As NASA’s 2001 Mars Odyssey spacecraft neared completion Wednesday of its 100th orbit of the Red Planet, scientists had to contend with a surprisingly fickle Martian atmosphere in guiding and slowing the robotic probe. Odyssey entered orbit around Mars on Oct. 23. Since then, scientists have guided the spacecraft on a series of controlled skims through the atmosphere, using the drag provided by the carbon dioxide-rich shroud to slow the spacecraft and shape its orbit. From orbit to orbit, however, scientists have discovered wider than expected swings in the density of the Martian atmosphere as the probe passes over the planet at varying latitudes, longitudes and altitudes. The changes in density seen so far have been up to 100 percent and have been most dramatic over the north pole.