Prior, BS 65, was the NASA Principal Investigator for PAGEOS, still the second largest satellite by volume to be injected into earth orbit. He published a paper on orbital drag measurements at altitudes above 1,000 kilometers, from observations of the balloon satellite's orbital decay (The Use of Artificial Satellites for Geodesy, Byrd Press, 1972, pp. 197-207), and found hydrogen levels to be 200 percent greater than given in the U.S. Standard Atmosphere. Prior's results were so unexpected at the time that his own NASA supervisor decided not to cite the paper or recommend inclusion of his hydrogen observations in the next upcoming version of the U.S. Standard Atmosphere, published four years later in 1976.
According to Prior, hydrogen has been difficult to measure in the upper atmosphere because its concentration is so low that few instruments can measure it accurately. At the high altitudes where PAGEOS orbited, hydrogen predominates over the other heavier atmospheric gases. Researchers in England had been attempting to measure drag effects at such altitudes from the orbital decay of balloon satellites as a way to determine the concentrations of hydrogen in the upper atmosphere, but none had succeeded. The problem was complicated by the fact that even minute earth-reflected radiation (earthshine) forces gently push balloon satellites out of their orbits, an effect that has to be accounted for in order to isolate hydrogen drag.
Prior was familiar with the work of his Illinois astronomy instructor, Prof. Stanley P. Wyatt (now deceased). Wyatt was an expert on earth-reflected radiation forces and Prior successfully incorporated Wyatt's modeling approach into his hydrogen drag research with PAGEOS. The result was the first hydrogen drag measurements from a satellite orbit, which were so much greater than expected that Prior concluded hydrogen concentrations were three times greater than given in the U.S. Standard Atmosphere.
In 2008, the Air Force Space Command announced Prior's results had finally been accepted and incorporated his 36-year-old hydrogen measurements into its new atmospheric drag model, used by the AF Space Battlelab for missile trajectories and for satellite orbit predictions.
Prior, who holds advanced degrees in astronomy from the University of Virginia and in information systems from George Washington University, retired from NASA in 2005, ending a 40-year career. His research has been variously cited in The Stratosphere: Past and Present, the COSPAR International Reference Atmosphere, the U.S. National Report on Geodesy, and the various U.S. Standard Atmosphere publications. There are a total of six references to his research in three different annual editions of Significant Achievements in Space Science.