Odyssey expects to accomplish its aerobraking in one continuous period lasting between sixty and seventy-five days. There is a solid scientific imperative for this comparative haste. If Odyssey is to satisfy its principal science objectives BEFORE the four month time period, in early 2004, when it will be serving primarily as a relay satellite for data acquired by the twin Mars Exploration Rovers (MER), then Odyssey must reach its desired observational orbit sometime in January 2002. This timing requirement for the completion of the aerobraking phase provides another challenge for the Odyssey flight controllers.

If there are significant differences between the predicted Odyssey spacecraft response and the actual response seen during the early calibration drag passes, it might be necessary to make major changes to the mission plan. For example, a critical component or subsystem of the spacecraft may become much hotter during a calibration drag pass than the models predicted. If such an untoward event occurs, then the Odyssey flight team may need to limit the depth in the Martian atmosphere to which the spacecraft descends during drag passes, or even change the nominal attitude of the spacecraft during aerobraking events. Any major changes in the mission plan that are required as a result of the calibration phase will almost certainly extend the overall duration of aerobraking and make it impossible to achieve the desired science orbit before the end of January 2002.

As mentioned before, the Martian atmosphere itself is a large source of uncertainty for the Odyssey flight controllers. During the MGS mission, the density of the atmosphere at the altitude of the drag passes fluctuated wildly from orbit to orbit. MGS experienced several hundred drag passes before its aerobraking was completed. Statistics were maintained that expressed, in quantitative terms, how much the observed density varied from the predicted density on a pass by pass basis. Ten percent of the time the density actually encountered by MGS on a drag pass sweep through the upper atmosphere was off by more than seventy percent from the predicted density! As a result both the velocity change and the resultant spacecraft heating imparted by the drag pass were often significantly different from what was predicted only several hours earlier. These large variations from prediction meant that the MGS flight team often had to scramble, for spacecraft safety reasons, to implement a propulsive maneuver that would alter the altitude of closest approach on the next orbit, or the orbit following.

Fortunately, the Odyssey flight team includes many key personnel from both the Jet Propulsion Laboratory and the prime spacecraft contractor, Lockheed Martin Aerospace, who participated in the entire MGS aerobraking experience. They have developed operational procedures and staffing plans that reflect the lessons they learned from MGS. A critical cadre of trained flight controllers will be in position in the Odyssey operations centers for twenty-four hours a day, every day, throughout the aerobraking activity. These personnel will monitor each and every drag pass in real time, and will be prepared to take any action that is necessary, including implementing a propulsive maneuver before the next Martian closest approach, if either the Martian atmosphere fluctuates dramatically or the Odyssey spacecraft encounters a problem during the pass.

The twin failures of the Martian Climate Orbiter and the Martian Polar Lander in 1999 have significantly increased the attention that has been focused on the Odyssey mission. Key personnel on the project are well aware that the Odyssey mission has now become the flag-bearer for the entire, revamped Mars Exploration Program. Unusual measures have been taken to improve the probability of mission success. Nevertheless, the perils of the aerobraking phase are well understood by the flight team, and there will doubtless be a celebration, and a huge sigh of relief, once aerobraking is completed and the spacecraft begins its science observations.