The pilot was contracted to fly alongside weather balloon launched from Northern Minnesota and flew WESTWARD towards Ellsworth AFB. This was completed multiple times without incident. On one flight, as the balloon and aircraft were nearing the "drop zone" for the instrumented package on the balloon, near the Air Force Base, the pilot notices a disk-shaped object near the balloon. He photographed the object, the balloon and the object, and the wingtip of the aircraft with the balloon and the object in the background. He radioed the sighting to the air base and asked if they saw the object on radar. The air base controller instructed the pilot to complete some maneuvers, which the pilot did. the controller stated that he did not see the object on radar. The object remained near the balloon. A moment later, when the pilot looked towards the balloon again, the object had disappeared. After the instrumented package was dropped form the balloon, the pilot circled the area until the package was picked up on the ground. Then the pilot requested permission to land at Ellsworth AFB to refuel and return to his base in Minnesota. This was the typical procedure at the end of each balloon-tracking event. However, this time the pilot was ordered to land at another airport. The pilot checked his air navigation map and discovered that the airport was shown to be closed. The pilot replied that the airport was closed. He was again instructed to land there. On the final approach to the closed airport, the pilot noted abandoned hangers with no one on the ground. Once he landed on the runway, jeeps with personnel in them came out of the abandoned hangers and blocked his Navion aircraft in front and in black of his aircraft. He was interviewed in his aircraft. His camera was taken and the film removed. His camera was given back to him, without the film. He was told that if he ever mentioned this to anyone, he would lose his contract - and more...Apparently, it was several years before he told a couple of his friends who were at the our local airport, sitting in the lobby during a blizzard outdoors. They were "hanger flying". The airport was empty, except for the four of us. I was a teenager who worked at the airport and was sweeping the lobby floor. For whatever reason, the pilot suddenly looked around, and then in a very hushed tone, began to describe the UFO event. Although there was no one else around, he talked in almost a whispered tone, and kept looking around to make sure no one else was listening. He told everyone not to mention this to anyone else. The pilot was the Manager of the our local airport, in addition to his aircraft charter service. he was also the Executive Pilot for a large company in our town which had an Aero Commander as their company plane, I viewed a "Hanger One" series last night that discussed an incident near Ellsworth AFB, which caused me to contact you. I have another second-hand account of a UFO landing on a golf course, about 1963 - 1964, which a good friend who worked at the golf course at night watering the golf course greens,told me of his experience.