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Aerobraking began after Mars Orbit Insertion and a check of spacecraft health. It lasted about six months. During aerobraking, the spacecraft used friction from the Martian atmosphere to slow down and to trim its initial 35-hour, elliptical orbit around the red planet to a two-hour, near-circular orbit. The final orbit was more suitable for science operations and ideal for making highly detailed observations of the surface.

As on Earth, the upper atmosphere of Mars is somewhat different from day to day and place to place. These variations are difficult to predict. Because the orbiter flew through this varying atmosphere during aerobraking, operations were risky for the orbiter and difficult and intense for its operators.

Phases of Aerobraking

Aerobraking occured in three primary phases:

Walk-in:	Lasted about a week or 5 orbits.	Engineers commanded the spacecraft to lower the periapsis (the closest point to Mars in its orbit) one orbit at a time, taking the spacecraft from its Mars orbit insertion altitude to its aerobraking altitude. This phase was used as a calibration period to understand atmospheric densities and the way in which the orbiter behaved in and out of aerobraking.
Main phase:	Lasted about 5 1/2 months and fewer than 500 orbits.	Once the orbiter reached its operational altitude (where the desired atmospheric densities were found), the main phase of aerobraking began. Engineers commanded the orbiter to make large-scale reductions in its orbit. If the altitude got too low, the spacecraft would be in danger of overheating; if the altitude got too high, then aerobraking would finish too late. Therefore, small propulsive maneuvers were occasionally required to keep the orbiter within a specified "corridor" by raising or lowering its periapsis altitude.
Walk-out:	Lasted about 5 days or 64 orbits.	The walk-out phase occurred during the last few days of aerobraking. Engineers commanded the orbiter to increase its periapsis (the closest point it came to Mars in its orbit), causing the orbit to shrink more slowly. When the apoapsis (the farthest away from Mars the spacecraft reached in its orbit) was reduced to 450 kilometers (280 miles), the periapsis was raised out of the atmosphere and aerobraking was complete.

Post-aerobraking activies that preceded science operations included:

Orbit trim maneuvers:	Lasted about two weeks.	When aerobraking was complete, the orbiter performed three small orbit trim maneuvers (OTMs) to place itself in the primary science orbit.
Spacecraft and instrument checks:	Lasted through solar conjunction, which occurred between Oct. 7 and Nov. 8, 2006.	Science operations could not begin until after solar conjunction, when the Sun came between Earth and Mars and communications were limited by solar interference. During that time the orbiter remained in a safe and quiet operational mode. On Earth, however, engineers configured the orbiter. They powered on, checked out, and calibrated the Instruments to be ready for the collection of science data. Instrument deployments such as the SHARAD antenna deployment and the CRISM cover release occurred in this period. All instruments made initial observations of the surface to verify observing modes, image quality, and processing performance.