 The Apollo 8 crew were the first humans to pass through the Van Allen radiation belts , which extend up to 15 @,@ 000 miles ( 24 @,@ 000 km ) from Earth . Scientists predicted that passing through the belts quickly at the spacecraft 's high speed would cause a radiation dosage of no more than a chest X @-@ ray , or 1 <unk> ( during a year , the average human receives a dose of 2 to 3 mGy ) . To record the actual radiation dosages , each crew member wore a Personal Radiation <unk> that transmitted data to Earth as well as three passive film dosimeters that showed the cumulative radiation experienced by the crew . By the end of the mission , the crew experienced an average radiation dose of 1 @.@ 6 mGy . 
