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Earth's atmosphere and magnetic field are a shield you never notice until you climb above them. Pick a flight or a space mission and see how much cosmic radiation gets through, measured in chest X-rays.
Cosmic rays are mostly protons and atomic nuclei flung across the galaxy by exploding stars, moving at a good fraction of the speed of light. Almost none of them reach you directly. When one slams into the top of the atmosphere, it shatters air molecules into a cascade of secondary particles, and the air soaks up most of that shower on the way down. At sea level you're sitting under the equivalent of a 10-meter-thick wall of water.
At 35,000 feet, roughly three quarters of that shield is below you. The cosmic-ray dose rate is about 100 times higher than on the ground: a few microsieverts per hour instead of a few hundredths.
Latitude matters because of Earth's magnetic field. Near the equator, field lines run parallel to the ground and deflect all but the most energetic particles. Near the magnetic poles, the field lines dive straight down into the Earth, and charged particles can spiral right along them. That's the same effect that lights up the auroras. A polar route from New York to Tokyo can deliver more than twice the hourly dose of a flight across the equator at the same altitude.
The International Space Station orbits about 400 km up, far above the atmosphere but still inside Earth's magnetic field, which deflects a good share of the cosmic rays. Beyond low Earth orbit, that protection is gone too.
| Location | Typical dose rate | Source |
|---|---|---|
| Ground level, world average (all natural sources) | ~0.0066 mSv/day | UNSCEAR, 2.4 mSv/yr |
| Airliner at 35,000 ft, mid-latitudes | ~0.1 mSv/day | if you stayed up 24 h |
| International Space Station | ~0.6 mSv/day | NASA, 80–160 mSv per 6 months |
| Surface of Mars | ~0.64 mSv/day | Curiosity rover RAD |
| Surface of the Moon | ~1.37 mSv/day | Chang'e 4 lander |
| Deep space between planets | ~1.84 mSv/day | Curiosity, during its cruise to Mars |
A round trip to Mars with a 500-day stay adds up to roughly 1,000 mSv, more than NASA's entire career limit for an astronaut. That's why radiation, not rockets, is often called the hardest problem in sending people to Mars.