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Taking the Mystery Out of Mercury
El planeta Mercurio aparece como un circulo negro contra la superficie del Sol

The black dot in this picture of the Sun is Mercury. The planet made a rare pass in front of the Sun on May 7, 2003. The SOHO and TRACE spacecraft were watching with all instruments. The spacecraft websites are among many with Mercury transit galleries. Image Credit: NASA
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On Nov. 8, 2006, telescopes in North America and other parts of the world captured the image of a little dot crossing the face of the sun. That dot was Mercury, and its passing between the Earth and sun is known as a transit.

Transits of Mercury take place about 13 times each century. The event highlights how small the planet is compared to the sun: During the transit, Mercury appeared to be 194 times smaller than the sun. The transit is also a reminder of how little is known about what is now the smallest planet in the solar system, after tiny Pluto was downgraded from planetary status.

Because of its orbit, Mercury is hidden from Earth-based optical telescopes except for brief periods just after sunset or just before sunrise. Even then, the haze of Earth's atmosphere obscures the view. In fact, ground-based observations have raised more questions than answers -- questions such as whether the bright areas that show up at Mercury's poles in radar images are ice, and how ice could exist on a planet where daytime temperatures climb as high as 450 degrees Celsius (840 F).

NASA's MESSENGER spacecraft entered into orbit around Mercury in March 2011 and started a yearlong, in-depth study of the planet. The spacecraft orbits Mercury twice every 24 hours, flying as close as 200 kilometers (about 125 miles) from the planet's surface. A total of seven instruments collect data aimed at answering the following questions:

Un hombre con traje, guantes y una redecilla blancos trabaja en una sala aseptica

Neal Bachtell of the Johns Hopkins University Applied Physics Laboratory attaches the middle section of the fabric "sunshade" to MESSENGER. Image Credit: NASA/JHUAPL/Carnegie Institution of Washington
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-- Why is Mercury so dense?
-- What is the geologic history of Mercury?
-- What is the structure of Mercury’s core?
-- What is the nature of Mercury’s magnetic field?
-- What are the unusual materials at Mercury’s poles?
-- What volatile elements are important on Mercury?

Scientists hope that by learning more about the elements, minerals and processes that helped form Mercury, they will gain a better understanding of the early evolution of the solar system and why Earth is so different from nearby planets.

One of the most important features of MESSENGER is not an instrument. Rather, it's a protective cloak of heat-resistant fabric. This "sunshade" will keep the instruments and entire spacecraft from overheating as MESSENGER travels to, approaches and orbits Mercury, the closest planet to the sun.

Adapted from: http://www.nasa.gov/audience/foreducators/k-4/features/F_Taking_Mystery_Out_of_Mercury.html