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Mercury Revealed: Smallest Planet Is Further Shrinking

By Alice Turner
23:00, July 4th 2008

NASA revealed images sent by its MESSENGER probe in January, when it snapped up some 1,200 photos taken from some 120 miles from the planet's surface. MESSENGER, the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft launched by NASA in 2004, raced past Mercury on January 14 in the first visit in almost 33 years to the mysterious small planet, in the first of four sweeps over Mercury by the probe.

Now, scientists have analyzed these unprecedented images and have come up with 11 papers published yesterday, July 3, in Science. It appears that the innermost planet of our solar system has shrunk by more than a mile in diameter over its history. It appears that the planet's metal core is still active, but its cooling causes the upper-most layers to solidify and sink into the molten layers below.

The analysis also indicates an abundance of silicon, sodium and sulfur in Mercury's core. Mercury, named after the Roman god Mercurius, is orbiting the Sun once every 88 days, with a volume of 0.054 Earths and a surface of 0.108 Earths.

Launched on a Boeing Delta II rocket, MESSENGER lifted off from Cape Canaveral Air Force Station, Florida at 02:15:56 EDT on August 3, 2004 with the goals to determine the chemical composition of Mercury's surface, its geologic history, the nature of the planet's magnetic field, the size and state of the core, the volatile inventory at the poles, and the nature of Mercury's exosphere and magnetosphere.

However, MESSENGER's most important mission is yet to come: its Mercury orbit insertion will be on March 18, 2011, beginning a year-long orbital mission which will see a lot more data sent to Earth.

The space probe is also interesting because its navigation team is lead by KinetX, the first private company to be responsible for navigation of a NASA deep space mission. Their experts are fully responsible for determining all trajectory adjustments throughout the probe's flight through the inner solar system ensuring that MESSENGER arrives at Mercury with the proper velocity for orbit insertion.

The robotic space probe Mariner 10 was the only spacecraft so far to approach Mercury, and managed to map about 40 percent of its surface. Its mission ran between 1974 and 1975 and the probe was the first spacecraft to make use of an interplanetary "gravitational slingshot" maneuver.

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