Limitations
Limitations of the tractor concept include the exhaust configuration. With the most efficient hovering design (that is, pointing the exhaust directly at the target object for maximum force per unit of fuel), the expelled reaction mass hits the target head-on, imparting a force in the exact opposite direction to the gravitational pull of the tractor. It would therefore be necessary to use the orbiting-tractor scheme described below, or else design the hovering tractor so that its exhaust is directed at a slight angle away from the object, while still pointing "down" enough to keep a steady hover. This requires greater thrust and correspondingly increased fuel consumption for each m/s change in the target's velocity.
Issues of the effect of ion propulsion thrust on the dust of asteroids have been raised, suggesting that alternative means to control the station keeping position of the gravity tractor may need to be considered. In this respect, solar sails have been suggested.
Read more about this topic: Gravity Tractor
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