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We present a concept for a spacecraft that can controllably alter the trajectory of an Earth threatening asteroid using gravity as a towline. The spacecraft hovers near the asteroid with thrusters angled outward so the exhaust does not impinge on the surface. This deflection method is insensitive to the structure, surface properties, and rotation state of the asteroid.The collision of an asteroid as small as ~200 m with the Earth could cause widespread damage and loss of life 1 . One way to deflect a threatening asteroid is to dock a spacecraft to the surface and push on it directly 2 . The total impulse needed for rendezvous and deflection is too large for chemical rockets, but is achievable by 20 ton class nuclear-electric propelled spacecraft proposed by NASA 2 . Regardless of the propulsion scheme, a docked asteroid tug needs an attachment mechanism since the surface gravity is too weak to hold it in place. Asteroids are likely to be rough and unconsolidated, making stable attachment difficult. Furthermore, most asteroids rotate, so an engine anchored to the surface would thrust in a constantly changing direction.Stopping the asteroid's rotation, reorienting its spin axis 3 , or firing the engine only when it rotates through a certain direction adds complexity and wastes time and propellant.Our suggested alternative is to have the spacecraft simply hover above the surface. The spacecraft will tow the asteroid with no physical attachment using gravity as a towline.

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