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The graphic below  [on original]  – created using data provided by NASA, the EPA, FDA, NRC, scientific journals, and other sources – compares various exposure levels in scenarios both familiar and far-flung.

Hover over a category box to see how it compares.

Musk has “aspirational” hopes to launch a round-trip mission to the red planet with humans in 2024, but the trip could total a year, and astronauts may spend about 500 days on Mars’ surface.

The whole journey would expose astronauts to about 1,000 millisieverts – depending on how many solar storms belch high-energy particles toward Mars, and whether the first entity to reach the planet actually lands on it.

This means the first Martian explorers could get roughly eight times the amount of radiation per year of a radiation worker’s annual exposure limit. In total, the space travellers would get about one-third of the way toward hitting a NASA astronaut’s maximum lifetime exposure limit (2,500-3,250 mSv).

Where the radiation comes from – and why cancer isn’t the only danger

Two main types of radiation in space are extremely harmful to humans: protons spewed out by the sun and cosmic rays. These high-energy particles and the secondary radiation they create penetrate deep into cells, promoting chronic and sometimes deadly diseases such as cancer.

Earth’s magnetic field and atmosphere protect us by deflecting and absorbing most of this energy.

“The background radiation rates on the ground are 100 times to 1,000 times smaller than they would be above the atmosphere in free space,” Edward Semones, a radiation health officer at NASA’s Johnson Space Center, previously told Business Insider.

Cancer is a major risk of radiation exposure, but there are more immediate and surprising symptoms. Deep-space radiation might promote cataracts and impair eyesight. Even high-flyingcommercial-airline workers face that risk because of the thinner atmosphere.

Animal-based experiments also suggest radiation could damage the nervous system, including the brain, which might impair astronauts’ focus and memory.

“You’re somehow losing cognitive ability,” Semones said, adding that, over the years, this “may impact conducting the mission.”

Ultimately, colonists may try to terraform Mars – a deliberate and unprecedented act of climate change.

Frozen carbon dioxide at the Martian poles could be turned into greenhouse gases in order to create a radiation-absorbing atmosphere that would insulate the surface. Plants could then convert the thin air into oxygen and, over hundreds of years, temperatures may warm enough to melt hidden water and make it again flow on Mars’ surface. One day, that could even permit spacesuit-free excursions.

Jim Green, the former head of NASA’s planetary science division, has proposed building anartificial magnetic shield for Mars to protect a hypothetical nascent atmosphere from the sun’s proton radiation, which might otherwise blow the air into space.

“This may sound ‘fanciful’ but new research is starting to emerge revealing that a miniature magnetosphere can be used to protect humans and spacecraft,” Green and other researchers wrote in a brief study of the concept in 2017. “If this can be achieved in a lifetime, the colonization of Mars would not be far away.”