So how do you build a tractor beam? The three NASA scientists are looking at three different methods. The first--the "optical tweezers" method--relies on two counter-propagating beams that create a kind of ring-like geometry that confines particles inside. In lab tests, researchers have shown that by alternately strengthening or weakening one of the beams they can create a backward pull along the ring's center where the particles are trapped, moving the particles opposite the direction of the beams. This method, however, requires an atmosphere because it relies on temperature differences to move the particles. It won't work in the vacuum of space or on moons or asteroids with no atmospheres to speak of.