1. A projectile tracking device comprising: detector apparatus for converting into electronic form images in at least two infrared wavebands; optics for projecting onto the detector apparatus an image of a scene in the at least two infrared wavebands; logic operative to obtain from the images in electronic form apparent brightnesses of the projectile at the optics in at least two infrared wavebands; logic operative to estimate the speed of the projectile from the ratio of the two measured apparent brightnesses; logic operative to obtain from the images in electronic form an azimuth of the projectile from the optics at successive times; and logic operative to estimate the direction of the trajectory of the projectile from the measured azimuths in combination with the ratio between the measured apparent brightnesses.