A European Space Agency (ESA) experiment aboard the International Space Station (ISS) suggests that space travelers may have less to worry about when it comes to radiation ... thanks to a phantom. Called the Matroshka, the "phantom" is a plastic mannequin that is the key component of the first comprehensive study of the effects of radiation on astronauts on long-term space missions that indicates that the hazard may not be as severe as previously thought.

Of all the perils of space travel, the most pervasive as it is intangible is radiation. Each day that an astronaut spends outside the protective confines of the Earth's atmosphere brings an increased chance of cancer and other conditions. According to ESA, a person on the ground soaks up about 2.5 mSv/year, while an astronaut on the space station can receive up to 1 mSv/day. This is the reason the European Astronaut Corps limits its members to 500 mSv/year and 1Sv for an entire career. (Sv or sievert is a unit used to measure of the health effect of small amounts of radiation on the body).

Surprisingly, despite this awareness, very little is actually known about exactly how much and what kind of radiation an astronaut is actually exposed to. It's to fill this gap that the Matroshka was sent to the ISS. Named after the famous Russian nesting dolls, it was built and operated by ESA in cooperation with Roscosmos and various European institutions, and was flown to the station in 2004. Its purpose was to measure the type and amount of radiation astronauts are exposed to both inside and outside the space station over a period of several years.

Matroshka covered by a container simulating a spacesuit (Photo: DLR)

The Matroshka is technically a phantom. That is, a radiological doll designed as a stand-in for a human being while testing radiation equipment or, in this case, space radiation. It consists of a head and torso made of 33 horizontal cross sections of plastic, each measuring 2.5 cm (1 in) thick. Layers are used, so the mannequin can be assembled around a central dowel, which makes it easy to install and remove sensors. Each layer is made of a special plastic that simulates the soft tissues of the body with different densities standing in for the muscles, liver, spleen, lungs, and so forth. In addition, there are pieces of real human bone inserted into the cross sections to provide the proper radiological properties and a battery of sensors.

Space radiation is composed mainly of cosmic rays made up of protons and other heavy ions instead of the more common gamma rays found in terrestrial radiation sources. Since there are many different kinds of radiation, a number of different active and passive sensors are needed to detect them. The Matroshka includes about a dozen different sensors, including detectors for recording pressure and temperature.

Among the sensors are six thousand passive thermoluminescent detectors – many of which were made by the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Kraków, Poland. They are constructed of doped lithium fluoride placed in plastic tubes set in a 3D lattice. The dopants upset the detector's crystalline structure, which sets up "forbidden" energy levels that capture electrons generated by cosmic rays. When the tubes are returned to Earth and heated in a laboratory, they release light in proportion to the amount of radiation they've absorbed.

The Matroshka phantom with astronauts S. Krikaliew and J. Philips on board of the International Space Station (Photo: NASA)

Over the Matroshka is a not very fashionable jacket, which isn't just there for looks. It also acts as a mounting for cables and additional detectors. The latter need to be placed on the outside of the mannequin in order to measure incoming radiation and skin exposure, and to simulate the dosimeters carried by all space station personnel.

From 2004 to 2009, the Matroshka sat inside one of the Russian modules on the ISS, but radiation hazards outside the station are many times greater than inside, so the phantom made a spacewalk for the first such exposure measurements ever made. Like a human astronaut, the Matroshka was clad in a spacesuit – or, at least, a simulated one made of layers of carbon fiber and plastic and filled with dry oxygen gas.

According to IFJ PAN, the Matroshka measurements indicate that exposure estimates for inside the station were 15 percent too high, while those outside the station were off by 200 percent. This indicates that travel to the Moon and perhaps Mars may be safer than previously thought. However, the team emphasizes that the ISS sits in a very protected orbit.

“We must remember that measurements within the Matroshka experiment were performed at low Earth orbit where the Earth’s magnetosphere significantly reduces the number of charged particles from cosmic radiation. In interplanetary space there is no such shielding,” says Dr. Pawel Bilski of IFJ PAN.

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