How Stage II works
Stage II (unlike the Atlas Stage I) is re-startable, and fires twice.
First engine firing: The Centaur engine fires for the first time shortly after separation from the Stage I booster to boost the spacecraft into a parking orbit of about 100 nautical miles (185 kilometers or 115 miles) altitude. A "parking orbit" is the name used for the Earth orbit in which the spacecraft and Centaur coast between burns. The parking orbit is needed to allow the Centaur to be in the right position relative to both Earth and Mars for each of its two burns.
The duration of the first burn is about nine-and-a-half minutes.
Coast and Second engine firing: The Centaur (with the spacecraft still attached) then coasts in the parking orbit until it has reached a point over the Indian Ocean where the Centaur engine fires a second time to accelerate the spacecraft out of Earth orbit and on its way towards Mars. The coast period between the two Centaur burns lasts for about 33 minutes. During this coast period, the Centaur points the orbiter toward the sun and slowly rolls along its axis (like a rotisserie) to control the temperatures on all parts of the Centaur and the spacecraft.
Separation: After the second burn (about ten minutes duration), the Centaur automatically releases the clamp band that holds the spacecraft on top of the Centaur and the spacecraft separates. Separation occurs when the spacecraft is just North West of Australia. After a short period of time for the spacecraft to drift far enough away from the Centaur, the Centaur performs a maneuver to move away from the flight path of the spacecraft. This maneuver is important since, if it were not done, the Centaur would try to follow the spacecraft to Mars and might eventually collide with the spacecraft or impact Mars. As soon as this maneuver is done, the job of the launch vehicle is complete and the spacecraft is on its way to Mars.
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