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Space Facts: Part 6 - Apogee and Perigee

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What is Apogee and Perigee ?

The point of orbit nearest to Earth is Perigee and farthest from Earth is called Apogee. Satellites sent from PSLV rockets follow this system as the speed of PSLV rockets is less than 11.2 Km/sec. Here spacecrafts have to change different orbits (also known as Orbit Maneuvers) around Earth to get speed of 11.2 Km/sec. By achieving speed higher than 11.2 Km/sec we spacecrafts can escape Earth's gravity, this velocity is called "Escape Velocity".
GSLV MK III rockets have speed 11.2 Km/sec, so spacecrafts do not have to change different orbits and spacecrafts are directly inserted into space. 
Mangalyaan had to undergo Orbital Maneuvers as it was launched from PSLV rocket. If it was to launch from GSLV MK III, Mangalyaan would take less time to reach Mars than it took with PSLV.
In Apogee and Perigee the orbit is of elliptical shape. When spacecraft travels from Apogee to Perigee it gains velocity die to gravity of Earth. Orbit of spacecraft becomes more elliptical. After completion of one round, again when spacecraft travels from Apogee to Perigee it gains more velocity than earlier giving more elliptical orbit. This transition is called Orbital Maneuver, where the orbits are changed. 
These Orbital Maneuvering is done till escape velocity is achieved and spacecraft leaves Earth's gravity.
When spacecraft enters in Mars' gravity, it has to be slowed down, otherwise it will fly-by across Mars and leave the desired trajectory. For slowing down the spacecraft, engine thrusters are used and spacecraft is settled in desired orbit.

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