Mercury to Get Its Best Close-Up Yet

Studies of Mercury from Earth are difficult because from our planet's vantage point, Mercury never strays more than 28 degrees from the sun. That's too close even for orbiting telescopes like Hubble to image for fear of damaging the sensitive optical equipment by overexposure to harmful solar rays.

Ground-based observatories are developing new techniques to work around Mercury's tough neighborhood, but the science planned for MESSENGER's year-long mission has the potential to open entirely new perspectives on how the planets formed.

For example, one of the most debated questions about Mercury is why it is about as dense as Earth, though it has just 40 percent of the real estate. Scientists estimate Mercury's core must be about 75 percent iron, or iron mixed with other metals. Yet the planet also has a magnetic field, which on Earth is generated by a conductive molten core.

In a planet as small as Mercury, however, a core should have cooled and solidified a long time ago.

One possible explanation is that Mercury wasn't always number one in the parade of planets. Some astronomers suspect Mercury migrated to its present position after a cosmic collision by the Mars freeway. The solar system was a nasty place back then, an over-crowded autobahn before Jupiter arrived, with its great massive gravity, to impose order like a traffic cop.

The crash may have cost Mercury dearly, arriving at its new location stripped of crust and missing mantle, a fraction of its former size and mass. The heavy metal core remaining would have then found balance in one of the harshest environments imaginable, so close to the sun that an average day on the Tropic of Capricorn registers more than 400 degrees Fahrenheit.

MESSENGER may solve the mystery. Its science instruments include mineral detectors and chemical analyzers scientists can use to determine, for example, how materials on Mercury's surface formed and if they have been ground cover since inception or if they bear signs of a more internal origin.

During Monday's close approach, MESSENGER will soar 146 miles over the planet's equator, closer than Mariner ever flew and in position to study the 55 percent of Mercury never seen. Scientists will have to wait about a week to get all the data collected during the flyby relayed back to Earth.

A second pass is expected in October and a third and final flyby in September 2009. When MESSENGER returns in March 2011, it will be going slow enough to be captured by Mercury's relatively puny gravity and slip into orbit to become its first artificial satellite. A joint European/Japanese mission slated to launch in 2013 will add a second companion six years later.

NASA is spending about $446 million for MESSENGER, which is designed to last about a year.

"It is an understatement to say that the science team is extremely excited," said Solomon, the lead MESSENGER researcher. "It's our first glimpse of Mercury in 33 years. It's been a really long road and we're about to see some payoff."

Related Links:

Irene Klotz's blog: Space Diary

The Messenger Mission

The Planet Mercury

How Stuff Works: Mercury Explained

Mariner 10