With a diameter of 4,879 kilometers and an average orbital distance of only 57,909,100 kilometers, Mercury,
named after the Roman god of trade and travellers,  is both the
smallest and the closest to the
of all the planets  (Ceres
newly promoted to dwarf planet, Pluto newly demoted to dwarf
planet and Haumea, Makemake
all dwarf planets and all newly discovered are smaller). 
life on Mercury would have to be hardy indeed. In 1974 and 1975 three
flybys by the Mariner 10 spacecraft (the first probe to reach Mercury)
revealed a world even more inhospitable than the Moon,
satellite, heavily cratered in some areas smooth in others a barren
pitilessly blasted by the Sun's radiation with a mean surface
temperature of 179 °Celsius.
spacecraft Messenger (the second probe to reach Mercury and the first
in orbit) performed flybys on January 14, 2008, October 6, 2008, and
September 29, 2009, before final orbit insertion on March 18, 2011. The
fresh, more detailed, and somewhat surprising information it
transmitted home is being added to what till now has been a rather
skimpy knowledge base (e.g., water ice and frozen organic compounds found inside permanently shadowed
polar craters, water vapor in the exosphere and
signs of intense past volcanic activity on the surface).
All things come to an end, however, and after
surpassing expectations, Messenger, out of
fuel and out of time, its mission extended twice, crashed into the Solar System's innermost planet on April 30, 2015, at 19:26 GMT. A final tweet "It is time to say goodbye." An exceptional finish to an exceptional mission.
a joint venture between ESA and JAXA is the next Mercury mission on the
horizon. It's presently scheduled for launch in October 2018 (there's
been a lot of rescheduling) and following two Venus and five Mercury
flybys will arrive in orbit
around Mercury in 2024. It consists of the Mercury Planetary Orbiter
which will examine the surface and internal structure of our system's
innermost planet and the Mercury Magnetospheric Orbiter which will
probe its magnetosphere. The spacecraft will use a combination of
gravity assists and solar-electric propulsion to reach its destination.
 Later to be
equated with Hermes, the Greek messenger of the gods.
A "planet" is a celestial body that (a) is in orbit around the Sun, (b)
has sufficient mass for its self-gravity to overcome rigid body forces
so that it assumes a hydrostatic equilibrium (nearly round shape) and
(c) has cleared the neighborhood around its orbit.
A "dwarf planet" is a celestial body that (a) is in orbit around the
Sun, (b) has sufficient mass for its self-gravity to overcome rigid
body forces so that it assumes a hydrostatic equilibrium (nearly round
shape), (c) has not cleared the neighborhood around its orbit and (d)
is not a satellite.
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