomical journals and web sites. (You may also contact the club for help). After that, you pick a spot with a good view of the appropriate horizon and hope for clear skies! These difficulties have also plagued telescopic studies of the Mercury’s surface. Until the space age, the best telescopic images of the planet’s surface were vague and patchy with little verifiable detail. The first clear view came with NASA’s Mariner 10 probe.

Launched in late 1973, Mariner 10 was the first interplanetary probe to use the gravitational ‘slingshot’ method to modify its speed and direction, using Venus, whose clouds it photographed in ultraviolet in unprecedented detail. In one year it made three close passes of Mercury, imaging about half its surface. The surface was surprisingly like the moon, with pockmark evidence of meteor impacts everywhere. One major impact feature emphasised its violent past: the Caloris Basin, about 1350 km across, is the largest known crater in the solar system. Another hint of this past was found in Mercury’s unusually large iron core, about 42% of the planet’s volume (compared to the Earth’s 17%), with a significant magnetic field. It is thought that Mercury was once twice its present size, until a collision with a smaller planet nearly destroyed it billions of years ago, leaving the remnant we see today, with the original core intact.

NASA’s successor to Mariner 10, MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging), was launched in 2004. Using complex gravity assists, it will arrive at Mercury in 2011, observing much more of its surface in much greater detail from orbit, furthering the discoveries of its predecessor. A Japanese/European probe, BepiColombo, scheduled for launch in 2013, will examine the planet from orbit in 2019.

Long neglected by earthlings, the planet with midday temperatures over 400°C has finally come in from the cold!