Last updated December 19, 2017 at 4:02 pm
In September Adelaide hosted the 2017 International Astronautical Congress. Elon Musk spoke about his SpaceX Falcon Heavy rocket to Mars while Lockheed Martin unveiled their plans for a Mars Base Camp. A multitude of countries and companies who are active in the international space industry displayed exhibits. Anyone who attended would leave with a sense that travel by human beings to Mars is changing from a dream to a goal.
An expedition to the surface of Mars is hazardous enough as it is, with the low gravity, freezing temperatures, sparse atmosphere and social isolation. But an added danger comes from exposure to high levels of ionising radiation. NASA has long recognised this and with its collaborators is devoting a large amount of research towards understanding the nature of the radiation risk, the potential health implications, and the control measures which could be implemented during the journey to and back from Mars and while exploring the surface of Mars.
This article follows on from two Science Updates on nuclear issues. These covered the radiation health consequences from use of a nuclear weapon and from a major nuclear power plant accident. Here we look at ‘nuclear’ issues and the associated radiation hazards we deliberately confront as we explore our environment in space.
Unlike the involuntary victims of an atomic bomb or the plant operators responding to a nuclear reactor in melt-down, travellers to Mars know the dangers and have consented to take the risk. Even so, there are uncertainties about the level of risk. It might be higher than originally thought. Further, NASA and others who undertake Mars missions still need to adhere to international standards for radiation protection for the astronauts in their care.
Back on Earth we are all exposed to terrestrial and extraterrestrial sources of ionising radiation from our environment. But the levels are relatively low, we can’t feel it and any health consequences are not obvious (unlike from exposure to ultraviolet radiation from the sun). So we tend to ignore it. Indeed for those of us managing radiation risks in the occupational setting we deliberately exclude general environmental exposure from measurements of personal radiation dose of workers.
Earthlings are fortunate to be protected from most extraterrestrial radiation by a dense atmosphere and the Earth’s magnetosphere. For all our talk of space travel it’s an extraordinary fact that since the completion of the Apollo missions in the early 1970s no human being has ventured beyond the protective cocoon of lower Earth orbit. The International Space Station (ISS) resides within this geomagnetic sphere.
When we travel beyond the low Earth orbit of the ISS and enter the radiation environment of deeper space there are three main sources of radiation exposure:
- The Van Allen belts, consisting mainly of protons and electrons trapped by the Earth’s magnetic field. The ISS in lower earth orbit is located well below this region of space.
- Galactic cosmic rays (GCR), produced by supernovae or exploding stars and consisting of protons (88%), nuclei of helium atoms (‘alpha particles’) (10%) and the remainder ionised nuclei of heavier elements from throughout the period table.
- Solar particle events (SPE), produced by coronal mass ejections or solar flares from the Sun and consisting mainly of protons.
On Earth we aim to harness some of these types of radiation, for example, the first proton therapy unit in the Southern Hemisphere is about to be constructed at the South Australian Health and Medical Research Institute. But in the wilds of interplanetary space an astronaut on a space walk can be caught in a veritable proton storm if there is an SPE, with the potential for suffering very high radiation doses.
Describing the amount of radiation dose
In the two Science Updates on nuclear issues an attempt was made to quantify the levels of radiation exposure and from there describe the main health effects. The principal unit of radiation dose was used, namely, the Gray (Gy), a measure of the amount of energy absorbed per kilogram of body tissue. This unit is appropriate in the case of electromagnetic radiation like gamma radiation and X-rays, the dominant types of radiation in medical settings and nuclear accidents. But in space the radiation is particulate and particulate radiation has a much greater biological impact for the same amount of energy absorbed. For this reason the dose in Grays is multiplied by a ‘Quality’ factor to give the dose in Sieverts (Sv), this giving a better indication of the relative potential for biological damage. For example, for X-rays and gamma rays 1 Gy gives 1 Sv, while for alpha particles 1 Gy results in 20 Sv.
The Curiosity Rover’s role in gathering data on radiation levels
Launched in November 2011 and landing in the Gale crater on Mars in August 2012 the Curiosity rover has a Radiation Assessment Detector (RAD) mounted on its top deck. The RAD measured radiation levels during the flight to Mars and then on the Martian surface [link], identifying potential future astronaut radiation doses from SPEs and GCRs during their mission.
The journey to Mars
After blasting off to Mars the expedition will traverse the Van Allen belts as quickly as possible. The belts therefore won’t contribute much to radiation dose.
Throughout the journey the spacecraft will be consistently immersed in GCR and these are the source of the highest exposures. A Mars mission would take typically 180 Earth days to reach Mars and 180 days to return. The radiation dose received will be about 0.33 Sv each way, or a total of 0.66 Sv. Referring to the earlier articles in this series, this dose is less than that received by the most-exposed operators after the Chernobyl nuclear accident (134 operators received from 0.8 Gy to 16 Gy) and more than that received by the most exposed plant workers at Fukushima (few workers had doses > 0.1 Gy). A dose of 0.66 Sv is well above the international occupational dose limit of 0.02 Sv per year. To further place this in perspective it was quoted earlier in this series that the total lifetime dose (80 years) from environmental radiation in Japan is about 0.17 Gy. The astronauts would certainly face an increased lifetime risk of chronic health effects such as cancer and reduced life expectancy.
