"Contra-Grav 

Many spacecraft have contra-grav (CG) lifters as fuel-efficient means of landing and taking off from a planet surface, and CG lifters are also used on grav vehicles. CG lifters do not provide thrust and so cannot physically lift a craft or vehicle. Instead, they neutralize most of the gravitational attraction of a world (approximately 99% of gravitational force, beyond which power use becomes prohibitive). This, combined with atmospheric pressure, will provide buoyancy in very dense atmospheres and so allow the craft to float at low altitudes, but usually CG is used only as an adjunct to the ship's thrusters. By neutralizing most of a world's gravitational field, a ship with 

only 1G of thrust can still escape the world's gravity well."