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Nasa satellite may find ice on hottest planet

Reuters, Florida
A satellite that heads off to Mercury, the hottest planet in the solar system, yesterday has Nasa scientists buzzing because it is expected to journey there and find, of all things, ice.
The Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft takes off three decades after the last time Nasa took an up-close look at Mercury, where the temperature at noon is just a shade under 900 degrees F.
The $427 million mission starts with lift-off aboard a Boeing Delta 2 rocket from the Cape Canaveral Air Force Station in Florida, the space agency said.
If ice does exist on Mercury, it manages to stay out of the sun, inside
the shadowy interior of craters at the poles of the nearly airless planet, where the temperature may not rise above minus -300 degrees F.
Radio telescopes on Earth have detected the signature of ice in those craters, scientists say, though they caution it might also be super-frozen silica or something else.
Equally intriguing is Mercury's composition. The planet is about two-thirds iron and as dense as a planet the size of Earth, although it is much smaller, scientists say. Mercury is slightly larger than the Earth's moon.
MORE EARTH-LIKE?
"Did Mercury start off more Earthlike, then lose its rocky exterior?" asked Sean Solomon of the Carnegie Institute of Washington and principal scientist for the Messenger mission. Both long exposure to solar winds or some giant impact might account for that, he said.
"What are the processes that contributed to the inner planets (Mercury, Venus, Earth and Mars) turning out so differently?" To understand that "we really have to study the most extreme of those outcomes, and that's Mercury," Solomon said.
Messenger will be the first satellite to visit Mercury since Mariner 10 a generation ago. It photographed about 45 percent of the planet's surface during three fly-bys in 1974 and 1975.
But Mariner was traveling too quickly to enter orbit around Mercury, where MESSENGER will spend at least a year.
To do that, Messenger will be forced onto a long, meandering path, 15 times around the sun, flying past Earth once, Venus twice and Mercury
itself three times, using planetary gravity to slow the craft with each pass.
Finally, it will enter Mercury's orbit in 2011, almost seven years after liftoff.
A direct flight would take just three months, Nasa said, but it would also take more fuel for braking maneuvers, which would have required a bigger rocket than the Delta 2 and a higher mission price tag.
Messenger bristles with seven complex sensor systems. Beside cameras, there are spectrometers for remote chemical analysis, magnetometers to study Mercury's strong magnetic field and a laser altimeter to map the planet's topography.
Protecting the equipment from the sun is a curved, rectangular screen that will always face the sun. Sometime in 2012 or 2013, when its fuel is depleted, Messenger is likely to crash onto the planet's surface.