Documents / Report
This August 1949 technical report from the US Air Force Air Materiel Command presents Project Grudge's investigation of unidentified flying objects through January 1949. Drawing on consultants including Dr. J. A. Hynek, Rand Corporation, the Air Weather Service and Dr. P. M. Fitts, it explains most of 244 incidents as balloons, aircraft, astronomical phenomena, hoaxes or psychological errors. It concludes the objects pose no direct threat to national security and recommends reducing the investigation in scope.
must have been orphaned Arnold states that the object should be about 20 times as long as wide. Let us assume that the thickness was just discernible, which means that the object was just at the limit of resolution of the eye. Now, the eye cannot resolve objects that subtend an angle of approximately less than 3 minutes of arc, and, in general, for any detail to be seen at all, the angle subtended must be much greater. Even if we assume the limiting resolution or 3 minutes, then, if the distance was 20 miles, elementary calculations show that each object must have been at least 130 feet thick, and if, as Arnold's drawings indicate, the object was some 20 times longer than wide, it must have been about 2000 feet long. Looking at the matter in another way and assuming that Arnold's estimate of distance as 20 to 20 miles (12,000 feet) and his estimate of length as 6: to 30 feet are both correct, then it can be shown that the object will mustend an angle of only about 50 seconds of arc, which is definitely below the limit of resolution of the eye. If Arnold actually saw the objects, and if his estimate of distance is correct, that of size cannot be, and vice versa. It seems most logical to assume that his estimate of distance is far too great. In fact, assuming a reasonable limiting size to the objects of 400 feet, in order to show the detail that Arnold's drawings indicate, the distance must have been not over roughly six miles. At this distance the objects would have travelled 11 miles (rather than 47 miles) in 102 seconds, or at a rate of approximately 400 kPa. (Arnold's UNCLASSIFIE
Report, from the classic collection. The PDF is mirrored here; the original link is under it. The text was read from the page images by an OCR model; expect the odd misread word. 413 pages are in the text index: search them above, or from the library's search.