It's relatively trivial to spot comets, but spotting the dust they leave behind? That's no mean feat -- it's like tracking grains of fast-moving sand on a cosmic-scale beach. NASA, however, might just have a way of mapping that dust. It's developing a technique that would use the movement of the ESA's LISA Pathfinder probe to detect the minuscule "crumbs" left by comets and asteroids. The trick involves tracking the tiny thruster bursts Pathfinder uses to compensate for dust stream impacts. When you measure the direction and strength of those bursts, you can get a good sense of the dust's impact location, size and trajectory.