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 varying over time from the varying ratio at successive times of the measured apparent brightnesses in said at least two infrared wavebands; 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.