Hey Dr. Z – I read about the huge solar activity and I saw the Northern Lights for the first time in my life. But I know that these solar flares put out a lot of radiation – is this something I need to worry about? And what about the astronauts on the International Space Station?
Wow – the skies have been spectacular, haven’t they? And knowing the science behind the Northern Lights makes things even better…at least for geeks like me!
The short version is that solar flares spew massive amounts of hydrogen into space – the hydrogen is so hot that the electrons have been removed, leaving only the protons, which are charged particles that can damage our cells. If the flares are aimed at Earth when they go off, these protons will slam into our magnetic field; most of them will be diverted into Earth’s radiation belts, but some will leak through into the atmosphere. When they run into atoms in the upper atmosphere they add energy to the atoms, causing them to give off light by the same process that causes fluorescent lights to work.
I’ve written before about how cosmic radiation can give rise to cosmic ray air showers (link) that can give radiation dose to those of us on the Earth’s surface – but here I’m more interested in how the radiation affects people in space. At the moment we have the astronauts on the International Space Station (ISS) but in the not-too-distant future we might have astronauts on the Moon, in transit to Mars, or even living on Mars’ surface.
The easy one is radiation dose to the astronauts on ISS. When protons and electrons run into Earth’s magnetic field they’re deflected by the magnetic lines of force; this is what diverts these electrically charged particles into Earth’s radiation belts. Some of the high-energy particles manage to enter the Earth’s atmosphere where they strike atoms and cause them to give off photons of visible light. Oxygen atoms, for example, give off green light and hydrogen atoms give off red. But what would happen to astronauts on the Moon or on their way to Mars?
The National Oceanic and Atmospheric Administration (NOAA) maintains a Space Weather web page (https://www.spaceweather.gov/communities/space-weather-enthusiasts-dashboard) that posts regular information on radiation conditions in space. As I’m writing this, for example, they’re showing that solar proton flux is right around 100 high-energy protons per square centimeter (cm) in space – right at the lower border of what they call S2 conditions. S3 conditions are called when the high-energy proton flux is 1000 protons per square cm per second, the flux for S4 is 10,000 protons, and S5 is anything more than 100,000 protons per square cm per second. And by “high-energy” protons NOAA means protons with an energy of 10 million electron volts (MeV) or higher.
The Sun recently blew massive amounts of ionized hydrogen (or protons) into space with densities of 10,000 to 100,000 protons per square centimeter and energies of 10 million electron volts. If an astronaut caught by this blast in deep space (outside the protection of Earth’s magnetic field) they’d receive a radiation dose as high as several hundred rem, depending on the duration of the exposure – enough to cause severe radiation sickness, a 10-20% risk of death in the short term as well as increasing the surviving astronauts’ risk of developing cancer by 5-10% over their remaining years of life. Astronauts exploring or living on the Moon or Mars would receive a similar dose if they were on the day side when it hit.
This is a pretty serious dose, which is why engineers at NASA and throughout the aerospace industry have been working on ways to protect astronauts from these events. Plans for long-duration space craft include building shielding into the outer walls of the craft as well as a more-heavily shielded “storm cellar” the crew can hunker down in during bad storms such as what the Sun unleashed recently. Luckily, water is a pretty good proton shield, so the crew’s drinking water can help protect them. There’s a great NASA slide deck (https://ntrs.nasa.gov/api/citations/20210009760/downloads/Clowdsley_Lunar_Radiation_Talk_v2.pdf?attachment=true) that shows some of NASA’s thinking on the subject.
NASA has collected radiation exposure data on every space mission since the Gemini missions (1965) and the highest-dose missions (unsurprisingly) tended to be those that flew furthest from the Earth, including the Apollo missions to the Moon. Having said that, the highest average daily dose was about 220 mrem per day, which is only about 10 mrem hourly – that’s about 1000 times as high as normal background radiation levels on Earth, but it’s not a dose rate that will cause short-term health problems and, for trips of less than a year or so, is unlikely to cause a significant long-term health risk.
And that brings us to another thought – the difference between exploring space and living and working in space.
Consider the risks faced by seafarers during the Age of Exploration – fatality rates of 25%-50% were not at all uncommon, and there were some missions (such as Scott’s polar expedition) that suffered 100% fatality rates. In comparison, I served on a submarine in the Navy and I’ve taken a few trips on cruise ships – with nary a worry about whether or not I’d survive the trip. Tourists (including my mother!) travel to the Antarctic every year, and millions of people routinely take long trips – all without any risk of death. In fact, it would be ridiculous for a tourist – or a worker – to regularly assume the same level of risk the early explorers took for granted; at some point, when we transition from exploration to residency we assume that the risks will drop.
So what’s this mean for our astronauts? Well, primarily that radiation dose limits for astronauts, which are currently rather high, will likely be reduced substantially when we begin living and working in space (https://www.scientificamerican.com/article/new-space-radiation-limits-needed-for-nasa-astronauts-report-says/).
At the moment we’re looking at resuming missions to the Moon within the decade, and missions to Mars…maybe in the next 20 years or so, and our first orbital or lunar residences are likely decades in the future. At the same time, we need to start thinking about these things now rather than waiting until we’re out there and realizing that we need to update our rules…and we need to make allowances for emergencies and unpredictable events as well.