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Moon safe for long-term human research, first measurements of surface radiation show
By Adam Mann September 25, 2020, 2 p.m.
Moonwalkers take heart – China's Chang & # 39; e 4 Lander has made the first detailed measurements of the intense radiation blasting the lunar surface and found it to be safe for human exploration. The results give researchers a better idea of how much protective shield future crews will need.
Astronauts on the Apollo missions of the 1960s and 1970s wore dosimeters to measure their radiation exposure, but the devices recorded total exposure throughout their journey – not just their time on the lunar surface. Since then, scientists have had to estimate the radiation doses of crews walking around on the lunar surface "by extrapolation and modeling," says physicist Robert Wimmer-Schweingruber from the University of Kiel, co-author of the study. "We have never measured them exclusively on the moon."
There is renewed interest in such measurements, however, as NASA's Artemis program plans to land crews for long term stays by 2024 and the Chinese space agency seeks human missions sometime in the 2030s. The Chang & # 39; e 4 robot made history last year when it landed in Von Kármán Crater on the other side of the moon and brought a number of instruments with it.
The measured dose is about five to ten times as high as that received by passengers on an intercontinental flight from New York City to Frankfurt when the aircraft is over parts of the protective atmosphere, says Wimmer-Schweingruber. Although radiation is high by Earth-based standards, it is one of the known dangers of space travel. NASA is legally prohibited from increasing the risk of their astronauts dying of cancer by more than 3%, and these levels stay below that.
In addition, the researchers calculated that lunar bases covered with at least 50 centimeters of lunar soil would be sufficient to protect them. A deeper chamber, shielded with about 10 meters of water, would be enough to protect against the occasional solar storm, which can lead to a dramatic increase in radiation levels. (Between the Apollo 16 and 17 missions, the sun rose in a way that could have caused radiation sickness, vomiting, and possibly death if astronauts had been exposed in space at that time.) Such a chamber should have been within 30 minutes the duration of the extended warning time that is now possible with the monitoring of satellites.
"Fig. 1 View of the Chang & # 39; E 4-Lander with the position of the LND sensor head indicated by the red arrow.
LND is mounted in the Chang & # 39; E 4 payload compartment. The lid at the tip of the red arrow is closed at night to protect LND from the cold moonlit night. Photo credit: China National Space Agency (CNSA) and China National Astronomical Observatories (NAOC). "
The full paper is available:
LND measured an average dose equivalent of 1369 μSv / day on the lunar surface. For the same period, the dose equivalent measured with the DOSIS 3D DOSTEL instruments (29) on board the International Space Station (ISS) was 731 μSv / day with contributions from GCR only of 523 μSv / day. The extra ~ 208 μSv / day is due to protons crossing the South Atlantic Anomaly. Therefore, the daily GCR dose equivalent on the lunar surface is a factor of 2.6 higher than the dose inside the ISS. Since the sun is currently still at an extended activity minimum (30), the dose rate given here from the GCR can be viewed as the upper limit for human exploration of the moon under conditions of low solar activity. Settlements on the moon provide additional shielding as they are buried under layers of lunar regolites. While this would decrease the dose rate of charged particles, it is expected that the absolute contribution of neutrons to shielding from in situ resources will increase, as measurements with the Apollo 17 lunar neutron probe experiment show. These showed that the flow of thermal and epithermal neutrons increases significantly up to a depth of approx. 150 g / cm2 (31).
LND has for the first time measured the radiation environment on the lunar surface with this accuracy. Due to the fact that we are now approaching the minimum solar conditions, the contributions of the GCR can be taken as upper estimates for the GCR dose. No SPE was observed from the lunar surface during the period given here. Such events can only add orders of magnitude to the dose by a thin shield (32).
Zhang et al., 2020
Chang & # 39; e 4 explores the "Von Kármán crater in the gigantic South Pole Aitken Basin" …
Figure 2. Farside Von Kármán's crater of the moon
The Apollo astronauts' dosimeters measured the total exposure over the duration of the missions. Therefore, it was not possible to separate the exposure from the lunar surface during space travel.
Figure 3. SP-368 Biomedical Results from Apollo
The Apollo 14 astronauts (Alan Shepard, Stuart Roosa, and Edgar Mitchell) received the highest dose of radiation of any Apollo mission. 1.14 rad corresponds to 11,400 μSv, which corresponds to about 8 full days of exposure on the lunar surface. The radiation exposure for Apollo 14 was high because the runway led the spacecraft through the "heart of the enclosed radiation belt" and a higher level of background radiation than the other missions. Despite the radiation exposure, Alan Shepard set the record for the longest golf swing on the moon.
Commander Shepard's record still stands today, almost 50 years later.
In December 2014, NASA conducted the first unmanned test flight of the Orion spacecraft, a type of spacecraft used for the Artemis Lunar missions. Exploration Flight Test One completed two orbits, including one high orbit, and crossed the heart of the Van Allen Radiation Belt twice.
Figure 4. NASA EFT-1
Three hours and five minutes after takeoff, Orion peaked and began its descent back to Earth, separating from the second stage about 18 minutes later. In the second phase, a one minute waste incineration was carried out to ensure that the spacecraft's flight path was not affected. During the return journey through the Van Allen Belt, Orion fired its engines for 10 seconds to adjust course for re-entry. At an altitude of 400,000 feet, the spacecraft encountered the first tendrils of Earth's atmosphere at a point called the Entry Interface and was moving at 20,000 miles per hour (mph). An accumulation of ionized gases caused by re-entry heating resulted in a loss of communication with Orion for about two and a half minutes. The spaceship experienced a maximum heating of about 4,000 degrees Fahrenheit, which proves the value of the heat shield. After Orion's front bay cover was released, two drogue parachutes were deployed to slow and stabilize the spacecraft. Next came the deployment of the three main parachutes, which slowed the spaceship to 32 km / h. Splashdown occurred 4 hours and 24 minutes after takeoff about 600 miles southwest of San Diego, California. A video of the Orion EFT-1 mission can be viewed here.
The cumulative radiation exposure corresponded to about 10,000 μSv. Most of the exposure occurred during the spacecraft's approximately hour-long journey through the radiation belts.
Bahadori AA, Semones EJ, Gaza R, Kroupa M, Rios RR, Stoffle NN, Campbell-Ricketts T, Pinsky LS, and Turecek D 2015. Battery powered independent radiation detector data report from exploration flight test 1. NASA / TP-2015-218575 NASA Johnson Space Center : Houston, TX http://ston.jsc.nasa.gov/collections/TRS/397.refer.html
S. Zhang et al. First measurements of the radiation dose on the lunar surface.Advances in science. Published online 25 September 2020. doi: 10.1126 / sciadv.aaz1334.