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Mercury is the planet closest to the Sun, and after the reclassification of Pluto from the status of planet to a dwarf planet, Mercury is now the smallest planet in our Solar System. It is situated between the Sun and Venus. Mercury's elliptical orbit takes the small planet as close as 47 million kilometers (29 million miles) and as far as 70 million kilometers (43 million miles) from the Sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the Sun, the Sun would appear almost three times as large as it does when viewed from Earth.

We know very little about Mercury. In fact, we know more about every other planet in the solar system (Pluto aside) than we do about one of our closest neighbours.

This is because it never strays far from the Sun. It can only be observed during the day, when scattered sunlight impedes clear views of the planet, or just before sunrise and after sunset, when the light we receive from it must pass through 10 times as much atmospheric turbulence as when it is overhead. This means that even the best ground based telescopes get a worse view of it than humans get of the moon with the naked eye each night. Even the Hubble Space Telescope cannot look directly at Mercury, in case Solar rays damage its sensitive instruments. As such all of the detailed pictures we have of Mercury are courtesy of the Mariner 10 spacecraft, which mapped 40% of the planet over 3 fly-bys in the mid 1970s. The rest of the planet has never been seen.

Temperatures on Mercury's surface can reach 430 degrees Celsius (800 degrees Fahrenheit). Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -170 degrees Celsius (-280 degrees Fahrenheit).

As Earth-based observation shed little light on Mercury, spacecraft is the way to go in understanding this little neighbour of Earth.

The first spacecraft to approach Mercury was NASA’s Mariner 10 (1974–75). Mariner 10 provided the first close-up images of Mercury’s surface, which immediately showed its heavily cratered nature, and also revealed many other types of geological features, such as the giant scarps which were later ascribed to the effect of the planet shrinking slightly as its iron core cools. However, Mariner 10 only managed to map less than 45% of the planet’s surface.

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the surface. At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury’s rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet’s magnetic field is much like the Earth’s, which deflects the solar wind around the planet. However, the origin of Mercury’s magnetic field is still the subject of several competing theories.

Just a few days after its final close approach, Mariner 10 ran out of fuel; since its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut itself down. Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004. The MESSENGER spacecraft will make several close approaches to planets to place it onto the correct trajectory to reach an orbit around Mercury. The first fly-by of Mercury occurred on January 14, 2008. Two more fly-bys of Mercury are scheduled, in October 2008 and September 2009. Most of the hemisphere not imaged by Mariner 10 will be mapped during the fly-bys. The probe will then enter an elliptical orbit around the planet in March 2011.

The mission is designed to shed light on six key issues: Mercury’s high density, its geological history, the nature of its magnetic field, the structure of its core, whether it really has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe is carrying imaging devices which will gather much higher resolution images of much more of the planet than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles. Detailed measurements of tiny changes in the probe’s velocity as it orbits will be used to infer details of the planet’s interior structure.

The European Space Agency is planning a joint mission with Japan called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere. A Russian Soyuz rocket will launch the bus carrying the two probes in 2013. The spacecraft bus will reach Mercury in 2019. The bus will release the magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for a terrestrial year.