PARIS — A decade-long experiment using a human-like mannequin to assess radiation absorption inside and outside the international space station has concluded that the human body is much better at protecting astronaut internal organs than previously thought.

The experiments, which used U.S. technology monitored by U.S., Russian, Japanese and European teams, conclude that previous radiation-intake measures, mainly dosimeters worn by astronauts in their pockets or on their chests, overstate the radiation exposure to internal organs.

For an astronaut working inside the space station, the overestimate was about 15 percent — a fairly close correlation given that the station’s exterior shell provides much of the protection needed.

But for astronauts working outside the station, the radiation absorption measured was substantially less than what had been registered by the personal dosimeters worn by astronauts.

“Measurements of a personal dosimeter dramatically overstate the exposure of an astronaut, in the worst case by a factor of three,” according to a summary of the results by a Euro-Russian team. “[I]n an outside exposure the self-shielding of the human body is very effective. … [T]he effective dose equivalent is less than 30 percent higher than in an inside exposure.”

The results were published

recently in Radiation and Environmental Biophysics, published by Springer Group.

“The result is important for planning long-term missions: It means that you can fly farther and fly longer,” Vyacheslav Shurshakov of the Russian Academy of Sciences’ Institute of Biomedical Problems said in an Oct. 21 statement.

Shurshakov cautioned that the results should not be used to minimize the risk to astronauts on long-term missions beyond Earth orbit, such as to Mars. “Even with these data, possible radiation doses for travelers to Mars is still too high … which creates an unnecessarily high risk of cancer. Experts have to look for ways to reduce the radiation dose or reduce the period of the flight.”

Orbiting at about 400 kilometers in altitude and inclined 51.6 degrees relative to the equator, the space station is subject to radiation levels that have been estimated at 100 times the exposure at sea level.

The experiments, called Matroshka, used a Rando mannequin provided by The Phantom Laboratory of Salem, New York. Rando is a human-skeleton torso padded with polyurethane to simulate soft human tissue and muscles. Other materials said to be “radiologically equivalent” to human organs were used for the lungs, brain, thyroid, pancreas, liver, colon, testicles and other organs.

The 84-centimeter-high, 40-centimeter-wide and 22-centimeter-deep Rando was crisscrossed with 1,634 detectors. Rando comes in male and female versions; the one used was male.

The Mastroshka facility was designed for the 20-nation European Space Agency by the German Aerospace Center, DLR. The Rando mannequin was placed outside the station for a year, to simulate radiation exposure by an astronaut performing an extravehicular activity. During its time outside, it was inside an airtight, carbon-fiber container built to approximate the radiation absorption of an astronaut’s spacesuit.

A similar experiment had been used in the U.S. space shuttle in 1989 but it was never placed outside the shuttle.

Peter B. de Selding was the Paris bureau chief for SpaceNews.