Cosmic Rays pose excessively high radiation risks to future human Mars missions

Measurements taken by NASA's Mars Science Laboratory (MSL) mission as it delivered the Curiosity rover to Mars in 2012 are providing NASA the information it needs to design systems to protect human explorers from radiation exposure on deep-space expeditions in the future.  MSL's Radiation Assessment Detector (RAD) is the first instrument to measure the radiation environment during a Mars cruise mission from inside a spacecraft that is similar to potential human exploration spacecraft. The findings will reduce uncertainty about the effectiveness of radiation shielding and provide vital information to space mission designers who will need to build in protection for spacecraft occupants in the future. The findings, which are published in the May 31 edition of the journal Science, indicate radiation exposure for human explorers could exceed NASA's career limit for astronauts if current propulsion systems are used.

Two forms of radiation pose potential health risks to astronauts in deep space. One is galactic cosmic rays (GCRs), particles caused by supernova explosions and other high-energy events outside the solar system. The other is solar energetic particles (SEPs) associated with solar flares and coronal mass ejections from the sun.
Galactic cosmic rays are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.

Cosmic rays are energetic particles originating from outer space that travel towards every direction and also impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are simple protons, with nearly 10% being helium nuclei (alpha particles), and slightly less than 1% is other heavier elements. Electrons (beta particles) or gamma ray photons with similar origin are also included under that denomination. The term ray is a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. In modern common usage high-energy particles with intrinsic mass are known as "cosmic" rays, and photons, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as “gamma rays” or “X-rays”, depending on their frequencies. Cosmic rays attract great interest practically, due to the damage they inflict on microelectronics and life outside the protection of an atmosphere and magnetic field, and scientifically, because the energies of the most energetic ultra-high-energy cosmic rays (UHECRs) have been observed to approach 3 × 10²⁰ eV, about 40 million times the energy of particles accelerated by the Large Hadron Collider (LHC). At 50 Joules, the highest-energy ultra-high-energy cosmic rays have energies comparable to the kinetic energy of a 90-kilometre-per-hour (56 mph) baseball.

Galactic cosmic rays (GCRs) are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft. Galactic cosmic rays are the high-energy particles that flow into our solar system from far away in the Galaxy. GCRs are mostly pieces of atoms: protons, electrons, and atomic nuclei which have had all of the surrounding electrons stripped during their high-speed (almost the speed of light, 299 792 458 m/s) passage through the Galaxy. Cosmic rays provide one of our few direct samples of matter from outside the solar system. The magnetic fields of the Galaxy, the solar system, and the Earth have scrambled the flight paths of these particles so much that we can no longer point back to their sources in the Galaxy. Cosmic rays also pose a threat to electronics placed aboard outgoing probes. In 2010, a malfunction aboard the Voyager 2 space probe was credited to a single flipped bit, probably caused by a cosmic ray. Strategies such as physical or magnetic shielding for spacecraft have been considered in order to minimize the damage to electronics and human beings caused by cosmic rays.

The second form of “radiation” to boost the levels of risk towards a Mars missions is represented by the Solar energetic particles (SEP). These are high-energy particles coming from the Sun. They consist of protons, electrons, and alpha particles helium ions, and HZE ions (which have an electric charge greater than +2) that are highly energetic. They are of particular interest and importance because they can endanger life in outer space. Solar energetic particles can originate from two processes: Particles accelerated at a solar-flare site or by shock waves associated with coronal mass ejections (CMEs). However, only about 1% of the CMEs produce strong SEP events.

Radiation exposure is measured in units of Sievert (Sv) or milisievert (one one-thousandth Sv). Long-term population studies have shown exposure to radiation increases a person's lifetime cancer risk. Exposure to a dose of 1 Sv, accumulated over time, is associated with a 5 percent increase in risk for developing fatal cancer.

NASA has established a 3 percent increased risk of fatal cancer as an acceptable career limit for its astronauts currently operating in low-Earth orbit. The RAD data showed the Curiosity rover was exposed to an average of 1.8 milisievert of GCR per day on its journey to Mars. Only about 5 percent of the radiation dose was associated with solar particles because of a relatively quiet solar cycle and the shielding provided by the spacecraft. “In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," said Cary Zeitlin, a principal scientist at the Southwest Research Institute (SwRI) in San Antonio and lead author of the paper on the findings. "Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep space destinations is critical for planning future crewed missions.”

The RAD data will help inform current discussions in the United States medical community, which is working to establish exposure, limits for deep-space explorers in the future.

References:

1.      Radiation Measured by NASA's Curiosity on Voyage to Mars has Implications for Future Human Missions; Access: (accessed on: May 30, 2013);

2.       Cosmic ray; http://en.wikipedia.org/wiki/Cosmic_ray Access: (accessed on: May 31, 2013);

3.      Cosmic rays: What are cosmic rays?; Access: http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html  (accessed on: May 31, 2013);

4.      HZE ions; Access: http://en.wikipedia.org/wiki/HZE_ions  (accessed on: May 31, 2013);

5.      Image: Access: http://www.popsci.com/science/article/2013-04/apply-now-one-way-trip-mars  (accessed on: May 31, 2013)