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.
The Radiation Assessment Detector switched on inside Curiosity's body on 6 December 2011, ten days after launch: silicon diodes, a caesium-iodide crystal and a slab of plastic scintillator, riding inside the aeroshell behind roughly the shielding a crew vehicle would carry. Every earlier estimate of a crew's dose came from 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 Zeitlin's team reported about 1.8 millisieverts of dose equivalent per day, which Zeitlin compared to a whole-body CT scan every five or six days: roughly 466 millisieverts one way, and 0.66 ± 0.12 sieverts for the shortest plausible round trip with chemical rockets. Five solar particle events en route contributed 24.7 millisieverts, about five per cent; the rest was galactic cosmic rays, iron nuclei near light speed, which shielding shatters into secondary showers rather than absorbing.
RAD resumed measuring on the crater floor on 7 August 2012 and averaged 0.64 millisieverts a day through June 2013, over 95 per cent of it cosmic rays: eight millibars of carbon dioxide overhead and the bulk of the planet underfoot cut the deep-space rate by about two thirds. Solar particles the thin air does handle — the first event seen from the surface, on sol 242, delivered 0.025 millisieverts.
Hassler's paper carrying that number appeared on 9 December 2013. Against NASA's reference crewed profile — 180 days out, 500 on the surface, 180 home — the trip totals a little over a sievert, on the order of 1,000 millisieverts, as NASA said that day. A sievert carries roughly a five per cent increase in lifetime fatal cancer risk, and since 2022 NASA has used a single career ceiling of 600 millisieverts regardless of age or sex.
Both measurements came at moderate solar activity, near 600 megavolts of modulation: a busy Sun's magnetic field sweeps cosmic rays out of the inner solar system. As activity fell toward 2020 RAD watched the Gale surface dose climb about 50 per cent, and Guo's Hohmann round trip works out at 0.65 sieverts at solar maximum against 1.59 at minimum. The models converting heavy-ion dose to cancer probability are extrapolated from Hiroshima and Nagasaki survivors, who were hit by gamma rays.