On October 19, 1989, a massive solar flare triggered an intense geomagnetic storm, causing auroras to light up the skies across many countries. Flying around the Moon at that time would have exposed you to a lethal dose of radiation. The Orion spacecraft for a Moon fly-by mission is equipped with a heavily shielded storm shelter to protect the crew, but this wouldn’t be sufficient for a journey to Mars. Galactic cosmic rays, which constantly bombard space travelers, are particularly challenging to shield against. Earth’s natural shielding system, including the magnetic field and atmosphere, protects us from the majority of space radiation. Proposed solutions to protect astronauts from radiation on long space missions include using active shields to deflect charged particles, though implementing this has proven difficult.
https://arstechnica.com/science/2024/03/shields-up-new-ideas-might-make-active-shielding-viable/