The dose will be greater if there are solar flares or coronal mass ejections during the journey to produce SPEs. SPEs occur unpredictably from time to time throughout the solar cycle. The extra dose from SPEs would usually add about 0.028 Sv to the above journey dose. However, occasionally SPEs have been recorded with much greater magnitudes, such as in 1972, 1989, 2000 and 2001. If an astronaut were caught on a space walk or in an unprotected section of the spacecraft these exceptional SPEs would give doses ranging from 1 Sv up to 3 Sv from the single event. Such a dose is within the range of the highest doses experienced by plant operators at Chernobyl and could result in the similarly severe health impacts such as Acute Radiation Syndrome.
On the surface of Mars
Mars has a weak magnetic field and therefore lacks the protective force shield of the Earth. The sparse atmosphere (<1% of that on Earth) provides some limited shielding, but compared with Earth a person on the surface of Mars is pretty much laid bare to the ravages of radiation streaming from the sky.
An anticipated 500 day sojourn on Mars before returning to Earth would be associated with an estimated dose of 0.32 Sv – almost double the typical environmental dose of an Earthling accumulated over a lifetime. However this is for a short stay. For long term expeditions lasting many years or for permanent colonists the dose accumulated would exceed many Sieverts. Because this dose is spread over a long period it will not result in Acute Radiation Syndrome, but it would increase the risk of cancer. A question: Astronauts and the first colonists may exercise individual choice to accept these risks, but what of children born to Mars colonists?
Special radiation dose limits for astronauts
Radiation doses received during a long journey in deep space will easily exceed yearly international dose limits specified for radiation workers. In the special case of astronauts NASA aims for an individual career dose limit of around 1 Sv, the precise value depending on age and gender. The career limit represents an estimated 3% risk of exposure-induced death from cancer. On a long journey the risk of non-cancer effects also needs to be assessed. In a mission to Mars and back as described above the total dose will be close to 1 Sv, right on the edge of astronauts’ career dose limits.
Problems with estimating radiation doses
It was mentioned above that in the case of particulate radiation a factor is used to arrive at Sieverts of dose to take into account relative biological effects. One problem is that in space we encounter types of particulates that are not usually relevant on Earth – such as from heavy ions of elements with high atomic number. We have a lack of human research data and therefore don’t know the correct quality factors to use. There are further uncertainties in relation to the types of chronic effects. A high research priority has been given to resolving these uncertainties. Recent scientific reports suggest that health risks from radiation exposure may be significantly higher than first anticipated. So people volunteering today to go on missions to Mars may be accepting estimated radiation risks that are actually underestimating the true risk.
Preventing radiation exposure
On space missions or on the exposed surface of Mars or the Moon some of the usual practices for minimising exposure to radiation are not available. The standard practices are to: minimise the time exposed to the radiation; maximise the distance from the radiation source; and shield the source of the radiation at the source. When the source of the radiation exposure is the environment itself in which you are continuously immersed these control measures are difficult to apply.
Rather than shielding the radiation source, shielding needs to be placed around any and every person at risk of exposure. Research shows that innovative shielding materials in novel configurations can give a high degree of protection to space travellers when incorporated into transport vehicles, habitats and space suits.
On the surface of Mars it may be necessary to live underground. A person living at a depth of 3 metres underground would have their radiation dose reduced to something similar to what is normally experienced on the surface of the Earth. I picture a living environment reminiscent of the underground habitations at the Coober Pedy opal fields in South Australia (but with little gravity and with way below freezing temperatures!). It has been suggested that future residents of Mars could shelter in huge lava tubes beneath the surface. For surface activities proton storm warnings may be as important as weather forecasts on Earth.
To sum up
Research on potential exposures to radiation on Mars missions show that the mission members can in rare circumstances—such as from a high level solar proton storm (SPE)—receive radiation doses comparable to the doses received by some plant operators in the worst nuclear power station accident in history, with the associated risk of radiation sickness and even death. The longer term dose arising from one return mission to the surface of Mars would be well in excess of international occupational dose limits, and close to special career limits set for astronauts. For Mars explorers and colonists undertaking the long haul they might need to live 3 metres or more underground to achieve acceptable exposures.
The protons, neutrons, alpha particles and heavy ions that constitute this radiation are produced by a nuclear universe – either from our own sun or in distant galaxies. When we are tempted to venture above low earth orbit into the wilds of space, we encounter a hostile environment where high radiation levels are the norm. This undoubtedly will not stop people going to Mars. But radiation poses a significant health risk that must be carefully managed by NASA, SpaceX and others sending people to Mars.