How Much Radiation Would a Crew Get on the Way to Mars?

Short answer: a lot more than on Earth, and most of it is hard to block. On its way to Mars, NASA’s Curiosity rover carried a radiation detector inside its spacecraft. It measured an average of 1.84 millisieverts per day from galactic cosmic rays. At that rate, the transit legs of a round trip alone add up to about 0.66 sievert, which is close to NASA’s whole-career limit for an astronaut.

In SOLEN: The Eden Archive, the crossing takes one hundred and eighty-two days. Carter & Alina: The Bond follows the two founders “through 182 days alone in deep space,” and SOLEN: The First Record covers the “eighty days alone in the cryosleep phase of the transit.” This post sets out what real measurements say a crossing like that would mean for the human body. The science is real. The story details are the novels’, and they are labelled that way.

What Curiosity measured on the way to Mars

The Mars Science Laboratory, the spacecraft carrying NASA’s Curiosity rover, launched on 26 November 2011. For most of its 253-day, 560-million-kilometre cruise, the Radiation Assessment Detector (RAD) recorded the radiation inside the spacecraft. That is roughly the position a crew would be in. The results were published in Science in 2013 (Zeitlin et al.).

The headline figure was an average dose equivalent of 1.84 millisieverts (mSv) per day from galactic cosmic rays. Lead author Cary Zeitlin put it in everyday terms in NASA JPL’s announcement: “In terms of accumulated dose, it’s like getting a whole-body CT scan once every five or six days.”

The team then scaled the measurement to a human mission. For “even the shortest round-trip with current propulsion systems and comparable shielding,” the paper gives a dose equivalent of 0.66 ± 0.12 sievert for the transit legs alone. The Southwest Research Institute, which led RAD, notes that time on the surface “might add considerably” to that total, depending on shielding and how long the crew stays.

What would 182 days cost?

Our own back-of-envelope arithmetic, not a published figure: 182 days at Curiosity’s average cruise rate comes to about 335 mSv one way. That assumes shielding and solar conditions like those during Curiosity’s cruise. The authors stress that the result depends strongly on both. It also leaves out large solar storms, which can’t be predicted. Two crossings of that length come to roughly 670 mSv, in line with the paper’s 0.66 sievert estimate.

For comparison, NASA’s human-system standard (NASA-STD-3001, Volume 1) says an astronaut’s total career effective dose from spaceflight “shall be less than 600 mSv,” for all ages and sexes. That limit is set to keep the mean added risk of dying from cancer below 3%. Curiosity’s figure (dose equivalent) and NASA’s limit (effective dose) are related measures but not identical, so treat the comparison as an order of magnitude. Even so, the transit alone would use up most or all of a career’s allowance.

Two kinds of space radiation

JPL’s write-up describes two main threats in deep space:

• Galactic cosmic rays (GCRs) come from supernova explosions and other high-energy events outside the solar system. They are “highly energetic, highly penetrating particles that are not stopped by the modest shielding provided by a typical spacecraft.” They made up nearly all of Curiosity’s cruise dose.
• Solar energetic particles (SEPs) come from solar flares and coronal mass ejections. They have lower energy, and current spacecraft shield against them much better. SwRI says only about 5 percent of RAD’s cruise dose came from solar particles, partly because the Sun was relatively quiet during the cruise.

This is the uncomfortable part. The radiation that is easiest to block is the one that arrives in bursts. The radiation that hardly stops is the one that never lets up.

How crews shelter: the Orion approach

NASA’s answer for solar storms on its Orion capsule is to build a shelter from things already on board. Orion carries a sensor, the Hybrid Electronic Radiation Assessor, that warns the crew when to take cover. According to NASA’s Orion crew systems reference, the crew then has up to one hour to empty two storage bays beneath their seats. They stack the stowage bags against the less-shielded parts of the cabin and climb inside, two astronauts per bay, with food, water, medical supplies, air lines and computers. They may stay there for up to 24 hours.

The choice of material matters. A NASA Orion radiation handout notes that some of the best shielding materials have a high concentration of hydrogen atoms, “such as water.” Food, water and supplies do double duty. Against galactic cosmic rays, though, no practical amount of stowage is a complete answer, and NASA’s Human Research Program has a dedicated Space Radiation element whose job is keeping crews within acceptable radiation health risks.

And when they arrive?

Landing doesn’t end the exposure. Mars has a thin atmosphere and no global magnetic field. On the surface at Gale Crater, Curiosity measured about 0.64 mSv per day from galactic cosmic rays (Hassler et al., 2014). That is lower than in cruise, but it never stops. We covered the options underground in Could Humans Build a City Inside a Mars Lava Tube?

The series has its own answer, and it is fiction. In NovaSeed: Eden Rising, the habitat sits on the floor of the Hellas Planitia basin, “lower than the surrounding terrain by almost eight kilometers, which provided additional atmospheric pressure and reduced radiation exposure.” The direction of that idea is sound: more air overhead means more shielding. The habitat and its readings are the novel’s.

Why it matters for the story

A crossing like this has a cost the crew carries for the rest of their lives, and no one sees it. Every day in transit is spent from a budget that does not refill. In SOLEN, that half-year belongs to two people who chose to go anyway, with an AI watching over them. If you want to read it in order, start with NovaSeed: Eden Rising or see Where to Start with SOLEN. The in-world view of the cryosleep phase is in What Cryosleep Keeps: 80 Days Alone.

Reader question: if you knew the trip would use up most of a lifetime’s safe radiation dose, would you still go?

SOLEN: The Eden Archive is a fiction series by Warren Pulley. Scientific statements in this post are linked to their sources below; story details and quoted lines come from the books and their published descriptions and are labelled as the novels’.