far eastern unit was based at the Naval Air Facility Atsugi, Japan. Part of the reason for the May reauthorization was that the CIA promised that improvements from Project RAINBOW would make the majority of U-2 flights undetected. On 2 April 1957, a RAINBOW test flight crashed in Nevada, killing the pilot. The U-2's large wingspan slowed its descent during crashes, often leaving its remains salvageable; Lockheed was able to rebuild the wreckage from the incident into a flyable airframe, but that it could do so should have been evidence to the CIA that its cover story might not be viable after a crash in hostile territory. The RAINBOW anti-radar modifications were not very successful, and their use ended in 1958. Soviet overflights resumed in June 1957 from Eielson Air Force Base in Alaska to the Russian Far East, which had less effective radar systems. Others originated from Lahore, Pakistan. A Lahore flight on 5 August provided the first photographs of the Baikonur Cosmodrome near Tyuratam: the CIA had been unaware of its existence until then. Other flights examined the Semipalatinsk nuclear test site and the Saryshagan missile test site.Pedlow and Welzenbach 1992, pp. 135–139. After a few more overflights that year, only five more took place before the May 1960 incident because of Eisenhower's increasing caution. The president sought to avoid angering the Soviets as he worked to achieve a nuclear test ban; meanwhile, the Soviets began trying to shoot down U-2 flights that never entered Soviet airspace, and the details in their diplomatic protests showed that Soviet radar operators were able to effectively track the aircraft. To reduce visibility Lockheed painted the aircraft in a blue-black color that helped them blend in against the darkness of space, and the CIA aircraft received the more powerful Pratt & Whitney J75-P-13 engine that increased maximum altitude by , to . In April 1958, CIA source Pyotr Semyonovich Popov told his handler George Kisevalter that a senior KGB official had boasted of having "full technical details" of the U-2, leading Bissell to conclude the project had a leak. The source of the leak was never identified, although there was speculation that it was Lee Harvey Oswald, then a radar operator at a U-2 base in Japan. The Soviets developed their own overflight aircraft, variants of the Yak-25, which in addition to photographing various parts of the world through the early 1960s