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In a general sense, Mercury is no different than our moon, and is only 40% larger. Mercury has no atmosphere, and therefore erosion does not occur, so it has a heavily cratered surface, as seen in the composite picture to the right. Unlike the moon, it was not recently (recently as in 3.8 billion years ago) resurfaced in parts, and so all the original craters are still preserved.

Because of its atmospheric absence, the temperatures of the side facing the sun and that fa
cing away from the sun are very different: The side towards the sun is a sizzling 350 °C (662 °F), while the night side is a cold -180 °C (-292 °F). The reason for this huge difference is that it is the atmosphere (and water in the case of Earth) of planets that moves the heat around, and allows night and day temperatures to remain relatively constant.

Contents:

1. Explorations

2. Unique Characteristics

3. External Links

Explorations

Reaching Mercury from Earth poses significant technical challenges. Mercury orbits three times closer to the Sun than does Earth, so a Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers down into the Sun's gravitational potential well. From a stationary start, a spacecraft would require no delta-v or energy to fall towards the Sun; however, starting from the Earth, with an orbital speed of 30 km/s, the spacecraft's significant angular momentum resists sunward motion, so the spacecraft must change its velocity considerably to enter into a Hohmann transfer orbit that passes near Mercury.

In addition, the potential energy liberated by moving down the Sun's potential well becomes kinetic energy, increasing the velocity of the spacecraft. Without correcting for this, the spacecraft would be moving too quickly by the time it reached the vicinity of Mercury to land safely or enter a stable orbit. The approaching spacecraft cannot use aerobraking to help enter orbit around Mercury since it has no atmosphere and must rely on rocket boosters. Because of this, a trip to Mercury requires even more rocket fuel than to escape the solar system completely. As a result of these problems, there have not been many missions to Mercury to date.

The only spacecraft to approach Mercury has been the NASA Mariner 10 mission (1974–75). The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, and it provided the first close-up images of Mercury's surface. It made three close approaches to Mercury, the closest of which took it to within 327km of the surface. Unfortunately, the same face of the planet was lit at each close approach, resulting in the restriction of images to less than 45% of the planet's surface. Mariner 10 also found the first evidence for Mercury's magnetic field, and measured temperatures across its surface.

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004 from the Cape Canaveral Air Force Station in Florida, USA, aboard a Boeing Delta 2 rocket. The MESSENGER spacecraft will make three flybys of Mercury in 2008 and 2009 before entering a year-long orbit of the planet in March 2011. It will explore the planet's atmosphere, composition and structure.

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Unique Characteristics

Mercury is different from all other planets in three main ways. First, it is the smallest non-controversial planet after Pluto. Therefore, it has very weak gravity, which leads to the second unique characteristic: Of all the planets in the solar system, Mercury is the only one that has little to no atmosphere. This is due to its weak gravity, which allows the solar wind to literally blow away its atmosphere. The third distinguishing feature is that its surface is completely covered with craters - even more so than Earth's moon. Since it has no atmosphere, it has no way for the craters to erode the way Earth's and Mars' craters do. Once formed, they have and will remain for billions of years, preserved by the vacuum of space.

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External Links:

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