What would happen to Earth if a wandering star careered through the solar system? If the moon were farther away from Earth, would the sun's gravity snatch away our satellite and spoil all those romantic songs? Students can take a crack at these questions with Astronomy Workshop, a set of Java applets and calculators designed to help users better understand solar system structure and dynamics. Astronomer Douglas Hamilton of the University of Maryland, College Park, and his students stocked the site with exercises for levels from junior high to college. To test whether an extrasolar planet could hold onto a moon, for example, students can try adjusting the planet's size, distance from its star, and other characteristics. And according to the simulations, a near miss from a rogue star less than half the size of the sun could bounce Earth and Mars out of their normal trajectories.