The Phoenix Lander, first of NASA's Mars Scout missions, arrived at the Red Planet on May 25, 2008. From

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The Mars Reconnaissance Orbiter performed aerobraking at Mars from April through September, 2006. Aerobraking includes directing a spacecraft to pass through the atmosphere of a planet in order to dissipate a spacecraft's orbital energy. This allows a spacecraft's orbital period to be dramatically reduced without the expenditure of a significant amount of fuel. Aerobraking operations tend to be very labor intensive, with command products constructed frequently to control the spacecraft, including performing regular maneuvers to maintain a desired altitude as the spacecraft passes through the atmosphere. The Mars 2001 Odyssey spacecraft also utilized aerobraking prior to settling into its operational orbit in 2002, as did Mars Global Surveyor from 1997 to 1999. Many lessons were learned in the operations of these two precursor missions that were leveraged in behalf of the Mars Reconnaissance Orbiter. This paper will outline some of those lessons learned, compare and contrast the operational strategies between Odyssey and the Mars Reconnaissance Orbiter, and report on specific statistics regarding the frequency of commanding that was required to perform aerobraking. I. Aerobraking BasicsEROBRAKING is a spaceflight maneuver performed by a spacecraft near a planet with an atmosphere. The Mars 2001 Odyssey spacecraft and the Mars Reconnaissance Orbiter (MRO) utilized this strategy to reduce their orbital period from about 18½ hours and 35½ hours, respectively, to roughly 2 hours in both cases. Aerobraking involves passing a spacecraft through the upper levels of the atmosphere of a planet. The heat generated by the friction of the spacecraft against the atmosphere dissipates the kinetic energy of the spacecraft, thus imparting a change in velocity, or delta-V. The spacecraft radiates this heat to space between successive passes through the atmosphere, thus reducing its specific mechanical energy.For these two spacecraft, Mars was the target planet, and its tenuous atmosphere was used as the medium through which the vehicles passed. More heat was generated when the spacecraft passed deeper into the Martian atmosphere. At the altitudes where both spacecraft performed aerobraking, the atmosphere was so thin that it was better treated as a free molecular medium, rather than as a continuous fluid; the delta-V imparted is more a function of the surface area presented in the ram direction of the vehicle than it is by the aerodynamic shape of the vehicle.Nevertheless, both vehicles were designed to be aero-stable during aerobraking. Were it not so, the dynamic forces experienced would have imparted an undesirable torque on them, complicating aerobraking activities or potentially putting the vehicles at risk. Studies also illustrated that were either vehicle to perform a pass through the atmosphere in the wrong orientation, the aero-stable nature of the vehicles would "right" them. By maintaining the same face of the vehicles in the ram direction, it was possible to ensure that thermal limits for the various spacecraft compon...

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