While thinking about trying to book a rocket ride to the Moon and Mars, besides the habitability of the planets, another point to be added to the specific checklist is space radiation. Of course, if the plan is to the Moon- and-Mars-and-back, radiation consideration might be somewhere at the bottom.
But to persuade friends and family about longer stays, one has to factor in sporadic solar energetic particles (SEPs) on lunar and martian surfaces generated from extreme solar eruptions. The levels of radiation vary, but it can touch potentially hazardous values. The hand luggage, if allowed to carry, should have radiation-proof suits.
On October 28, 2021, a SEP event occurred that emitted such high levels of energy that it triggered ground-level-enhancement (GLE) on the surfaces of Earth, the Moon, and Mars. For the first time, a GLE event was observed on three planetary surfaces. The study published in the Geophysical Research Letters enumerates potential SEP radiation risk to humans envisaging shuttling between the two planets in the next few decades.
The outburst was detected by a fleet of spacecraft, including ESA’s ExoMars Trace Gas Orbiter (TGO), NASA’s Curiosity Mars rover, the CNSA Chang’e-4 moon lander, NASA’s Lunar Reconnaissance Orbiter (LRO), and DLR’s Eu:CROPIS Earth orbiter reports phys.org.
GLE events are rare and this is only the 73rd between February 1942 and October 2021. But only recently, multi-point observations at vantage points within the heliosphere have been available.
During GLE, charged particles from the Sun in the aftermath of a solar outburst breach the layers of the Earth and its magnetic bubble. With Earth and Moon in each other’s vicinity, the spectra derived at Earth were applied to the Moon. But the Moon does not have a global magnetic field or an atmosphere, so SEPs can reach its surface directly and interact with the lunar soil. The Red Planet’s radiation dose is much smaller during the SEP event because of the atmospheric shielding that prevents low-energy protons from penetrating. High-energy SEPs are mainly accelerated during the initial eruption phase.
Astronauts are exposed to radiation sickness, and anything above 700 milligrays – an absorption unit of radiation – can destroy bone marrow, resulting in symptoms like infection and internal bleeding. More than 10 gray can be fatal.
Results suggest that extreme SEP events on Mars and Moon can induce significant radiation risks. On Mars, the radiation threshold has never been crossed, but in 12 of the 67 events on the Moon, the levels have climbed above the mark. NASA’s Lunar Reconnaissance Orbiter measurement showed the dose in the lunar orbit is only 31 milligray. ExoMars TGO showed it was 9 milligray on Mars for the October 28 event.
Two mannequins - Helga and Zohar – were equipped with radiation sensors provided by the German Aerospace Centre and NASA. Helga flew unprotected while Zohar had a newly developed protection vest. DLR is comparing the two datasets.