the main Japanese warship force to send an aircraft attack force against Henderson Field. The Ryūjō mission was likely in response to a request from Nishizo Tsukahara, the naval commander at Rabaul, for help from the Japanese combined fleet in neutralizing Henderson Field. At 12:20 and northeast of Guadalcanal, the Ryūjō launched six "Kate" bombers and 15 A6M Zero fighters to attack Henderson Field in conjunction with an attack by 24 "Betty" bombers and 14 Zero fighters from Rabaul. Unknown to the Ryūjō force, the Rabaul aircraft encountered severe weather, and returned to their base at 11:30am. The Ryūjō'''s aircraft arrived over Henderson Field at 14:23pm, and tangled with 14 Marine Wildcats and four Army P-400s while bombing the airfield. In the resulting engagement, three Kates, three Zeros, and three Marine fighters were shot down... but no damage was done to Henderson Field. Two Marine pilots were killed in the engagement as well as eight Japanese aircrewmen. All of these Japanese aircraft were eventually lost because, while they were attacking Henderson Field, the Ryūjō was sunk by aircraft from the aircraft carrier USS Saratoga, forcing the Japanese aircraft to ditch upon returning to the previous location of their carrier. On 31 August, the aircraft carrier USS Saratoga was torpedoed by a Japanese submarine. Returning to Pearl Harbor for drydock repairs, most of the Saratoga's aircraft and aircrewmen remained behind at Espiritu Santo. Admiral McCain planned to send some of these aircraft to reinforce the Cactus Air Force at Guadalcanal. September On 2 September, the U.S. Marine 3rd Defense Battalion began operating an air search radar at Henderson Field, which, along with reports from the coastwatchers, helped provide early warning of incoming Japanese warplanes. By 3 September, the day of Geiger's arrival, the CAF consisted of only 64 flyable airplanes. Due to the heavy losses that the CAF had sustained, Admiral McCain decided to immediately deploy the USS Saratoga's fighter squadrons to Guadalcanal. On 4 September 24 F4Fs of VF-5 flew from Espiritu Santo to Henderson Field. From 1 September through 8 September the Japanese aviation units at Rabaul concentrated on providing air cover for Japanese Army forces operating along the Kokoda Track on New Guinea, during the Kokoda Track campaign. On 9 September the Japanese resumed air operations against Henderson Field, with the objective of destroying the CAF and isolating the American forces on Guadalcanal. Between 21 August and 11