The MARIE (Martian Radiation Environment Experiment), aboard the 2001 Mars Odyssey spacecraft, was launched on April 7, 2001, and arrived at Mars on October 24, 2001. Data were collected intermittently during the cruise phase, starting in late April and ending in late July. A problem with MARIE's onboard computer occurred in early August, and the instrument was turned off until early March 2002, after Odyssey 's mapping orbit had been established. Data have been collected from that time to the present without major interruption. Routine minor interruptions of up to 48 hours have occurred during the orbital phase when the instrument's data is erased from local storage (after having been downloaded). MARIE is oriented to point in the direction opposite Odyssey's velocity vector. Space radiation is for the most part isotropic, so the orientation of Odyssey is usually not critical and references to external coordinate systems are not a part of the data returned by MARIE. There is one exception to the statement that space radiation is isotropic: During the early stages of solar particle events, there can be directionality in the particle flux.