The Hot Rock: Mysterious Mercury

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SOME MERCURY MYSTERIES Is there ice on the surface? Radar imaging has detected what appears to be ice reflections in craters near the planet's poles. The walls of these craters may be so steep that sunlight never reaches their floors, putting them in permanent deep freeze

Why is there a magnetic field? Earth's magnetic field is believed to be generated by the movement of molten iron in the core. But Mercury is so much smaller than Earth that its core should have solidified long ago. MESSENGER will try to determine whether the field is just a dying remnant or is still being driven by a molten core

A ROUNDABOUT ROUTE

Launch--Aug. 2004 Earth--flyby Venus--flybys Mercury--flybys Arrival--March 2011

MESSENGER isn't due to enter Mercury's orbit until March 2011. But the probe will be busy all along the way, getting a gravity boost from Earth next year, then Venus twice in an ever tightening spiral toward the first planet from the sun

WE DON'T KNOW THE HALF OF IT This picture of Mercury is a mosaic of 500 frames taken by Mariner 10, which saw only half the planet up close during its three flybys. The Hubble Space Telescope has not dared to snap some pictures of its own (training its gaze so close to the sun could fry its electronics) MESSENGER will need just 12 hours to complete an orbit of Mercury. With the planet spinning slowly below, it should be able to survey the entire surface within six months

THE MESSENGER PROBE A nimble little spacecraft, MESSENGER weighs just 2,400 lbs. (1,100 kg), 55% of which is fuel. It carries a dual camera system and six other scientific instruments, all of which operate on solar power-something of which there is no shortage so close to the sun. A sunshade made of ceramic cloth keeps the hardware running at a balmy room temperature

--Energetic particle and plasma spectrometers Will measure charged particles in Mercury's magnetosphere

--Magnetometer Will map Mercury's magnetic field

--X-ray, gamma-ray and neutron spectrometers Will detect types of elements on Mercury's surface and within its crust

--Laser altimeter Will map Mercury's terrain

--Dual imaging system Wide-and narrow-angle cameras

--Atmospheric and surface composition spectrometer Will detect and measure atmosphere and surface composition

--Insulating blanket

--Sunshade

--Star trackers

--Propellant tank (1 of 3)

--Solar panel (back)

Sources: NASA; JPL; Mark Robinson, Northwestern University Center for Planetary Sciences; Johns Hopkins University Applied Physics Lab TIME