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Houston, We Could Have a Problem
Though thou art far away, thy rays are on earth;
Though thou art in their faces, no one knows thy going.
Egyptian Pharaoh Akhenaten, “The Great Hymn to the Aten”
It was supposed to be solar minimum, the time when the Sun rests. But Nature does not heed human schedules; it makes them. In August 1972 the Sun produced a “sudden and spectacular resurgence of solar activity,” as the staff at the National Oceanic and Atmospheric Administration’s (NOAA) Space Environment Center called it. It was just a matter of luck that Apollo astronauts were not caught up in that resurgence.
On July 29, 1972, an average-sized sunspot group spun into view over the eastern edge of the Sun. While not unusually large, the region was magnetically complex, with snarled and twisted fields and intense gradients. That tightly wound knot of energy began to come undone on August 2, when the region exploded
with three separate flares, two of them of the most potent “X-class.” By the latter part of August 2, radio noise from the Sun reached a crescendo, making radio communications on Earth much more difficult, and next to impossible at Arctic and Antarctic stations. This pattern would continue for much of the next week.
August 2 marked the beginning of 10 days of flares and explosions that would stir up the magnetosphere and radiation belts of Earth. The series of flares produced a steady shower of “solar protons,” hydrogen nuclei that are accelerated to extremely high energies by the solar blast. During solar proton events, these energetic particles travel from the Sun to the Earth within 20 to 30 minutes, and they can stream into the magnetosphere for hours. Upon reaching Earth, some of these particles spiral down Earth’s magnetic field lines, reaching the upper layers of the atmosphere.
Close behind the proton blast, a coronal mass ejection (CME) raced toward Earth, though no one knew it at the time because CMEs had not yet been discovered. A magnetic storm commenced around Earth on August 4 and auroras dove into the continental United States in the early morning hours, with official reports from Colorado, Oregon, Vermont, Pennsylvania, and Illinois. By early evening, AT&T reported that one of its underground long-distance phone cables between Chicago and Nebraska was knocked out of service. Power companies in Minnesota, Wisconsin, South Dakota, and Newfoundland suffered through tripped transformers and fluctuating currents in their lines. And to top it off, the Sun released another X-class flare late in the day to further fuel the turbulent space weather. On August 5, communication through several of the transatlantic cables was interrupted.
The largest and greatest flare of the entire solar cycle lit up the Sun on August 7. The mangled sunspot region generated a flare that lasted at least four hours (see Figure 14). Radio signals from the Sun screamed at hundreds to thousands of times higher than normal levels, and the X rays and energetic particles saturated the sensors on several spacecraft such that the peak of the event could not be measured. Protons from the solar blast rushed to Earth and bombarded the upper atmosphere. In the Canadian province of
FIGURE 14. The great solar flare of August 7, 1972, became known to solar physicists as the “seahorse flare” due to its unusual shape. The intense radiation and swarm of particles from the flare probably would have been fatal to astronauts of the Apollo missions had they been in space at the time. Courtesy of Big Bear Solar Observatory/New Jersey Institute of Technology.
British Columbia, a 230,000-volt transformer blew up, the result of too many days of too much magnetic storming around Earth.
By the time the last major flare shot out from the Sun on August 11, Earth had endured the most intense swarm of solar protons since the era of satellite measurements began. At least 12 power companies had suffered mishaps or damage from the storm. Brilliant auroras reached as far south as Washington, D.C. But perhaps the most important effect of the storm was not one reported by power companies, satellite operators, or auroral observers. It was noted—and keenly studied—inside NASA.
By pure good fortune, the August 1972 flares and the string of solar proton events fell right between NASA’s Apollo 16 (April 16 to 27) and Apollo 17 (December 7 to 19) missions to the Moon. With a simple change of launch dates, which can happen often in manned space launches, the astronauts could have easily been caught in the middle of the proton swarm. And for nearly three decades since the August 1972 event, scientists and flight surgeons have analyzed and reanalyzed the physics and the biology of the event, replaying it hypothetically in their minds and their computers. What if the astronauts had been outside of Earth’s protective magnetosphere and on their way to the Moon when the Sun acted up? How much radiation would the astronauts have absorbed during the event, and how would it have changed the mission? What most of those scientists have concluded is that the astronauts might not have survived the trip.
