John C. Houbolt, an engineer whose contributions to the U.S. space program were vital to NASA's successful moon landing in 1969, has died. He was 95. Houbolt died Tuesday at a nursing home in Scarborough, Maine, from complications from Parkinson's disease, his son-in-law Tucker Withington, of Plymouth, Mass., confirmed Saturday. As NASA describes on its website, while under pressure during the U.S.-Soviet space race, Houbolt was the catalyst in securing U.S. commitment to the science and engineering theory that eventually carried the Apollo crew to the moon and back safely.