According to the P'ri Chadash (89:6), six hours is not meant literally, and a person should just wait from one meal until the time for the next meal. In the winter, the P'ri Chadash writes, this could be approximately four hours. (Disclaimer: I haven't noticed many people follow this opinion in practice).
It seems reasonable to interpret the P'ri Chadash as understanding that you simply wait until the time for the next regularly scheduled meal. As such, let's say six hours passed on Earth (during the springtime, for example) and four hours passed for the astronaut. He might decide to eat at the scheduled meal time on Earth. Perhaps the P'ri Chadash would consider this acceptable.
A more immediate application of this question would perhaps involve an airplane trip from west to east (e.g. during spring). Let's say the flight is four hours and the destination is two time zones ahead the departure location. Perhaps the P'ri Chadash would allow the passenger to eat a dairy meal for supper in Chicago four hours after he ate a meat meal for lunch in Los Angeles.
Note also the reasoning given by the P'ri Chadash in 89:2, which suggests that there must still be a significant amount of time that elapses for the eater before he eats a dairy meal, so it is not possible to assume that the P'ri Chadash would permit any duration shorter than approximately four hours.