We only know about Sign's estimate because USAF Captain Edward J. Ruppelt who was once head of Project Blue Book, saw the physical document and revealed its existence in his classic 1956 book, The Report On Unidentified Flying Objects . Below are excerpts from his book along with unpublished notes that were acquired by researcher Michael Swords. Together they clearly indicate that during the late 1940s, the USAF task force assigned to evaluate UFOs concluded that UFOs are of extraterrestrial origin.
Unfortunately the Estimate of The Situation was rejected by Chief of Staff, General Hoyt S. Vandenberg. A few months later it was declassified and ordered destroyed. 1 It is somewhat curious that a document of such historical importance would be ordered destroyed. Many other less significant documents from Project Sign were archived and are now publicly accessible. According to Ruppelt, a few copies never made it to the incinerator, but if they still exist, it is not known where they are. Several attempts have been made to locate them, but FOIA requests have so far been fruitless and the USAF has consistently denied any knowledge of them.
However even if the Estimate were to be found, we are still only dealing with documents, and as such they are in and of themselves no proof of the ETH. Furthermore, the conclusion that UFOs are interplanetary suggests that the supporting research favored planets within our solar system. In 1949, science considered the most probable planets to be Mars or Venus. However the work of scientists, most notably astrobiologist Carl Sagan, has since proven beyond any reasonable doubt that neither Mars nor Venus possess any advanced life or technology, and that even if they once had, it must have been so long ago that no remnants remain. Add to this the specious claims of period contactees, and we can already hear the laughter coming from CSICOP.
Any documents from Project Sign that have been declassified and released to the public reflect the conclusions and recommendations that Sign was ordered to come to, not what those who did the study actually believed. Lastly, the released documents could not in-themselves be part of the original Estimate. Their appearance does not match the description provided by Edward J. Ruppelt, most conspicuously, they are not printed on legal sized paper and the report cover is not black. Proving this point is an actual microfiche image of the Project Sign folder released by the USAF to the NARA archives ( see below ).
Microfiche image of the sanitized Project Sign folder:2