A NASA Mars orbiter, launched in 2001 April. It went into polar orbit around Mars in 2001 October, using aerobraking to circularize the orbit. The craft carried three main instruments: a Thermal Emission Imaging System (THEMIS) to map the surface mineralogy; a Gamma Ray Spectrometer (GRS) to analyse the surface chemistry; and the Mars Radiation Environment Experiment (MARIE) to assess the radiation risk to future human explorers.
http://mars.jpl.nasa.gov/odyssey/ Official mission website.
Subjects: astronomy and cosmology.