How to Track a Flying Saucer

This paper deals with a problem in computer vision: how to recover the motion of a disk, thrown toward an observer, from a sequence of images acquired by a pinhole camera. Polynomial equations describing the motion are established, and techniques from algebraic geometry are used to show that in general a sequence of three images is sufficient for the recovery of motion of the disk when it is known to be moving along a straight line, and that five images suffice in the more general situation in which the disk travels in a gravitational field. Examples are worked out in detail to illustrate our results.