Guest “Let’s light this candle!” by David Middleton
Moon safe for long-term human exploration, first surface radiation measurements show
By Adam Mann Sep. 25, 2020 , 2:00 PM
Moonwalkers take heart—China’s Chang’e 4 lander has made the first detailed measurements of the intense radiation that blasts the lunar surface and found that it’s safe for human exploration. The results give researchers a better idea of how much protective shielding future crews will need.
Astronauts on the Apollo missions of the 1960s and ’70s carried dosimeters to measure their radiation exposure, but the devices captured total exposure from their entire journey—not merely their time on the Moon’s surface. Ever since, scientists have had to estimate the radiation doses of crews bounding around on the lunar surface “from extrapolation and modeling,” says physicist Robert Wimmer-Schweingruber of the University of Kiel, a co-author of the study. “We’ve never actually measured them exclusively on the Moon.”
But there is renewed interest in taking such measurements, with NASA’s Artemis program intending to land crews for long-term stays by 2024 and the China National Space Administration eying human missions sometime in the 2030s. The robotic Chang’e 4 made history last year when it touched down in Von Kármán crater on the Moon’s far side, bringing a suite of instruments along for the ride.
The measured dose is about five to 10 times what passengers on an intercontinental flight from New York City to Frankfurt, Germany, receive when the plane is above parts of the protective atmosphere, Wimmer-Schweingruber says. Though high for Earth-based standards, radiation is one of the known dangers of spaceflight. NASA is legally prohibited from increasing the risk of its astronauts dying from cancer by more than 3%, and these levels remain below that.
What’s more, the researchers calculated that Moon 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 occasional solar storms, which can cause radiation levels to spike dramatically. (Between the Apollo 16 and 17 missions, the Sun flared up in a way that could have caused radiation sickness, vomiting, and possibly death had astronauts been unprotected in space at the time.) Such a chamber would need to be reachable within 30 minutes, the amount of advanced warning time that is now possible with monitoring satellites.
“Fig. 1 View of the Chang’E 4 lander with the location of the LND sensor head indicated by the red arrow.
LND is mounted in the Chang’E 4 payload compartment; the lid at the tip of the red arrow is closed at night to protect LND from the cold lunar night. Photo credit: Chinese National Space Agency (CNSA) and National Astronomical Observatories of China (NAOC).”
The full paper is available:
LND measured an average dose equivalent of 1369 μSv/day on the surface of the Moon. For the same time period, the dose equivalent onboard the International Space Station (ISS) as measured with the DOSIS 3D DOSTEL instruments (29) was 731 μSv/day with contributions only from GCR of 523 μSv/day. The additional ~208 μSv/day is due to protons while crossing the South Atlantic Anomaly. Therefore, the daily GCR dose equivalent on the surface of the Moon is around a factor of 2.6 higher than the dose inside the ISS. Because the Sun is currently still in an extended activity minimum (30), the dose rate from GCR reported here may be considered as an upper limit for human exploration of the Moon during conditions of low solar activity. Settlements on the Moon will provide additional shielding because they will be buried beneath layers of lunar regolith. While this would decrease the dose rate from charged particles, the absolute contribution from neutrons is expected to increase for shielding constructed from in situ resources, as borne out by measurements with the Apollo 17 Lunar Neutron Probe Experiment. These showed that the flux of thermal and epithermal neutrons increases significantly up to a depth of approximately 150 g/cm2(31).
LND measured the radiation environment on the surface of the Moon at this precision for the first time. In addition, due to the fact that we are now approaching solar minimum conditions, the contributions from GCR can be seen as upper estimations for the GCR dose. In the time period reported here, no SPE was observed from the surface of the Moon. Such events can increase the dose by orders of magnitude behind only thin shielding (32).
Zhang et al., 2020
Chang’e 4 is exploring “Von Kármán Crater, within the gigantic South Pole-Aitken basin“…
Figure 2. The Moon’s Farside Von Kármán Crater
The Apollo astronauts’ dosimeters measured the total exposure over the duration of the missions. So, it was not possible to separate the exposure during spaceflight from the Lunar surface.
Figure 3. SP-368 Biomedical Results of Apollo
The Apollo 14 astronauts (Alan Shepard, Stuart Roosa and Edgar Mitchell) received the highest radiation dose of any Apollo mission. 1.14 rads is 11,400 μSv, equivalent to about 8 full days of exposure on the Lunar surface. The radiation exposure for Apollo 14 was high because the launch trajectory took the spacecraft through the “through the heart of the trapped radiation belts” and a higher background radiation level than the other missions. Despite the radiation exposure, Alan Shepard set the record for the longest golf shot on the Moon.
Commander Shepard’s record still stands today, nearly 50 years later.
In December 2014, NASA conducted the first unmanned test flight of the Orion spacecraft, the type of spacecraft that will be used for the Artemis Lunar missions. Exploration Flight Test One completed two orbits, including a high orbit, twice transiting the heart of the Van Allen radiation belts.
Figure 4. EFT-1 NASA
Three hours and five minutes after launch, Orion reached its apogee and began its descent back toward Earth, separating from the second stage about 18 minutes later. The second stage conducted a one-minute disposal burn to ensure it didn’t interfere with the spacecraft’s trajectory. During the passage back through the Van Allen belt, Orion fired its thrusters for 10 seconds to adjust its course for reentry. At an altitude of 400,000 feet, the spacecraft encountered the first tendrils of the Earth’s atmosphere at a point called Entry Interface, traveling at 20,000 miles per hour (mph). A buildup of ionized gases caused by the reentry heating resulted in a communications blackout with Orion for about two and a half minutes. The spacecraft experienced maximum heating of about 4,000 degrees Fahrenheit, proving the worthiness of the heat shield. After release of Orion’s forward bay cover, two drogue parachutes deployed to slow and stabilize the spacecraft. Next followed deployment of the three main parachutes that slowed the spacecraft to 20 mph. Splashdown occurred 4 hours and 24 minutes after launch about 600 miles southwest of San Diego, California. A video of the Orion EFT-1 mission can be viewed here.
The cumulative radiation exposure was the equivalent about 10,000 μSv. Most of the exposure occurred during the roughly one hour the spacecraft travelled 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-operated 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.Science Advances. Published online September 25, 2020. doi: 10.1126/sciadv.aaz1334.