“Although a great deal of thought and effort had gone into planning the Apollo missions, it was mostly luck that the crew was not subjected to excessive radiation exposure,” said Gautam Badhwar, NASA’s former chief research scientist for space radiation at the Johnson Space Center.1 “If they were in the command module, it would have been dangerous but not life threatening. But there was not much shielding in the lunar module, probably not much better than a space suit. If the astronauts had been on the Moon, they probably would have been instructed to find a crater and dig down into it, as there is no magnetic field to protect them on the Moon.”
In one study commissioned by NASA, researchers modeled the effects of the proton-accelerating flare of August 4, which began at 6:20 Universal Time (UT, more commonly known as Greenwich Mean Time). Within seven hours the Apollo astronauts would have absorbed the 30-day limit of radiation to their skin and eyes. Within eight hours the solar protons would have doused the crew with its 30-day limit of radiation for “blood-forming” internal organs and would have exceeded the yearly limit for exposure to the eyes. The limit for skin exposure would have been surpassed within nine hours, and one hour later the limit for a radiation dose to their organs would have been eclipsed. Within 11 hours after the flare the hypothetical Apollo astronauts would have exceeded the acceptable level of irradiation of their skin for an entire career.
In another study using measurements from the Solar Proton Monitoring Experiment on NASA’s Explorer 41 satellite, researchers found that the August 4 flare raised the radiation dosage for an astronaut outside the spacecraft at one point to 241 rem per hour, and it sustained a level of at least 45 rem per hour for more than half a day.2 Inside the Apollo command module, the rate would have approached 66 rem per hour. With a typical lunar round-trip lasting 11 to 14 days, each astronaut would have received a projected cumulative skin dose of 358 rem if he had stayed within the command module. According to the best projections available in 1972, astronauts who absorbed an acute dose of 340 to 420 rem over such a short period of time likely would have suffered from vomiting, nausea, and other symptoms of radiation sickness. At worst, each crew member would have been hospitalized for three to six months, with a 20 percent chance that they would have died from the accumulated radiation.
Radiation is one of the primary hazards of work in space. Regardless of storms from the Sun, astronauts receive a certain dose of radiation every time they go up, as the Earth is constantly bombarded by galactic cosmic rays from explosions and other phenomena that
occur well beyond our solar system. They also receive small doses from the radiation that is naturally trapped around Earth in the ionosphere and the Van Allen radiation belts, particularly near the South Atlantic Anomaly. Both of those forms of radiation behave in somewhat predictable patterns. Scientists know that cosmic rays penetrate the solar system better when the Sun is at a minimum of activity, and some of the patterns of activity in the radiation belts can be explained by daily variations due to the Earth’s rotation.
Solar proton events are significantly harder to predict. First of all, not every sunspot group produces a flare or coronal mass ejection, and neither event is necessarily tied to sunspots, so they are extremely hard to predict with any accuracy. Furthermore, not every flare or CME produces high-energy solar protons, and even when they do, those protons may not be directed at Earth. Even when observers manage to spot a proton-producing event on the Sun, it is difficult to determine the magnitude and intensity of the event until it is well under way, so that astronauts will have at best 10 to 100 minutes to take cover from the initial burst. Combine this unpredictability with the damage that can be caused by high-energy protons and you have perhaps the most dangerous space radiation hazard to astronauts and lightly shielded spacecraft.
The penetration of high-energy particles into living human cells and tissues can lead to burns, chromosome damage, cell death, and sometimes cancer. The potency of solar protons arises from what is called ionizing radiation—the proton, or ion, carries enough energy to eject an electron from an atom. When the energy from ionizing radiation is deposited in the human body, chemical changes can occur at the atomic level. The water in the body tends to absorb a large portion of the radiation, and that water can itself become ionized into highly reactive molecules called free radicals. Free radicals can react with and damage human DNA. Alternatively, radiation can collide directly with DNA molecules, damaging them directly.
