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Protecting Lunar Colonies From Space Radiation

2009; American Geophysical Union; Volume: 7; Issue: 8 Linguagem: Inglês

10.1029/2009sw000517

1542-7390

Mohi Kumar,

Spaceflight effects on biology

When Apollo 7 astronaut Walter Cunningham blasted off from Earth on 11 October 1968, the last thing he was thinking about was radiation risks or any risks at all. “Fear doesn't even enter your mind because you have confidence in yourself, your own ability, your training, and your knowledge,” Cunningham told Space Weather. As a crew member of the first manned mission in the Apollo program and the first three-man American space mission, Cunningham spent 11 days in Earth orbit, testing life-support, propulsion, and control systems on a redesigned command module. In retrospect, compared with immediate risks such as those associated with launch and reentry, “exposure to radiation, which could have long-term effects—we just never gave that a thought,” Cunningham said. Ironically, the fact that astronauts embrace the risks of spaceflight allows NASA to protect humans on future missions. The success of Apollo 7 helped pave the way for the first flight to the Moon on Apollo 8 two months later and the first manned lunar landing by Apollo 11 the next year. Additionally, data from the Apollo missions’ radiation monitoring programs continue to help scientists research, understand, and prepare for radiation hazards at and on the way to the Moon. Now, 40 years after U.S. astronauts landed on the Moon, NASA plans to send humans back to Earth's nearest neighbor in the next couple of decades, with an eye to establishing long-term lunar colonies. Leading this venture is the Lunar Reconnaissance Orbiter (LRO), a robotic mission launched 18 June 2009 to help locate future landing sites, survey for resources, and characterize lunar radiation hazards. This, combined with research on astronaut health and radiation shielding, will help protect astronauts and instrumentation on the Moon. During the manned Apollo missions (1967–1975), trapped particles in the Van Allen belts were the main source of radiation to astronauts—fortuitously, the Apollo crews encountered no large solar flares while in space. On the basis of data from the four dosimeters outfitted on each spacecraft and the four dosimeters carried by each astronaut, NASA calculated that the radiation skin dose of the three-person flight crews for the Apollo missions ranged from 0.16 to 1.14 rems. One rem, short for “roentgen equivalent in mammals,” is the dose that will cause as much damage to human tissue as 1 roentgen of X-rays and is less than the dose delivered when a doctor checks for tumors with a computer-aided tomography (CAT) scan. In addition to monitoring dose, the Apollo program conducted scientific missions on the Moon's surface to help characterize radiation hazards. One experiment, the Solar Wind Spectrometer, analyzed the flux of electrons and protons streaming in the solar wind; it operated continuously between late 1969 and early 1976. Data from this experiment, NASA's Interplanetary Monitoring Platform and Pioneer spacecraft, and ground-based observatories in NASA's Solar Particle Alert Network recorded an extremely large solar flare event on 4 August 1972, within the 8-month interval between Apollo flights 16 and 17. According to Francis Cucinotta, a radiation health officer at NASA's Johnson Space Center, an astronaut caught in this storm without adequate shielding might have absorbed nearly 400 rems. If at least 300 rems arrive in one burst, “we estimate that 50% of people exposed would die within 60 days without medical care,” said Cucinotta. Such radiation bombardment would cause severe burns. Astronauts would also develop nausea and vomiting as tissues that line the esophagus and intestines slough off. “Cells become damaged, anemia develops, white blood cells stop forming, “ explained Richard Scheuring, a flight surgeon at NASA. “Any tissues that actively replicate—skin, the gastrointestinal tract, bone marrow—are the ones most sensitive to radiation,” he told Space Weather. Other effects may occur after the mission, such as early onset of cataracts or increased risk of cancers. If notified of an approaching solar event, Apollo crew members within their spacecraft were under instruction to hunker down in the more heavily shielded command and service module. If caught far outside their vehicle, they were told to cover themselves in the lunar regolith. However, more recent studies show that “you actually need up to 1 foot of regolith to protect you from significant radiation dose,” Scheuring explained. Nonetheless, an event could last for, say, 36 hours, and space suits have only enough oxygen and battery power for 7 or 8 hours. Thus, taking shelter outside the spacecraft can be fruitless—"Your suit is going to run out and you're going to die anyway,” Scheuring said. During the Apollo era, scientists also assumed that a dangerous solar flare “would take 15 to 20 hours after the detection of the X-rays for the protons and electrons to actually arrive on the Moon,” Scheuring said. However, a solar flare that occurred during International Space Station (ISS) operations in January 2005 gave quite different results. “We found out that the time could actually be as short as 15 to 20 minutes,” constraining the time available for astronauts to seek shelter, he said. Although much has changed in technology, including how scientists perceive radiation risks on the Moon, information collected during the Apollo era continues to guide research on space radiation hazards. For example, “data from the 1972 event help us to envision worst-case scenarios for astronaut exposure,” Scheuring noted. Recent space missions on shuttles and to ISS have kept humans within low-Earth orbit, shielded by Earth's magnetic field. But “once you get beyond the Earth's shielding you are fully exposed to solar protons and other particles,” David Everett, chief mission systems engineer for LRO at NASA's Goddard Space Flight Center, told Space Weather. “For long-duration human missions to the Moon and beyond, radiation will be a significant [mission] design issue.” LRO's goals include more fully defining the radiation environment at the Moon in preparation for lunar outposts. LRO's Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument will characterize the biological impacts of radiation at the Moon by measuring the dose rate within plastics designed to simulate human tissue, Everett explained. Driving LRO is the idea that in preparation for returning to the Moon, NASA plans