The term “cosmic cancer” refers to the risk of developing cancer due to exposure to the harsh radiation environment of deep space. For humans traveling beyond the protection of Earth’s magnetic field and atmosphere, this radiation exposure is one of the most significant health concerns, particularly for long-duration missions. The space radiation field is composed of highly energetic particles that pose a distinct threat to biological systems. Understanding and mitigating this threat is necessary for the future of human space exploration, especially for planned journeys to the Moon and Mars.
The Unique Radiation Environment of Space
The radiation field outside of Earth’s low orbit is different from the one experienced on Earth. Terrestrial life is shielded by the planet’s magnetic field, which deflects most charged particles, and by the thick atmosphere, which absorbs or breaks down the rest. Beyond this protective bubble, space travelers are exposed to a constant barrage of high-energy particles from two primary sources: Galactic Cosmic Rays (GCR) and Solar Particle Events (SPEs).
GCR originate from outside our solar system and are composed of highly energetic protons, helium nuclei, and heavier ions. Some of these particles, such as ionized iron nuclei, are accelerated to nearly the speed of light. This high energy allows GCR particles to penetrate the thick metal walls of a spacecraft and the tissue of the human body, depositing energy along their path.
SPEs are intermittent but intense bursts of radiation, primarily high-energy protons, ejected from the Sun during solar flares or coronal mass ejections. Spacecraft can often use a dedicated “storm shelter” to protect against these events, but the GCR component is a continuous threat. These particles classify as high-Linear Energy Transfer (LET) radiation, causing dense, concentrated damage as they pass through matter.
How Cosmic Radiation Causes Cellular Damage
Cosmic radiation damages the fundamental building blocks of life both directly and indirectly. When a high-LET particle traverses a cell, it leaves a dense track of ionization that can shatter the cellular machinery. This interaction can directly strike the DNA molecule, or it can ionize water molecules within the cell to create highly reactive free radicals.
These free radicals then chemically react with the DNA, leading to damage. The most detrimental form of this damage is the complex double-strand break (DSB), where both complementary strands of the DNA helix are severed. While cells have robust mechanisms to repair simple damage, the clustered nature of DSBs caused by high-LET cosmic rays makes repair significantly more difficult.
A single GCR ion track creates multiple lesions in a localized area of the DNA, known as clustered damage. This complexity overwhelms the cell’s repair enzymes, increasing the likelihood of misrepair or incomplete repair. Incorrectly repaired DSBs can lead to permanent changes in the genetic code, resulting in genomic instability, chromosomal rearrangements, and the initiation of carcinogenesis.
Quantifying the Cancer Risk for Space Travelers
Space agencies calculate the cancer risk for astronauts using a metric called the Risk of Exposure-Induced Death (REID). This calculation estimates the probability that an individual will die from a cancer caused by the radiation exposure accumulated during their career. NASA currently maintains a career exposure limit that shall not exceed a 3% REID for cancer mortality.
This 3% limit is based on the most susceptible case, converted into a total allowable dose. For most astronauts, the maximum allowable career dose is set at approximately 600 millisieverts (mSv). This dose is significantly greater than the 50 mSv per year limit for terrestrial radiation workers, reflecting the unavoidable nature of the space environment.
Current data shows that a six-month stay on the International Space Station (ISS) exposes an astronaut to about 72 mSv. In contrast, a three-year mission to Mars, which travels outside Earth’s magnetosphere for its entire duration, could expose the crew to a total dose exceeding 1000 mSv.
Long-duration missions face a projected risk that exceeds current limits, driving the development of mitigation strategies. These countermeasures include developing effective passive shielding materials and exploring concepts for active magnetic or plasma shielding. Agencies are also researching pharmaceutical countermeasures, known as radioprotectants, to boost the body’s natural DNA repair capabilities.