Gravitational Tractor . ... Credit Copyright: Dan Durda ( FIAAA , B612 Foundation ) Explanation: How would you change the course of an Earth-threatening asteroid? One idea - a massive spacecraft that uses gravity as a towline - is illustrated in this dramatic artist's view of a gravitational tractor in action. ... The spacecraft's ion drive thrusters are canted away from the surface. ... NASA Web Site Statements, Warnings, and Disclaimers . NASA Official: Jay Norris. ...