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Orbit insertion is the spaceflight operation of adjusting a spacecraft’s momentum, in particular to allow for entry into a stable orbit around a planet, moon, or other celestial body. This maneuver involves either deceleration from a speed in excess of the respective body’s escape velocity, or acceleration to it from a lower speed.
The first kind of orbit insertion is used when capturing into orbit around a celestial body other than Earth, owing to the excess speed of interplanetary transfer orbits relative to their destination orbits. This shedding of excess velocity is typically achieved via a rocket firing known as an orbit insertion burn. For such a maneuver, the spacecraft’s engine thrusts in its direction of travel for a specified duration to slow its velocity relative to the target body enough to enter into orbit. Another technique, used when the destination body has a tangible atmosphere, is called aerobraking, which uses the friction of atmospheric drag to slow a spacecraft. Generally aerobraking is used to circularize a highly elliptical capture orbit while minimizing the use of on board fuel. To date, only a handful of aerocapture maneuvers have been attempted by NASA.
The second type of orbit insertion is used for newly launched satellites and other spacecraft. The majority of space launch vehicles used today can only launch a payload into a very narrow range of orbits. The angle relative to the equator and maximum altitude of these orbits are constrained by the rocket and launch site used. Given this limitation, most payloads are first launched into a transfer orbit, where an additional thrust maneuver is required to circularize the elliptical orbit which results from initial space launch. The key difference between this kind of maneuver and powered trans-planetary orbit insertion is the significantly lesser change in velocity required to raise or circularize an existing planetary orbit, versus canceling out the considerable velocity of interplanetary cruise.
Alternatives to rockets 
Although current orbit insertion maneuvers require precisely timed burns of conventional chemical rockets, some headway has been made towards the use of alternative means of stabilizing orbits, such as ion thruster or plasma propulsion engine to achieve the same result using less fuel over a longer period of time. In addition, research into the use of electrically conducting space tethers to magnetically repel the Earth’s magnetic field has shown some promise, which would virtually eliminate the need for fuel altogether.