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 thrusters or plasma propulsion engines 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. References Spacecraft propulsion Spaceflight concepts