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Dear Diary, the astronauts write.

Well, maybe not in those exact words. But three times a week, the two U.S. astronauts aboard the international space station write down their secret thoughts in their personal journals.

They write about their moods, their whines, how they feel, what they miss, whether they're sick of the food or aren't getting along with their roommates up in space.

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It may sound like high school, but it's really for science.

These diaries will be reviewed by a researcher in California who wants to measure how spending six months cooped up with just two other people at a time, 220 miles above Earth, can affect outlook and morale.

"It comes out looking like a gossip column, I'm sure," said astronaut Sunita Williams shortly before she arrived at the space station in December. "But the point is to identify characteristics that will make expeditions successful."

Williams and Michael Lopez-Alegria, the station's other current U.S. crew member, are told to be brutally honest.

While astronauts also typically keep public journals that are available on NASA's Web site, these entries will be read only by Jack Stuster, a Santa Barbara, Calif.-based researcher who has been downlinking space station journal entries once a month since 2003.

Click here to read NASA astronauts' public journals.

The results of Stuster's investigation will help NASA and other space agencies plan for and train astronauts for even longer stays on the moon and Mars in the future.

According to the Bioastronautics Roadmap, the document NASA uses for identifying and reducing risks in space, some U.S. and Russian crew members periodically fail to work well with each other.

"Interpersonal distrust, dislike, misunderstanding and poor communication have led to potentially dangerous situations, such as crew members refusing to speak to each other during critical operations, or withdrawing from voice communications with ground controllers," the Bioastronautics Roadmap said.

Often the gripes have to do with logistical issues and inefficiencies on the ground and in space.

The Russian cosmonauts appear to have a more antagonistic relationship with their mission controllers on the ground than Americans do, the U.S. astronauts have commented.

"Some of [the Russians] feel obligated to argue about every little thing," Stuster said. "But that might be more of a cultural thing."

Stuster, who has a doctorate in anthropology, breaks down the prose descriptions into measurable data by dividing the entries into 18 categories and noting whether the tone is positive, negative or neutral.

He also notes how many days into the mission the entry is made so he can divide the information into quarters of time. Astronauts suffer what might be described as the third-quarter blues; their positive entries drop during the third quarter of their stay. Stuster noticed a similar pattern in an earlier study he did of French doctors living in Antarctica.

Former astronaut Leroy Chiao, who lived at the space station in 2004 and 2005, found the journal-writing helpful.

"I used it almost as a therapy for myself — if I were upset about something or frustrated, I'd write that out," Chiao said.

Sometimes those entries were long, he said.