Scientifically Speaking | Radiation is a deadly threat to human space travel
On July 20, American billionaire Jeff Bezos flew on a Blue Origin rocket past the Kármán line, which, at an altitude of 62 miles, is the widely accepted boundary of space. The spacecraft topped out at 66.5 miles above the Earth, and its crew experienced a few minutes of weightlessness. Billionaire Richard Branson had reached the NASA-designated space boundary of 50 miles only nine days earlier. The other billionaire interested in space travel, Elon Musk, heads SpaceX, a company which has taken astronauts up to the International Space Station (ISS). Though Musk has not been in space yet, he has made no secret of his desire to take humanity to Mars and back.
Some futurists think a permanent colony on Mars will be possible. I don’t expect to see one in my lifetime. The challenges of travel to Mars and survival on the planet are exceptional. Mars has thin air, frigid weather, and trace oxygen. And after 10 years on Mars at lower gravity, a spacefarer’s legs and bones would be so brittle that re-entry into the Earth’s atmosphere would render them useless.
But one of the greatest risks in space is from radiation. Ionising radiation causes damage to cells and to DNA inside them. In deep space, ionising radiation is of two main types — galactic cosmic rays that originate outside the solar system from exploding stars, and solar energetic particles from the Sun.
Radiation poses an existential threat to humans and to all other forms of life. Unsurprisingly, NASA considers radiation one of the major unresolved problems of sustained human spaceflight. Returning astronauts might face a greater risk of various cancers, eye ailments, and cardiac events.
The risks are not unique to humans either. Any organisms that accompany humans into space and to Mars would need to be able to withstand ionising radiation. As Christopher Mason writes in his eminently readable new book, The Next 500 Years: Engineering Life to Reach New Worlds, “Sending an Earth-evolved organism to another planet would result in almost-certain death.”
Earth is an incomparable planet. The magnetic field of Earth is created by currents of electricity that flow in the molten core. The Earth’s internal magnetism creates a region around the planet known as the magnetosphere, which protects us from the harmful effects of most of the radiation of space.
Several planets in our solar system have magnetospheres. Earth’s is the strongest of all the ones possessed by rocky planets. Our magnetosphere is a large, comet-shaped bubble, which has played an essential role in our planet’s habitability. Life would not exist on the planet without it.
The Earths’ magnetosphere extends to about 40,000 miles. The ISS is positioned at a higher altitude than Bezos’ or Branson’s flight, and normally maintains a planned altitude of 248 miles. But since it is in low Earth orbit, it is still within the earth’s magnetosphere.
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The ISS circles the Earth every 90 minutes at 17,500 miles. Even at that altitude, there are enough molecules of the atmosphere to change its velocity, and cause it to dip towards Earth. To maintain a constant orbit, propellant is fired to reposition the station.
Astronauts on the ISS also experience the effects of radiation on prolonged visits. When astronaut Scott Kelly, who spent nearly a year onboard, closed his eyes, he could see streaks of high-energy ions flash like shooting stars. But the radiation he experienced was much lower than the amount that might be encountered on a future mission to Mars.
The Moon is at a greater distance from the Earth than the ISS is. When Neil Armstrong went to the Moon, he wore foil plates on his ankles. Streaks of high-energy particles can be seen on these plates. But that was a relatively short mission.
Any attempts to colonise Mars would need to reckon not only with radiation exposure to the trip to the red planet, but on the planet itself as well. The current scientific consensus is that Mars has a molten core, but that the protection on the planet is much lower than on Earth.
NASA estimates that the radiation exposure from one mission to Mars might exceed the lifetime limit of astronauts. The actual limits vary by age and sex. But a 30-month round trip mission to Mars could result in 1000 millisieverts, which is roughly the same as the radiation exposure from 10,000 chest X-rays.
Shielding ships and protecting exposure of astronauts is a proposed solution to the massive amounts of radiation on a trip to Mars. It seems to work well against solar energetic particles. But currently there are no spacecraft that can shield against galactic cosmic rays. Clearly, solutions to the challenge are necessary if we are to contemplate prolonged human space flight.
In the more distant future, we might see the development of drugs that protect against radiation-induced damage. Another solution might be to genetically enhance humans to make them more resistant to DNA-damage. While this seems far-fetched now, scientists are currently studying animals like tardigrades that have high levels of radiation resistance to learn how they achieve it.
But even as we look to the planets and beyond, it is clear that Earth is our home and there is no place quite like it. We take our comforts for granted, but they will not be found on Mars or anywhere else.
Anirban Mahapatra, a microbiologist by training, is the author of COVID-19: Separating Fact From Fiction
The views expressed are personal