Curiosity measures the radiation dose of a Mars trip
The rover's dosimeter logged 1.8 millisieverts a day in transit and 0.67 on the floor of Gale Crater — a round trip comes to roughly one sievert, two thirds past NASA's career ceiling.
Ten days after Curiosity left Earth, on 6 December 2011, a small stack of detectors bolted inside the rover's body switched on and started counting. The Radiation Assessment Detector — silicon diodes, a caesium-iodide crystal, a slab of plastic scintillator — rode inside the aeroshell, behind roughly the shielding a crew vehicle would carry. Every previous estimate of what the journey does to a human body had come out of a model. Odyssey's MARIE had tried from orbit and was killed by a solar storm in October 2003.
The cruise ran 253 days and 560 million kilometres. On 31 May 2013 Cary Zeitlin's team published the result: about 1.8 millisieverts of dose equivalent per day, which NASA translated as a whole-body CT scan every five or six days. One way came to roughly 466 millisieverts; the shortest plausible round trip with chemical rockets, 0.66 ± 0.12 sieverts, before anybody sets foot on anything. Five solar particle events occurred en route, contributing 24.7 millisieverts between them — about five per cent. The rest was galactic cosmic rays, iron nuclei moving near light speed, which shielding does not absorb so much as shatter into secondary showers.
Landing helped less than people had hoped. RAD resumed measuring on the crater floor on 7 August 2012 and averaged 0.67 millisieverts a day through June 2013, more than 95 per cent of it cosmic rays. Six millibars of carbon dioxide overhead and the bulk of the planet underfoot cut the deep-space rate by about two thirds — not by the order of magnitude an atmosphere is supposed to buy you. Solar particles the thin air does handle: the first such event seen from the surface, on sol 242, delivered 0.025 millisieverts. A rounding error.
Donald Hassler's paper carrying that number appeared on 9 December 2013. Run the rates against NASA's own reference crewed profile — 174 days out, 539 on the surface, 201 home — and the trip totals a little over a sievert. NASA said as much that day: on the order of 1,000 millisieverts. A sievert carries roughly a five per cent increase in lifetime fatal cancer risk. Since 2021 NASA has used a single career ceiling of 600 millisieverts, regardless of age or sex. One Mars mission spends it and then some.
The unwelcome part came later. Both measurements were taken near the peak of solar cycle 24, and a busy Sun is the good case — its magnetic field sweeps cosmic rays out of the inner solar system. As activity fell away toward 2020, RAD watched the Gale surface dose climb about 50 per cent. A Hohmann round trip works out at 0.65 sieverts at solar maximum and 1.59 at minimum. The biology is shakier still: the models converting heavy-ion dose to cancer probability are extrapolated from Hiroshima and Nagasaki survivors, who were hit by gamma rays. RAD did not answer whether a crew can go. It turned shielding, transit time and acceptable risk into an argument with numbers in it.