On October 20, about 2000 Pacific daylight time, a Cessna 172, N6000A, registered to and operated by the pilot as a 14 CFR Part 91 personal flight, collided with a tree while maneuvering near the Siletz Bay State Airport, Gleneden Beach, Oregon. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft was substantially damaged and the private pilot and passenger were not injured. The flight departed from Hillsboro, Oregon, about 30 minutes prior to the accident with the intended destination of Siletz Bay State. During a telephone interview, the pilot reported that he was maneuvering around the airstrip to determine runway conditions. There was ground fog present and the lighting conditions were "very dark." The pilot circled the airstrip three or four times and he had the rotating beacon and runway lights in sight, however, he lost sight of the runway lights and when he passed by the rotating beacon he pulled up and heard something hit the right wing. The pilot believed that he was near the southwest end of the airport environment when this happened. The pilot stated that he was unsure if he hit a tree or a bird, but decided not to land and diverted to McMinnville, Oregon, a distance of 42 nautical miles to the northeast, where a landing was made without further incident. Inspection of the aircraft revealed that the right wing tip, to include the outboard rib and about half of the right side aileron had been torn away.