Orbit insertion

Inspaceflightanorbit insertionis anorbital maneuverwhich adjusts aspacecraft’s trajectory, allowing entry into anorbitaround aplanet,moon,or other celestial body.An orbit insertion maneuver involves either deceleration from a speed in excess of the respective body’sescape velocity,or acceleration to it from a lower speed.

When the result is atransfer orbit,e.g. adescent orbit insertion,the maneuver is anorbit injection.

Orbit types

Orbits are periodic or quasi-periodic trajectories, usually around a centralcelestial bodylike the Earth or the Sun. They may also be trajectories aroundLagrange pointlocations in a multi-body system like theEarth–Moon system.(For example NASA used ahalo orbitfor theCAPSTONEmission.)

Low orbits

Low orbits are trajectories deep within the 'gravitational well' of a central body. Examples includelow Earth orbitandlow lunar orbit.Insertion into a low orbit can require substantial deceleration with respect to the central body or, for launch from a planetary surface, substantial acceleration to reachorbital speed.

High and elliptical orbits

Higher energy orbits likegeostationary orbitare often reached via ellipticaltransfer orbits.

Deceleration

One type of orbit insertion is used when capturing into orbit around a celestial body.

Rocket propulsion

Excess speed of an interplanetary transfer orbit is typically shed with arocketfiring known as an orbit insertion burn. For such a maneuver, the spacecraft’s engine is used to slow its velocity relative to the target body.For example, each successfulApollo programlunar landing mission first usedApollo service modulepropulsion to enter low lunar orbit.

Low thrust insertion

For some arrival trajectories,low thrust propulsionis sufficient to achieve orbit insertion. TheHitenspacecraft used this approach first, in 1991.

Other techniques

Another technique, used when the destination body has a tangible atmosphere, is calledaerocapture,which can use the friction of the atmospheric drag to slow down a spacecraft enough to get into orbit. This is very risky, however, and it has never been tested for an orbit insertion. Generally the orbit insertion deceleration is performed with the main engine so that the spacecraft gets into a highly elliptical “capture orbit” and only later the apocenter can be lowered with further decelerations, or even using the atmospheric drag in a controlled way, calledaerobraking,to lower the apocenter and circularize the orbit while minimizing the use of onboard fuel. To date, only a handful of NASA and ESA missions have performed aerobraking (Magellan,Mars Reconnaissance Orbiter,Trace Gas Orbiter,Venus Express,...).

Acceleration

The second type of orbit insertion is used for newly launchedsatellitesand other spacecraft. The majority of spacelaunch vehiclesused today can only launch a payload into a very narrow range of orbits. The angle relative to theequatorand maximum altitude of these orbits are constrained by therocketandlaunch siteused. 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 the 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 asion thrustersorplasma propulsion enginesto achieve the same result using less fuel over a longer period of time. In addition, research into the use of electrically conductingspace tethersto magnetically repel the Earth's magnetic field has shown some promise, which would virtually eliminate the need for fuel altogether.

See also

• Ballistic capture