How many simultaneous chest X-rays would it take to kill you?
According to NASA, 45,000.
How many equivalent chest x-rays might an astronaut be exposed to on the Moon or on Mars in the case of a coronal mass ejection (CME)?
300,000. Which, in other words, is lethal.
As shown below, a coronal mass ejection involves an explosive belch of fiery hot plasma and radioactive particles from the outer atmosphere of the sun or corona, which resembles a volcano spewing lava. When the energy from a CME interacts with the Earth’s atmosphere it is called a solar storm.
Figure 1. A CME.
The danger to us on Earth is nil because the magnetosphere, the magnetic field that surrounds the Earth like a bubble absorbs these radioactive particles and shields us from their effects. The magnetosphere also partially protects astronauts on the International Space Station (ISS) because it is in low-Earth orbit.
Outside of the magnetosphere?
Forget the term, “solar storm”. Think total sh*t storm.
Astronauts on ~ 3-year missions to Mars would receive little radiation protection in transit and almost none on the surface because of the rarefied atmosphere. This effectively puts Mars out of reach, because of the health risks to astronauts that include Acute Radiation Syndrome, neurodegeneration, cataracts, and cancer not to mention bone and muscle atrophy from the prolonged microgravity.
The most effective countermeasure for the radiation risk is reduced transit time, but faster propulsion remains a work in progress. The second most effective countermeasure is thick shielding, but materials such as lead that protect against radiation on Earth are not practical for space travel. And the third is radiation countermeasures like the small molecule, nibrozetone (RRx-001), for example.
Elon Musk famously aspires to die on Mars, “just not on impact.”
Assuming a safe landing is possible, Elon would likely get his wish to die on Mars—sooner rather than later.
To quote Andy Weir from his novel, the Martian, “With no magnetic field, Mars has no defense against harsh solar radiation. If I were exposed to it, I’d get so much cancer, the cancer would have cancer.”