When the Mars Science Laboratory (MSL) launched November 26, 2011, for its historic trip to the Red Planet, it brought along an instrument to measure the radiation inside the spacecraft as it journeyed through space.

Scientists released the findings from that Radiation Assessment Detector (RAD) and gave an estimate of the radiation dose that humans might receive during a round trip from Earth to Mars in the May 31 issue of Science. Engineers waited about two weeks from launch before turning on the RAD.

From December 6, 2011, through July 14, 2012, the instrument measured the energies of particle radiation resulting from solar events (like solar flares and coronal mass ejections) and galactic cosmic rays (high-energy electrons, protons, and heavier nuclei that zip around space). Scientists measure radiation doses in a unit of Sieverts (Sv), which gives the amount of ionizing energy per unit of mass. For ref-erence, a CT scan of the head delivers about 2 milliSieverts (mSv) while a CT scan of the abdomen delivers about 8 tnSv.

The scientists say that a round-trip mission carrying humans to the Red Planet would take 360 days using today’s best propulsion methods. Thus, Mars astronauts would experience a dose of 662.4 mSv from galactic cosmic rays in addition to the radiation from solar events. This number factors in only the journey’s radiation exposure, not the amount of radiation that makes it to the martian surface, where they could be stationed up to a year.

A heavily shielded craft will decrease the cancer risk, but not as much as one might initially expect: Most cosmic rays pass right through protective material and can create particle showers. The same dosage delivered to a female and a male result in slightly different fatal cancer rates. A 360-day Mars trip increases a male astronaut’s chance of getting a fatal cancer by about 4 percent, but that same journey increases a female’s chance by some 5 percent.

A jetliner pilot typically receives about 5 mSv per year, a radiation worker’s limit is 50 mSv each year, and an astronaut aboard the International Space Station receives about 200 mSv per year. NASA limits an astronaut’s radiation dose to what would cause a 3 percent increase in fatal cancer, and that value can vary from 370 mSv for a 25-year-old female to 1.15 Sv for a 50-year-old male. The Zeitlin team study yet again shows that a human exploration trip to Mars will be a dangerous venture.