NASA's Mars Reconnaissance Orbiter has begun the final portion of its "aerobraking" process of using friction with the top of Mars' atmosphere to shrink its orbit. "We are in the end-game part of aerobraking from now until August 30, when we will fire our thrusters to move the spacecraft out of an atmosphere-grazing orbit," said Mars Reconnaissance Orbiter Project Manager Jim Graf, of NASA's Jet Propulsion Laboratory, Pasadena, Calif.

There is a cool multimedia simulation of this process at this website at JPL.