Here is a target, anyone care to try shooting a few holes in it? Just for sport you understand.
In a paper that will appear in the Journal of Geophysical Research - Space Physics, Dr. John Emmert and his colleagues, Drs. Michael Picone, Judith Lean, and Stephen Knowles, report that the average density of the thermosphere has decreased by about 10% during the past 35 years. The thermosphere is the highest layer in the atmosphere, and begins at an altitude of about 90 km (56 mi).
The study utilized orbital tracking data on 27 space objects that have been aloft for over 30 years and whose closest approach to the Earth ranges from 200-800 km (124-497 mi). The Space Shuttle typically orbits at 300-450 km (186-279 mi), and the International Space Station is at an altitude of about 400 km (248 mi). Although the atmosphere is extremely thin in this region (the air at the Earth's surface is a trillion times thicker), it is enough to exert a drag force on satellites, causing their orbits to decay slowly and ultimately resulting in a fiery disintegration at lower altitudes. By analyzing changes in the orbits of the selected objects, the scientists derived the yearly average density encountered by each object. After adjusting for other factors, the data from every object indicated a long-term decline in the density of the thermosphere.These new results verify and significantly expand a limited earlier investigation, by scientists at The George Washington University, which also used orbital data to derive a long-term decrease in thermospheric density. The new NRL study utilizes more orbital data over a longer period of time and employs more precise analysis methods. By carefully examining all potential sources of error, the NRL team has provided solid evidence that the trend is neither artificial nor the result of physical processes other than internal atmospheric cooling.
Based on the NRL analysis and projections of carbon dioxide levels in the atmosphere, the density at thermospheric heights could be cut in half by the year 2100. This change may present mixed blessings: while operational satellites will be able to stay aloft longer, using less fuel, so will damaging spacecraft debris, potentially increasing the frequency of collisions.