Gemini 4 (or Gemini IV) was the second manned space flight in NASA's Project Gemini, occurring in June 1965. It was the tenth manned American spaceflight. Astronauts James McDivitt and Edward H. White, II circled the Earth 66 times in four days. The highlight of the mission was the first space walk by an American, during which White floated free outside the spacecraft, tethered to it, for approximately 20 minutes. Both of these accomplishments helped the United States overcome the Soviet Union's early lead in the Space Race. The flight was the first American flight to perform many scientific experiments in space, including use of a sextant to investigate the use of celestial navigation for lunar flight in the Apollo program. It is rumored that Astronauts may have filmed a UFO with a movie camera, during this mission, while in orbit passing over Hawaii.