The most immediate effect of severe ionizing radiation is radiation sickness, a phenomenon that has been studied in depth since the Hiroshima and Nagasaki atomic bomb explosions. The gastrointestinal system, bone marrow, and eyes are perhaps the
most sensitive to radiation, and many victims suffer almost immediately from nausea and vomiting after exposure. Intense radiation can also lead to severe burns that are slow to heal, cataracts, and even sterility. The immune system can be severely depleted, with the suppression of immunity allowing infection to overwhelm the body while defenses are down. And while the nerves and brain are most resistant to radiation, acute exposure can damage the central nervous system. Finally, extreme doses of radiation can be fatal within days or weeks.
The slower and more insidious effect of ionizing radiation is genetic mutation. The free radicals created by direct radiation hits can cause unpredictable changes in DNA that can affect both the victim and their offspring (some of the worst effects of radiation likely occur in fetuses in the womb and even within still-to-be-fertilized eggs). Some genetic mutations are relatively harmless, or at least negligible in their effect, though mutations rarely occur for the better. In the worst cases, mutations can disrupt a tissue or cell’s ability to control its own reproduction, leading to the uncontrolled growth and division of cells that we know as cancer.
“Any radiation exposure results in some risk,” says Mike Golightly, who leads the Space Radiation Analysis Group at NASA’s Johnson Space Flight Center, the team that monitors space weather activity during space shuttle and space station missions. “The increase in cancer risk is the principal concern for astronaut exposure to space radiation.” With this in mind, NASA and other space agencies set annual and career limits on the amount of radiation an astronaut may endure while traveling and working in space. In fact, the Occupational Safety and Health Administration officially classifies astronauts as “radiation workers.” The limit for astronaut exposure to radiation is 25 rem within a 30-day period and 50 rem for an entire year, compared to the limit of 5 rem in a year for people who work with radiation on Earth and 10 cumulative rem over a five-year period. Above 450 rem is considered a median lethal dose—an amount at which 50 percent of victims would die. In terms of harmful radiation exposure, the worst of the Apollo missions—Apollo 14—exposed the astronauts to just
1.14 rem of absorbed radiation. The astronauts who lived on Skylab two years later received a dose of 17.8 rem, and the Mir cosmonauts are thought to have received much higher does due to the length of their stay in space. Researchers in the United States have estimated that a one-year stay on Russia’s Mir space station provided a cosmonaut with 21.6 rem, increasing the lifetime risk of cancer by 1 percent.
When compared with the radiation we receive in everyday life on Earth, the amount astronauts receive seems reasonable for such dangerous work. A simple chest X ray gives a patient 0.01 rem, while the natural background radiation that every human receives from rocks and minerals in Earth’s crust is 0.1 rem per year. A dose of 0.1 rem corresponds to an increased chance of 1 in 17,000 of contracting a cancer from such a radiation exposure (compared with the normal incidence of cancer, which is 57 cases per 17,000). The researchers and managers who run the human space flight programs at NASA and other space agencies consider a 3 percent higher risk of cancer—which is still below the risk taken in some earthbound occupations in agriculture and construction—to be acceptable for its flight teams. It’s also a fairly low risk when you consider that the average Earth dweller increases his risk of fatality by 1 percent simply by commuting in a car to work each day. And it is a risk that astronauts knowingly take, inasmuch as they are briefed before every mission about space radiation, how much of a dose they can expect for any given flight, and where they stand with regard to total lifetime exposure to radiation.
However, Golightly notes that some research suggests that radiation encountered in space may be more effective at causing biological damage than the gamma rays and X rays encountered by workers on Earth. Moreover, most of the projections and calculations of radiation risk in space are best guesses and theoretical models. Very little is actually known about the biological effects of low-level radiation exposure in space. The astronaut generation is still relatively young and most astronauts are physically robust and healthy compared to the average citizen, so it remains to be seen whether astronauts are more likely to develop cancer
than the rest of the population. Furthermore, the number of astronauts past and present is still too small to provide a useful statistical sample to compare with cancer rates in the normal population. Nonetheless, Golightly and his colleagues at the Johnson Space Center note that at least one astronaut has told them that he felt like “a walking time bomb” because of his exposure to space radiation.
