The existence of transient decreases of Earth's ultraviolet dayglow intensities with spatial dimensions in the range of 50 to 100 kilometers was discovered in the pioneering global images of our planet's atmosphere as taken with a camera on board the Dynamics Explorer-1 spacecraft over ten years ago. This phenomenon is often referred to as "atmospheric holes" because of their appearance as black spots in these global pictures. These observations were interpreted in terms of a large influx of small comets, each with masses in the range of tens of tons, into the upper atmosphere. Water vapor from the disruption of a small comet is a very effective absorber of the atmospheric ultraviolet light. From the frequency of the occurrence of these atmospheric holes it was inferred that the water vapor clouds from about 20 small comets entered our planet's upper atmosphere each minute, or a total of about 10 million each year. The scientific community was startled by such a proposed cosmic rain into our atmosphere and unanimously rejected the observations as an artifact of the camera.

Recently researchers at The University of Iowa have again obtained global images of the ultraviolet light from the Earth's upper atmosphere, this time using cameras aboard the Polar spacecraft. In order to assure that an instrument artifact was not the cause of the atmospheric holes in the images the new camera, called the Earth Camera, was of entirely different design and of greatly enhanced capabilities relative to its predecessor on Dynamics Explorer 1. This camera was launched on the Polar spacecraft on 24 February 1996 into an eccentric orbit with perigee and apogee altitudes of 5170 km and 50,510 km, respectively, an orbital period of 17.6 hours, and an inclination of 86 degrees. Apogee position was located above Earth's northern hemisphere.

Remarkably the Earth Camera confirmed the previous results from the camera on Dynamics Explorer 1. The confirmation of these results was unexpected by the scientific community. The Earth Camera on the Polar spacecraft was capable of extending the observations to include detection of the same atmospheric hole in two consecutive images, the expansion rate of the atmospheric hole as it plunged toward the atmosphere, and the bright ultraviolet light trails of the initial disruption of the small comets. The camera continues to gather images of the atmospheric holes, as well as of the auroral lights.