An astronaut's bone density decreases ten times as fast as that of an elderly woman on Earth over the same time period, said Hiroshi Ohshima, also of JAXA's Space Biomedical Research Office.
That's because weightlessness interferes with an astronaut's ability to store calcium in his or her bones. In space the body excretes about 250 milligrams more calcium a day than it takes in.
In addition to weakened or even broken bones, excess calcium moving through the body's waste system means that astronauts risk developing excruciating kidney stones.
In an effort to fight bone loss, Wakata will take a weekly bisphosphonate pill, a medicine shown to increase bone density and reduce fractures in osteoporosis patients.
The density of bones in his legs, spine, and heels will be measured before and after the mission.
"Being in space is like having an x-ray every day," said Aiko Nagamatasu of JAXA's Space Environment Utilization Center.
"[And the radiation] is quite different to that on the ground. Space radiation consists of heavy ions, which cause a remarkably high degree of DNA damage and mutation."
Long-term cosmic ray exposure increases the chances of developing cancer and eye cataracts.
To help measure the risk to ISS residents, Wakata will wear a small personal device called a dosimeter and position 12 similar gauges around Japan's Kibo module, where he and the other astronauts will reside.
For two 24-hour periods aboard the ISS, Wakata will wear an electrocardiograph (EKG) to monitor his cardiovascular functions.
Flight surgeons on Earth will pay particular attention to his heart during space walks, which require great exertion. The EKG will also monitor changes to his body clock caused by the absence of dawn and dusk.
After the electrodes are removed, a high-definition camera will be used to check Wakata's skin for rashes. A similar system could one day help doctors on Earth remotely examine injured astronauts.
The People's Challenges
Not all of Wakata's tasks are quite so weighty. On March 5 JAXA announced 16 special challenges for the astronaut suggested by members of the public.
Wakata has been asked to fly on a "magic" carpet, use eye drops, fold clothes, and arm wrestle in space—among other offbeat tasks selected from more than 1,500 entries.
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