Mariner 10 has been the only spacecraft to approach the planet (2006), during the three flybys in February and September 1974, and again in March 1975. Here we have gained most of our meagre knowledge about Mercury. Disappointingly, this spacecraft saw only 45% of the entire surface. Planetary astronomers also detected its magnetic field that hints of a large iron core around 70% of the planetary mass. In 1994, Earth-bound radar observations have also suggested that within some craters of Mercury are ancient ice sheets near the north and south poles that have presumably never seen sunlight.


The next interplanetary visit to Mercury will be by the NASA 1.1 tonne Messenger spacecraft. (MErcury Surface, Space ENvironment, Geochemistry, and Ranging.) Launched on 03rd August 2004, this lengthy roundabout trip mission will achieve final orbit around Mercury in March 2011. Many gravity assists via planetary encounters are required to make the correct trajectory for the rendezvous and final orbit. By 2011, Messenger should have orbited the Sun fifteen times, approached the Earth once (August 2005), Venus twice (24th October 2006 and June 2007), and is planned to pass Mercury three times (January and 6th October 2008 and September 2009). The first of Messenger encounters happen on the 14th January 2008 and for the first time viewed the majority of the part of the planet missed by Mariner 10. This first fly-by revealed improved images from the far superior cameras, showing a heavily cratered surface that at first glance seems more like the Moon. Several close-up images were obtained showing many crater and various geological and impact features. Readers here might like to look at the N.A.S.A. Messenger Website, especially as further details and images comes to hand.

mercury mess
Fig. 2.: Image of Mercury Taken by Messenger
During Its First Fly-By
on 14th January 2008

Credit: NASA/Johns Hopkins University
Applied Physics Laboratory/Carnegie
Institution of Washington.

From September 2009, this dedicated mapping mission will explore the surface of Mercury for a whole year. It will analyse the planetary magnetic field, mineral deposits, composition and surface geology. Like Earth, we believe Mercury has a highly dense metallic core, which makes up more than 70% the diameter of the planet.

Another important goal is to examine Mercurys very thin gaseous atmosphere discovered by Mariner 10. The cause of the thin atmosphere is likely the intense solar radiation striking the surface material. It seems to be composed of only five main elements; being oxygen, hydrogen, neon, sodium and potassium. The actual process is presently not well understood.

Other aims are to look for evidence in the polar regions of ice in craters not exposed to the solar radiation. Investigations will also explore the effects of high temperature on the surface rocks, and if these have been transformed into heavier and denser metallic materials.

BepiColombo and Gaia

Also currently another Mercury dedicated spacecraft, which is now the final planning and construction stages by the European Space Agency (ESA). This twin spacecraft mission is named BepiColombo and Gaia, and is scheduled for launch in August 2013, However, this ion-propelled mission will not reach the innermost planet until September 2019. This dual mission has two quite different specialised spacecraft, are presently known as the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO), and individually weigh 500kg and 250kg, respectively. This advanced spacecraft will investigate for a year or two the nature of Mercury surface from the polar orbits of 400×1500 and 400×12 000 kilometres. Like NASAs Messenger, these two craft have to have there instruments carefully protected from the powerful energies and high temperatures so close to the Sun.


Telescopically, Mercury appears like a tiny featureless rosy-red disk. Like Venus, observations are best made in the daylight hours on the local meridian. By offsetting the telescope from the Sus position or using setting circles can make finding Mercury easy with a little practice. Should you use either of these methods, use general care and safety precautions as there is the real risk of accidentally exposing the optics of the main telescope to the rays of the Sun. Amateurs sometimes can estimate the observed time of the half phase or dichotomy but this requires sizeable apertures and reasonably good seeing conditions. Some have claimed to see odd-looking greyish features from time to time, but these are certainly contrast or optical effects.

NEXT PAGE : Transits of Mercury


Last Update : 1st December 2012

Southern Astronomical Delights © (2012)

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