to launch a new human spaceflight program, called Constellation, by the middle of the next decade. The first push of the Constellation initiative will be to shuttle crews and supplies between Earth and ISS on a new crew exploration vehicle, called Orion. These early Orion missions will “test-drive” Constellation's space exploration vehicle before it moves beyond low-Earth orbit at ISS, into the lunar environment, and eventually on to Mars. Prior to traveling to the Moon, Orion will rendezvous with the Altair lunar lander in low-Earth orbit. Once at the Moon, Altair will detach from Orion and land, similar to Apollo's lunar module. Altair can also be deployed as a cargo lander, which will help ship the building blocks of a long-term lunar outpost. While in lunar orbit or on the Moon, the crew will be exposed to solar flare particles and galactic cosmic rays (GCRs). Because of uncertainties in the amount of risk associated with GCRs, NASA's current guidelines maintain that astronauts cannot receive a radiation dose that would expose them to any greater than a 1% lifetime risk of developing cancer because of their occupational exposure, explained Scheuring, who is one of Constellation's medical operations flight surgeons. “If you exceed this level, you're done. You can't fly anymore.” Forecasting solar flares is continually improving, according to Andrew Holmes-Siedle, a space radiation scientist who consults on dosimetry for NASA and the European Space Agency. Better forecasts will allow astronauts time enough to shield themselves during radiation storms. Moreover, “lunar outposts are bound to have a large piece of machinery or a large vehicle; the more massive it is, the more there is a shielded region in the middle where you can put humans and the most sensitive electronics,” Holmes-Siedle told Space Weather. Areal density (grams per square centimeter) of shielding materials will also be an important factor. The hulls of Apollo command modules were about 8 grams per square centimeter; modern space shuttles are nearly 11 grams per square centimeter, and the most heavily shielded areas of ISS are 15 grams per square centimeter. By contrast, storm shelters in lunar colonies may be designed to exceed 20 grams per square centimeter, Cucinotta explained. On the Moon, though, GCRs are bombarding targets continually, unlike solar flares. To protect against GCRs, Holmes-Siedle recommended that future lunar colonists tunnel underground to find the best shielding. “Burying buildings in the Moon and building an underground network will stop a large portion of the high-energy cosmic rays,” he said. Additionally, extravehicular activity and moon walks should be kept to a minimum and avoided entirely when solar flares threaten. “A space suit is just a few layers of cloth—the least effective radiation shield you can imagine,” said Holmes-Siedle. “For a given weight of material, you can't increase the shielding much. There is no magic material that will soak up the X-rays or particles.” But the spacesuit can be designed to help monitor radiation hazards in real time. “Apollo astronauts had passive radiation dosimeters—they recorded dose but didn't transmit anything back to Earth in real time, which doesn't do the astronaut any good,” Scheuring said. Instead, NASA is studying ways that tissue dose levels can be monitored by mission control. NASA is also developing noninvasive blood analyzers, which will probe a human finger and tell exactly how white and red blood cell counts are faring, Scheuring said. Additionally, NASA is researching drugs called radioprotectants, which may moderate radiation damage to blood-forming organs and DNA. Technology on all mission components will be key, Scheuring indicated. Above and beyond physical problems associated with radiation dose, “you can't forget the fact that electronics of the suit and vehicle are as susceptible, if not more susceptible, to space radiation,” he said. “You might not have sustained a large enough dose to cause an acute problem physically, but if your electronics fail and your suit starts to fail, it doesn't make a difference.” The beginning of the Apollo era was marked by tragedy; Apollo 1 was destroyed by a fire that swept through the cockpit during a test and training exercise on 27 January 1967. All three astronauts selected for this first manned Apollo program mission—Edward White, Virgil “Gus” Grissom, and Roger Chaffee—perished in the fire. Cunningham, a member of the Apollo 1 backup crew, told Space Weather that for astronauts, “risk becomes a part of your life. Nobody sat us down and said, ‘Here are the risks you're facing. Do you want to take them or not?’ They selected us because they knew that it wasn't ever even a question for us.” After the Apollo 1 tragedy, which was attributed to a spark from a short circuit that was fueled by flammable material in the cabin and a highly concentrated oxygen environment, NASA quickly redesigned the Apollo spacecraft to improve safety. The switch from the shuttle mission to Constellation is also intended to improve safety. “With every mission, we get smarter about exploration, learning the capabilities of our hardware, learning about the environment…and ultimately reducing the risks of spaceflight through our knowledge gained,” said Pamela Melroy, a former NASA space shuttle pilot and shuttle commander. At the same time, becoming obsessive about the risks of spaceflight, including those related to radiation, can be destructive, Cunningham believes. On the disk of messages written by the leaders of 72 free-world nations that was sent to the Moon during Apollo 11, Cunningham recalled one particular message, written by John Gorton, Australia's then prime minister: May the high courage and the technical genius which made this achievement possible be so used in the future that mankind will live in a universe in which peace, self expression, and the chance of a dangerous adventure are available to all. Cunningham worries that as a society, Americans have lost that appreciation of dangerous adventure, which he feels is reflected by the drive to send astronauts back to the Moon before going on to Mars. “We have allowed our country to turn into a risk-averse society,” he said. “We need to get back to the point where…we recognize that there are some things in life worth taking a chance for. There are things worth risking your life for.” Space radiation is just one of the risks astronauts face. Balancing these risks with the human drive for exploration will be important to meeting NASA's exploration goals. Mohi Kumar is a staff writer for the American Geophysical Union.