Using navigation measurements, navigators were prepared to make final corrections to the path of the orbiter known as trajectory correction maneuvers (TCM's). These corrections could have been as small as 1/2 meter per second (about 1 mile per hour) to make sure the spacecraft reached its desired destination 300 kilometers (190 miles) above the surface of Mars, with a maximum error of only 25 kilometers (15 miles). Considering the spacecraft traveled hundreds of millions of miles to reach Mars, this "error" was incredibly small.
The spacecraft carried enough fuel for two final trajectory correction maneuvers that were ultimately not necessary because of the remarkable accuracy of the spacecraft's path:
If needed during the spacecraft's final approach to Mars, a trajectory correction maneuver would have targeted the orbiter to its final aim point for Mars orbit insertion on a southern approach trajectory. TCM-4 would have been used to remove any errors from the previous trajectory correction maneuvers (TCM-1, TCM-2, and TCM-3) during cruise. It would have targeted the final aim point for Mars orbit insertion and would have occurred about 10 days prior to that event.
This maneuver would have served a different purpose than the other TCM's. It was essentially an insurance policy for the orbiter. In case the orbiter came too close to Mars upon arrival, TCM-5 would have been used to increase the orbiter's altitude for the Mars orbit insertion burn. The TCM-5 commands would have been stored on board the orbiter several days in advance. The orbiter had two burn opportunities -- at 24 hours and at 12 hours before the Mars orbit insertion burn. During the last day before arrival, navigators determined the orbiter was not too close to Mars. Because the orbiter arrived at Mars on the expected trajectory, TCM-5 was never needed.