In the early evening of October 23, 2001, Pacific Daylight Time, the Odyssey spacecraft, now cruising through interplanetary space, will reach the planet Mars. At that time it will activate its main propulsion engines and perform a Mars Orbit Insertion (MOI) maneuver that will place the spacecraft in orbit around the red planet. Even though a successful MOI is absolutely essential for the achievement of the objectives of the Odyssey mission, it is not by itself sufficient to guarantee that the spacecraft will attain the tight circular orbit around Mars that is required for the science payload. To transfer from the loose, elliptical, post-insertion orbit to the tight orbit necessary for the science observations, a long period of aerobraking has been designed into the nominal Odyssey mission profile. This aerobraking period, beginning shortly after MOI and scheduled to last sixty days or more, represents both the biggest challenge and the biggest risk of the Odyssey mission.