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Revisiting Mercury, new sights abound

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By Roger Snodgrass

The first fly-by of Mercury has thrilled the international team of space scientists who have waited 30 years for their virtual return to the smallest and nearest planet to the sun.Completing the first leg of a 2 billion-mile trip, featuring a gravity boost from Venus, the MESSENGER spacecraft zipped past its destination this month, snapping pictures and grabbing data.It was the first of three fly-bys for NASA’s MErcury Surface, Space ENvironment, GEochemistry and Ranging  (MESSENGER) spacecraft that will slow the trajectory down and drop it into a long-term orbit two years from now.“This fly-by allowed us to see a part of the planet never before viewed by spacecraft, and our little craft has returned a gold mine of exciting data,” said Sean Solomon, MESSENGER’s principal investigator of the Carnegie Institution in Washington, D.C. In an announcement, he said, “From the perspectives of spacecraft performance and maneuver accuracy, this encounter was near-perfect, and we are delighted that all of the science data are now on the ground.”Much of the excitement had to do with new geological information and the discovery that the planet’s magnetic field had changed since it was last measured.Instrument scientist David Lawrence, now retired from Los Alamos National Laboratory, will continue to work on the project with the Applied Physics Laboratory at Johns Hopkins University.“We’ve looked at the neutron data,” he said Wednesday. “All the instruments worked well; certainly the neutron data looked great.”His piece of the project involves the gamma ray and neutron spectrometers on the spacecraft that will identify elements of the surface and atmosphere.Even from the earth, bright areas within polar craters on Mercury have aroused scientific curiosity about the possibility of finding water-like deposits.A NASA fact sheet on the MESSENGER website said it might seem unlikely that there could be any ice on a planet where temperatures can reach a maximum of about 800 degrees F. But, the “nighttime” temperature is about -300 degrees F., suggesting that temperatures in permanently shaded places might be cold enough to form ice for long periods.A member of the group that found unlikely ice deposits on the moon and Mars, Lawrence said, “The very strong outlines of anomalous materials at the poles of Mercury pose an interesting science question in its own right.”Although each of the fly-bys will gather data, the prime mission for the neutron analysis will be accomplished during the year in orbit.The orbit itself will be a much more complicated pattern than the orbit the lunar and Mars missions followed.The trick is to “undo all the effects” of that complicated motion, to get at the composition of this planet.Operating so close to the sun has hazards, too, Lawrence said. The spacecraft deploys a parasol, made out of high-temperature-resilient materials and always shading the instruments so they can remain at a reasonable temperature. The spacecraft’s elliptical orbit is also designed to get away from the planet periodically to cool off.