NASA astronauts had their closest brush with radiation in October 1989, when a large, long-lived X-class flare produced a swarm of protons that lasted nearly six days. The crew of the space shuttle Atlantis was aloft and hard at work for the duration of the October 1989 storm. Though flying well within the protective magnetosphere and at relatively low latitude (away from the auroral zones and the footprints of the radiation belts), the astronauts reported burning in their eyes, a reaction of their retinas to the solar particles. The crew was ordered to go to the “storm shelter” in the farthest interior of the shuttle, the most shielded position. But even when hunkered down inside the spacecraft, some astronauts reported seeing flashes of light even with their eyes closed. On Russia’s Mir space station, cosmonauts received a significant increase in radiation dose during the solar particle events. According to Badhwar, the total increase in cosmonaut exposure was about 6 or 7 rem, a dose equivalent to 100 to 150 days of additional radiation exposure. And one NASA researcher estimated that there was a 10 percent chance that astronauts on a deep-space mission beyond the magnetosphere or working on the Moon would have died during the October 1989 event.
The close calls may come closer and more often over the next few years. The United States, Russia, and their partners began construction of the International Space Station (ISS) Alpha in 1998, and the latest plans call for the work to continue through 2004. More than 40 astronauts are expected to fly 33 space shuttle missions and 10 Russian rocket flights and to work in 6-hour shifts for more than 1,500 hours outside the orbiting station—known as extravehicular activity, or EVA. Much of that activity will occur during the Sun’s most violent years, around the peak of the solar
cycle. “If you’re timing space station construction and the solar cycle, you couldn’t have done a worse job,” said Mark Weyland, a project manager at Lockheed Martin who studies the effects of radiation with Golightly.
The poor timing is exacerbated by a decision made in 1993 to fly the space station in an orbit that is more accessible from Russia. Under normal conditions, a space shuttle flying below 45 degrees latitude is almost completely shielded from solar flare protons. During large solar events and magnetic storms, the pressure on the magnetosphere and the interconnection of the Earth’s field with the interplanetary magnetic field can cause the magnetosphere to be compressed and can allow solar flare particles, trapped radiation near Earth, and cosmic rays to reach latitudes that are typically safe. Since the space shuttle rarely flies higher than 42 degrees— about the latitude of New York City—and since the largest storms from the Sun occur only a few times per solar cycle, shuttle flights are usually pretty safe from space weather. However, the space station Alpha flies in a much higher inclination, drifting as far north as 51.6 degrees. That orbit regularly takes the station near the auroral zone and the “horns” of the outer radiation belt— where the trapped particles descend toward the atmosphere. The space station orbit makes astronauts much more vulnerable to solar protons and to other accelerated particles during the largest solar storms.
In 1999 a panel convened by NASA and the National Research Council (NRC) reviewed the radiation risks associated with construction of the space station and criticized the space agencies for their planning and lack thereof in some cases. The group of 10 scientists predicted there is a 100 percent chance that at least two of the 43 missions to the space station will overlap with a solar proton event and a 50 percent chance that at least five missions will be affected. While these space radiation events may not be immediately life threatening, the panel noted, astronauts could be exposed to unacceptable doses of radiation that could put them in danger of exceeding short-term limits for overall exposure and increase their risk of cancer. Even if the health of the astronauts is
not affected by one event, the exposure limits could require that missions be interrupted, which would throw off the schedule for future flights and could lead to problems in the rotation of astronauts. Plus it is also a career issue. “No astronaut wants to reach the short-term radiation limits, much less the career limit” because it could end their time as an active astronaut, the panel noted. “The results would seem to call for an aggressive program aimed at reducing solar radiation risk to astronauts during ISS construction.”
