As the 50th anniversary of humankind’s first moon landing approaches, the conspiracy theories that claim the Apollo missions were a hoax refuse to die. One perennial anomaly pointed to by moon landing deniers is that the Apollo astronauts could never have survived their passage of two belts of intense radiation partly surrounding the Earth at heights of several thousand kilometres.
Although some fairly straightforward physics can dispense with the idea of a barrier of deadly radiation imprisoning us on our planet, like all good conspiracy theories it is built on a kernel of truth. There is potentially harmful radiation in space. So how did the astronauts survive it?
The term “radiation” is used to describe energy that is emitted in the form of electromagnetic waves and/or particles. Humans can perceive some forms of electromagnetic radiation: visible light can be seen and infrared radiation (heat) can be felt.
Meanwhile, other varieties of radiation such as radio waves, X-rays and gamma rays are not visible and require special equipment to be observed. Worryingly, when high energy (ionising) radiation encounters matter, it can cause changes at the atomic level, including in our bodies.
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There are a several sources of ionising radiation in space. The sun continuously pours out electromagnetic radiation across all wavelengths – especially as visible, infrared and ultraviolet radiation. Occasionally, enormous explosions on the solar surface known as solar flares release massive amounts of X-rays and gamma rays into space, as well as energetic electrons and protons (which make up the atomic nucleus along with neutrons). These events can pose a hazard to astronauts and their equipment even at distances as far from the sun as Earth, the moon and Mars.
Potentially dangerous radiation in space also originates from outside our solar system. Galactic cosmic rays are high energy, electrically charged atomic fragments that travel at nearly the speed of light and arrive from all directions in space.
On Earth, we are protected from most of this ionising radiation. The Earth’s strong magnetic field forms the magnetosphere, a protective bubble that diverts most dangerous radiation away, while the Earth’s thick atmosphere absorbs the rest.
But above the atmosphere, the magnetosphere traps energetic subatomic particles in two radiation regions. These “Van Allen belts” comprise an inner and outer torus of electrically charged particles.
So how did NASA solve the problem of crossing the Van Allen belts? The short answer is they didn’t. To get to the moon, a spacecraft needs to be travelling quickly to climb far enough away from the Earth such that it can be captured by the moon’s gravity. The trans-lunar orbit that the Apollo spacecraft followed from the Earth to the moon took them through the inner and outer belts in just a few hours.
Although the aluminium skin of the Apollo spacecraft needed to be thin to be lightweight, it would have offered some protection. Models of the radiation belts developed in the run-up to the Apollo flights indicated that the passage through the radiation belts would not pose a significant threat to astronaut health. And, sure enough, documents from the period show that monitoring badges worn by the crews and analysed after the missions indicated that the astronauts typically received doses roughly less than that received during a standard CT scan of your chest.
But that is not the end of the story. To get to the moon and safely back home, the Apollo astronauts not only had to cross the Van Allen belts, but also the quarter of a million miles between the Earth and the moon – a flight that typically took around three days each way.
They also needed to operate safely while in orbit around the moon and on the lunar surface. During the Apollo missions, the spacecraft were outside the Earth’s protective magnetosphere for most of their flight. As such, they and their crews were vulnerable to unpredictable solar flares and events and the steady flux of galactic cosmic rays.
The crewed Apollo flights actually coincided with the height of a solar cycle, the periodic waxing and waning of activity that occurs every 11 years. Given that solar flares and solar energetic particle events are more common during times of heightened solar activity, this might seem like a cavalier approach to astronaut safety.
There is no doubt that the political imperative in the 1960s to put US astronauts on the moon “in this decade” was the primary driving factor in the mission timing, but there are counterintuitive benefits to spaceflight during solar activity maxima. The increased strength of the sun’s magnetic field that permeates the solar system acts like an umbrella – shielding the Earth, moon and planets from galactic cosmic rays and therefore lessening the impact on astronaut radiation doses.
MORE ON THE MOON AND BEYOND
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History tells us that the gamble of flying during the years of high solar activity during the Apollo era paid off. None of the Apollo flights were blasted by powerful solar flares or engulfed by clouds of solar energetic particles. But there could have been a different outcome.
On August 4, 1972 – mid-way between the safe return to Earth of the Apollo 16 crew and the launch of Apollo 17 – a solar energetic particle event was detected. Had this struck a crew en route to the moon, or working on the lunar surface, it is likely that the astronauts would have needed to make an emergency return to Earth for prompt and potentially life-saving medical treatment, all while suffering from acute radiation sickness.
Even now, forecasting “space weather” is a challenge. Astronauts working on board the International Space Station in low Earth orbit benefit from much of the protection offered by the Earth’s magnetosphere, but they can also take shelter in the best shielded areas of the station if required.
But for crews on future lunar missions, or beyond the moon to Mars, dealing with the space radiation risk remains a key challenge. When your flight lasts months rather than days, the odds of dodging space radiation bullets are simply not as favourable.