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But as reports accumulated, some proved difficult to explain. Most UFO reports could eventually be connected to familiar objects or natural phenomena. Aircraft could look unusual at a distance. Balloons could appear to move strangely. Planets and stars could be mistaken for moving objects. Weather conditions could distort perception. Human beings are also imperfect observers, particularly when attempting to judge distance and speed in an open sky. Yet a smaller number of reports remained difficult to resolve. Some involved multiple witnesses. Some included radar observations. Others described apparently unusual movements. These were the cases that attracted serious attention. If an object was simply a misidentified aircraft, there was little reason for concern. But if a genuinely unknown object was capable of extraordinary speed or manoeuvrability, the implications were very different. The Air Force faced an uncomfortable contradiction. Its job was to know what was flying over the United States. Yet some objects appeared to resist identification. That was a difficult situation for an organisation built around control of the skies. In 1948, Project Sign produced an assessment that became famous as the "Estimate of the Situation." Later accounts have described the document as favouring an extraterrestrial explanation for at least some of the unexplained sightings. The precise history of the document has been debated, and many details of the episode come from later recollections rather than a complete surviving record. What is clear is that the idea of extraterrestrial craft was taken seriously enough within parts of the early investigation to become a subject of official discussion. That alone was remarkable. The United States military was not announcing that aliens had arrived. It was considering possible explanations for observations that remained unexplained. That distinction is important. An investigation can consider an extraordinary possibility without proving it. General Hoyt Vandenberg, the Air Force Chief of Staff, reportedly rejected the extraterrestrial interpretation because he wanted stronger evidence. That position was understandable from a military perspective. A claim that unknown objects were spacecraft from another world required extraordinary evidence. Radar data, photographs, physical material or other independently verifiable information would be far more convincing than eyewitness reports alone. The early investigators therefore found themselves caught between curiosity and caution. They had reports they could not always explain. But they did not have the evidence required to establish what the objects actually were. Project Sign eventually gave way to Project Grudge. The tone of the investigation became more sceptical. Where Project Sign had attempted to examine the phenomenon, Project Grudge placed greater emphasis on conventional explanations. Sightings were assessed against possibilities such as aircraft, balloons, astronomical objects, atmospheric effects and human error. That approach had advantages. A serious investigation must eliminate ordinary explanations before considering extraordinary ones. But there was also a danger. If investigators began with the assumption that every report must have a mundane explanation, they could simply replace one form of bias with another. The purpose of an investigation should be to determine what the evidence supports. It should not begin by deciding what the answer must be. This became one of the central problems of the early UFO investigations. The military was attempting to protect national security while also dealing with a phenomenon that had become a public obsession. Officials worried that public reports could create panic. They also worried that genuine enemy technology could be hidden among the thousands of reports. The Cold War made the situation even more complicated. The Soviet Union was developing its own military capabilities, and the United States feared technological surprises. An unidentified aircraft was therefore not simply a curiosity. It could potentially be