Preventing the exposure to severe space weather should be simple, as NASA and NOAA both have fleets of spacecraft, ground observatories, and computer models that monitor and predict space weather. If a crew were working outside the station, mission controllers would have about 30 minutes to an hour to get the astronauts inside. While there is no officially designated radiation shelter on the space station, Badhwar noted, “we are mapping the areas using a proportional counter and we have a generally good idea from the shielding distributions—developed from computer-aided design drawings of the ISS modules—to advise the crew of ‘safe’ areas.” So if a particularly egregious solar event is under way, NASA could warn the crew to stick to the most shielded areas to wait out the storm (the first station crew was advised to do so for parts of a solar storm from November 12 to 14, 2000).
But according to George Siscoe, a Boston University physicist and chair of the NRC panel, “an unofficial NASA flight rule specifies that changes in flight plans must be based on current data that reflect the weather immediately around the space station. Information about the size and shape of a solar storm and data on its occurrence, intensity, and duration can be retrieved from other sources, but under current guidelines, this information could not be used by flight directors to take immediate action.” Essentially, the flight director for a manned space mission is only supposed to consider real-time conditions in the immediate vicinity of the spacecraft, ignoring the possibility that models and observations might say that solar protons or excited radiation belt particles might be on their way. “These rules unnecessarily restrain ground-based
flight directors because other valid data could be used to assist in avoiding radiation exposure,” Siscoe says.
Among other things, the panel recommended that NASA install dosimeters on the outside of the space station to provide real-time information about the amount of radiation the crew is receiving while working outside. It also recommended that flight directors ignore the “real-time, onsite” rule and instead rely on the Space Radiation Analysis Group and NOAA’s Space Environment Center to provide specific information about the incoming space weather and its potential effects. Finally, NASA, NOAA, and other groups should convene a meeting and find some funding to gather more and better data and to develop more useful, health-specific models of the radiation environment.
“It’s not a matter of if radiation enhancements will occur while crews are aboard the International Space Station, but when and how serious,” says Mike Golightly. “During construction of the ISS, there is a reasonably large probability that extravehicular activity will coincide with radiation enhancement.”
Long after the space station is complete, storms from the Sun could hamper more advanced space exploration efforts. Future missions to set up stations on the Moon or to explore Mars and the outer planets will be impossible until researchers can figure out a way to protect the crew from all of the harmful radiation flying about the solar system. On shuttle flights and many of the manned space missions of the 1960s and 1970s, the astronauts have spent most of their time tucked inside Earth’s protective magnetosphere. Though space weather events and severe radiation exposure is possible near Earth, they are almost always mild. But outside of Earth’s magnetic field, there is no natural shield from cosmic rays and solar protons. The sheer length of the trip to Mars and back— anywhere from two to three years—ensures that astronauts would face at least one major space weather event. The more insidious threat comes from the long-term, low-level doses of radiation that the crew would take every day for several years. Would the biologically useful bacteria in the human gastrointestinal tract be destroyed? Would the central nervous system be affected, as in
clinical studies where long exposures to radiation have affected the function of nerves in lab rats? No one can venture more than a hypothetical guess about the effects of living in unshielded space for so long. “The astronauts are going to have to stay on the Martian surface for as long as a year-and-a-half,” Badhwar said, “so they must be healthy or the mission is in real trouble.” So as engineers, architects, and doctors figure out how to fly the crew to Mars and how to keep them well fed and mentally stable, physicists must figure out how to keep them from being irradiated to death.3
During that October 1989 solar proton event that sent the shuttle astronauts into hiding within the spacecraft, special sensors that measure cosmic radiation were triggered on a Concorde supersonic airliner. Though the airplane flies much lower in the atmosphere and well below the allegedly radioactive parts of Earth’s space, the passengers received the equivalent of a chest X ray from the solar protons. It was an unusual event, as the monitors on the Concorde have rarely been triggered. But it was enough to create interest in the health risks for aircraft crews and passengers who often fly on routes that approach Earth’s geographic poles. As with the space station, flights that take polar routes risk greater exposure to particles and radiation from space.
