Als Ende November 2022 das amerikanische Raumschiff Orion zu seiner ersten Mondumrundung aufbrach, saßen im Cockpit die beiden Dummy-Astronauten – „Helga“ und „Zohar“. Die beiden Fluggäste sollten während der 25 Tage dauernden Artemis-I-Mission die Strahlung registrieren, der lebende Astronauten während eines Flugs zum Erdtrabanten an Bord einer Orion-Kapsel ausgesetzt wären. Nach einer Analyse der gewonnenen Daten präsentieren die an der Mission beteiligten Wissenschaftler nun die ersten Messergebnisse in der Zeitschrift „Nature“.
Zwar hat es schon während früherer Weltraummissionen Strahlenmessungen gegeben, sie fanden aber hauptsächlich auf dem amerikanischen Space Shuttle und der Internationalen Raumstation an Bord des europäischen Columbus-Laboratoriums im niedrigen Erdorbit statt. Die ISS kreist in 400 Kilometern Höhe um die Erde. Dort wird der Löwenanteil der Strahlung aus dem Weltraum vom Erdmagnetfeld abgeschirmt. Einige Strahlungsmessungen stammen auch von den Apollo-Mondmissionen aus den 1970er-Jahren.
Weiblicher Anatomie nachempfunden
Mit der Mission Artemis I wurde erstmals die kontinuierliche Strahlenbelastung in einem Orion-Raumschiff jenseits des erdnahen Orbits erfasst. Dort dominieren die ionisierende Strahlung des Van-Allen-Gürtels – dies sind zwei ringförmige Bereiche um die Erde, die sich in einer Höhe zwischen rund 700 und 6000 Kilometern befinden – der Sonnenwind, der schnelle geladene solare Teilchen ins All weht, sowie die kosmische Strahlung, die vorwiegend aus energiereichen Wasserstoff- und Heliumkernen besteht.
Um Informationen über die Strahlungsbelastung innerhalb der Orion-Kapsel zu erhalten, sind vor dem Start an verschiedenen Stellen des Raumschiffs Dosimeter angebracht worden. Die beiden Puppen – sie stammen vom Institut für Raumfahrtmedizin des Deutschen Zentrums für Luft- und Raumfahrt in Köln und der israelischen Raumfahrtagentur Isa – hatte man mit unzähligen Sensoren bestückt. Diese sollten Daten liefern, welcher Strahlenbelastung die menschliche Haut, die Knochen und die inneren Organe im Weltraum ausgesetzt sind.
The scientists have modelled both measurement dummies on the anatomy of female astronauts. Christina Hammock Koch, the manned moon mission to Artemis II, which is planned for next year, will be the first woman on board. In order to protect the astronaut as well as possible from radiation during the flight, NASA needs more knowledge about the potential exposure of women during space flights. We know from the dropping of atomic bombs that they have a higher risk of cancer from ionising radiation than men. Zohar also wore a radiation protection vest developed by an Israeli start-up. This is filled with a special liquid that is designed to protect the body against high-energy electrons in particular.
Different radiation exposure in the capsule
The study, in which researchers from the American and European space agencies NASA and ESA were also involved, shows how high the radiation exposure was in each phase of the flight to the moon and back. Half an hour after launch on November 16, the Orion spacecraft flew through the inner Van Allen radiation belt. The concentration of high-energy protons is particularly high there. Accordingly, the radiation dose increased noticeably inside the capsule. However, the measured values were not the same everywhere. In some cases, they differed by as much as four times, depending on whether the sensors were installed in protected or less shielded areas.
The highest measured exposure was in the cockpit, the lowest in the shelter. The crew can retreat to this shielded area when there is a high level of radiation. According to NASA, the astronauts are safe there even from violent radiation outbursts from the sun, such as those experienced this spring. Strong solar storms had dose rates similar to those of the inner Van Allen radiation belt, write the researchers led by Stuart George from NASA’s Johnson Space Center in Houston and Thomas Berger from the DLR in their study.
However, no solar events were detected during Artemis I’s 25-day mission. In addition to the protons and electrons of the radiation belt, galactic cosmic radiation dominated. However, this was reduced as the moon orbited due to its shielding effect, according to the researchers.
And the two dummies? They would have been exposed to a radiation dose of between 26.7 and 35.4 millisieverts throughout the mission – the unit sievert is used to measure the risk of ionizing radiation. These values would be well below the limit of 600 millisieverts that NASA astronauts are allowed to absorb during their entire career. In Germany, the radiation limit for the general population is one millisievert, and for people occupationally exposed to radiation it is 20 millisieverts per year.
Flight maneuvers reduced radiation exposure
According to the scientists, the radiation exposure of the planned Artemis missions, which will last from a few days to weeks, will probably not exceed the current NASA limits for astronauts – provided that similar mission conditions as for Artemis I are maintained. The researchers are also optimistic about the radiation exposure of a Mars flight. The astronauts are mainly exposed to galactic cosmic radiation, and this is constant outside of the Earth’s magnetic field. The total dose will probably remain well below the aforementioned 600 millisievert limit.
The orientation of the spacecraft during the Artemis I mission also had an effect on the radiation levels inside the capsule. This was evident when it made a 90-degree turn in the Van Allen radiation belt for its flight to the moon. The radiation dose unexpectedly fell by 50 percent. The incoming radiation now hit the spacecraft primarily where the strongest shielding is located, the researchers explain the effect. “This shows us that with such flight maneuvers, the radiation exposure for the crew inside the spacecraft can be significantly reduced,” says DLR radiation physicist Berger.
The Orion capsule is also equipped with the HERA (Hybrid Electronic Radiation Assessor) radiation measuring and warning system. It consists of three radiation sensors that are installed in different shielded areas of the spacecraft. If the crew is exposed to a high-energy radiation event, such as a strong solar flare, a warning signal sounds. In this case, the crew would move to a better shielded part.
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