No, astronauts do not return from space with less mass than before they left Earth. According to the principle of conservation of mass, the total mass of a closed system remains constant unless there is an exchange of matter with the surroundings.
However, it is worth noting that astronauts may experience a loss of weight while in space due to the absence of gravity. In the microgravity environment of space, astronauts float freely, and their bodies are no longer subject to the constant force of Earth's gravity. This can give the impression of reduced mass or weight, but their actual mass remains the same.
The phenomenon you mentioned, traveling faster than light, is not achievable according to our current understanding of physics. The theory of relativity, proposed by Albert Einstein, sets a universal speed limit at the speed of light. Thus, it is not related to any mass loss experienced by astronauts during space travel.
However, it is possible for astronauts to experience a loss of body mass while in space due to changes in their physiology. Prolonged exposure to microgravity can lead to muscle atrophy, bone density loss, and fluid redistribution in the body. These physiological changes can result in a decrease in overall body mass, but this is distinct from a change in the fundamental mass of an individual.
To summarize, while astronauts may experience a loss of weight in space due to the absence of gravity, their actual mass remains the same. Any decrease in body mass is a result of physiological changes caused by living in a microgravity environment and not due to traveling faster than light.