In 1978 a study of the biological effects of air travel on the U.S. population suggested that the exposure to cosmic rays and space weather would be so small as to not be directly observable. Researchers calculated that it was likely that there would be 3 to 75 cases of genetic defects and 9 to 47 cases of cancer over a period of years. In a population of 250 million people, those numbers are so low as to be considered negligible, lower than the number of people who will be struck by lightning. On the other hand, in 12 medical research studies conducted over the past 3 decades, 6 studies found that the risk of cancer was higher for frequent airline flyers (the other 6 studies were inconclusive).
What all of the studies do show is that space radiation is more of a threat to pregnant women. The Federal Aviation Administration (FAA) estimates that the risk of birth defects among pregnant women who fly ranges from 1 in 680 to 1 in 20,000, depending on the frequency of travel and routes flown. Still, that rate is 0.5 percent higher than the rate of birth defects in children of women who do not fly during pregnancy, according to Dr. Donald Hudson, an aviation medicine advisor for the Air Line Pilots Association. “The risk for cancer from low-dose radiation is very low except for female crew members of childbearing age,” Hudson notes. “But unfortunately that risk to the fetus is most severe in the first trimester, when women often don’t know yet that they are pregnant.”
In Europe, where flights to higher latitudes occur more frequently, a law of the European Union went into effect in 2000 that requires all European airlines to educate flight crews about radiation issues. In the United States, the FAA published a directive in 1994 stating: “Air carrier crew members are occupationally exposed to low doses of ionizing radiation from cosmic radiation and from air shipments of radioactive material. . . . It is recommended that workers occupationally exposed to ionizing radiation receive exposure (to the issue) and appropriate radiation practices.” Yet very few commercial airlines in North America warn their pilots and flight attendants—much less the passengers—about the issue. The cynical view is that since so few people are aware there is any radiation risk, and no legal judgments citing “radiation exposure” have been made against the airlines, it is not in the industry’s interest to address the risks.
Dr. Robert Barish, a medical physicist who has studied inflight radiation, notes that an airline passenger’s exposure to cosmic radiation doubles with every 6,500 feet of altitude, and solar flares can increase radiation exposure by 10 to 20 times. On a typical round-trip flight from New York to Hong Kong—which takes a route closer to the poles and auroral zones of Earth— passengers would receive a radiation dose of 20 millirem (about two chest X rays). The recommended yearly dose of radiation for
a person on the ground is 100 millirem (radiation workers are permitted 5,000 millirem [5 rem] per year in the United States, 2,000 in Europe). So after just five flights to Asia from New York, a passenger has already reached the yearly radiation limit without accounting for medical X rays or any other exposures he or she might accumulate.
In his book on the radiation risks of flying, Barish notes that the rates of several types of cancer among pilots “are high—in some cases much higher than average. Radiation levels in a jetliner are occasionally so high that if it was a nuclear power plant, the levels would require signs warning employees not to spend any more time in the area than necessary to do their jobs.” Statistics show that the radiation exposure among airline crews is far from lethal, and in most cases the exposure does not exceed the occupational limit. “But their bodies certainly have to handle more than the ground limit exposure for the general public,” Barish said. While radiation risk is a relatively minor issue now, Dr. Hudson notes that it will be a much more important issue in the future, when high-speed planes are expected to fly at much higher altitudes
“We have thousands of flight attendants and pilots who receive a radiation dose in the top 5 percent of allowable radiation exposures,” says Barish, “yet they are not even counted as radiation workers.” On top of that, close to half a million people fly more than 75,000 miles per year, putting them into the same exposure levels as many of the flight crews.
Barish is by no means an alarmist. He just wants the public, or at least airline crews, to be given the right of informed consent. “There is no demonstrable harm at the levels of radiation received by airline crews and passengers,” he notes. Most people would not give up flying due to such a minimal risk. “But since environmental and nuclear regulators presume there is a risk, why don’t the airlines? The risks are small, but they are there. In every other area where people are exposed to radiation, they are informed. People are entitled to know about that risk.”
