A gravitational tractor is an object which influences the trajectory of an object in space through the force of gravity alone. It has been put forward as a method to deflect dangerous objects (such as asteroids or comets from Earth's orbit.)
It can be used to influence every object, independent from its chemical and structural properties. There are several methods of deflecting an icy comet, but only the gravitational tractor can be applied with a predictable result. Other methods are subject to some uncertainties arising from the unknown internal structure. For example, if an explosion is used to influence the trajectory, the result depends on the comet's ability to absorb the applied energy with the transformation into heat. The same unpredictable result applies to the attempt to influences the trajectory with the deliberate impact of a massive body.
This concept might be used in a plan for Terraforming Mars. Changing the orbits of comets on small moons so they eventually intersect with Mars and crash on it, to add volatiles and energy to the planet.
Such an object must be of sufficient mass to noticeably affect the target object.
The tractor needs a means of propulsion strong enough to counter the force of gravity between itself and the target.