Mercury is the nearest of the planets to the Sun, and the smallest (8th) in size, zipping around the Sun in just 88 days. Its surface has a 600°C variation (c. 100K (-173°C) to 700K (427°C)), as it rotates very slowly, with a day two Earth months long!
Baked slowly on its orbital spit, Mercury is hot and barren. Mercury has pyroclastic vents, and it still collecting fresh craters from meteoroid impacts. This image is colour enhanced to reveal its chemical diversity, following an indepth study over 4 years by the orbiting Messenger satellite, which ended its journey when NASA decided to let it crash into the surface in April 2015, after running out of propellant.
Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Flying to Mercury means travelling two-thirds of the way towards the Sun. Therefore, a spacecraft gains a lot of kinetic energy as it travels to Mercury, requiring it to brake to enter into orbit around the planet, or fly by. This requires a great deal of fuel (in fact less fuel is needed to send a probe out of the solar system!).
Launched in 2004, this NASA probe was the first and only to enter orbit around Mercury. From March 18, 2011, till April 30, 2015, Messenger scanned the surface of Mercury, until it was crashlanded, as planned, on April 30, 2015. Its mission included two fly-bys of Venus, and to study the chemical composition, geology, and magnetic field of Mercury.
One of the innovative features of this highly-successful mission was the use of a complex series of fly-bys, between Earth (x1), Venus (x2) and Mercury (x3), in order to minimise expenditure of fuel, while slowing enough to enter orbit around Mercury. Many probes fly past planets, in order to profit from the rapid acceleration from their gravity, known as the slingshot, so they can travel the vast distances between planets in reasonable time. However, the disadvantage is that it would take a great deal of energy, heavy fuel, to slow down enough to enter orbit. The technique used by NASA for Messenger proved that it is possible to have the best of both solutions - and get the data we so eagerly want before the original probe designers are in retirement.
Messenger spent 10 months taking nearly 100,000 pictures of Mercury's surface, which mapped the surface completely. Due to the closeness of its orbit, it required regular boosts to maintain orbit. When it finally ran out of propellant, after two year-long mission extensions, it was allowed to embrace its new home more intimately.
Among its discoveries was the confirmation that there is water ice at Mercury's North Pole. It also sent back data about how the planet formed, the chemical composition of its surface, the core, the poles, magnetic field, and its exosphere.
NASA launched the tenth mission in the Mariner series on November 3rd, 1973. This spaceprobe successfully flew by Venus (closest approach 5/2/1974, 5,768 km), and Mercury three times:
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