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High Altitude Radiation

March 2, 2017

Boeing introduced a new space suit design on January 25, 2017.[1] Space suits are a reminder that the only way that humans can survive in space is to take our earthly environment with us. Space suit design is quite closely coupled with materials science, since it's the rapid pace of materials development that's allowed the evolution of space suits from the bulky affairs that resemble robot exoskeletons to the lightweight suits of the present day.

Apollo 17 astronaut, Eugene Cernan, on the Moon, December 13, 1972.

Apollo 17 astronaut, Eugene Cernan, on the Moon, December 13, 1972.

Cernan and companion astronaut, Harrison Schmitt, were in the Moon's Taurus–Littrow valley for about 75 hours.

(NASA photo by astronaut, Harrison H. Schmitt, via Wikimedia Commons.)


While space suits and spacecraft provide a breathable atmosphere, sufficient atmospheric pressure, and protection from the cold of space, they don't shield against high intensity radiation. Astronauts first noticed this as random flashes of light caused by cosmic rays. Cosmic rays are protons and atomic nuclei zipping through space at energies up to the range of 1020 electronvolts (eV), which is several joules.

On lunar missions, the Apollo astronauts reported these flashes about once every three minutes, and other astronauts in Earth orbit have seen these about half as often. Such events were transient and sometimes annoying, but a more significant result is that a 2001 study showed a higher incidence of cataracts in former astronauts who have flown in space, some cataracts having appeared as soon as 4-5 years after the missions.[2]

There was a higher risk of cataracts at lens doses greater than 8 mSv than for lower does. The radiation appears to greatly increase the activity of Fibroblast Growth Factor 2 (FGF-2) that leads to the formation of abnormal fiber cells and cataracts.[3] Such a finding is important if we intend to send travellers to Mars, a journey that could take six months to a year. There's also the problem of cosmic ray exposure on Mars, which does not have a magnetosphere and an atmosphere for cosmic ray shielding.

Incidence of cataracts for NASA astronauts

Incidence of cataracts for NASA astronauts as a function of age.

The radiation dose effect is clearly evident, and getting cataracts in your thirties is unusual for ordinary people.

(Created with Inkscape from data in ref. 4.[4]


While it's unlikely that many of us will be astronauts, or will have enough money to be space tourists, ordinary people are exposed to cosmic radiation through air travel, and the crew of aircraft, who do much more air travel, are exposed even more. The cosmic ray flux increases significantly with altitude, a phenomenon discovered by physicist, Victor Hess, in a balloon flight experiment in 1912. Hess found a four-fold increase in cosmic ray flux at an altitude of 5,300 meters (17,000 feet), a discovery that resulted in his Nobel Prize in Physics in 1936. There's about a seven-fold increase at 10,000 meters (33,000 feet), which is less than the typical cruising altitude for a passenger aircraft.

Hess did his balloon experiment near the time of a near-total solar eclipse to ensure that the radiation that he measured came from sources other then the Sun. As can be seen from the figure, Hess's idea that the Sun at times can dump significant radiation onto the Earth was correct. The Earth's magnetosphere protects our planet by deflecting the solar wind around it, but it's less effective at the poles, as the aurora (northern lights) attests. Aircraft travelling great-circle routes over the North Pole will at times be exposed to greater radiation.

Polar radiation at aircraft cruising altitude after a 2003 solar storm

Radiation at the North Pole at aircraft cruising altitude after a 2003 solar storm.

The Earth's magnetosphere offers less radiation protection at the poles.

(Portion of a NASA/NAIRAS image.)


NASA's Radiation Dosimetry Experiment (RaD-X) is a stratospheric balloon flight mission designed to provide a basis for real-time monitoring of radiation exposure at high altitude to assist in route planning for aircraft.[5-7] Balloon-launched instrument packages characterize both cosmic ray primary, and cosmic ray secondary radiation, at aviation altitudes.[5] Secondary radiation is produced when cosmic ray particles strike nitrogen and oxygen nuclei in the atmosphere to produce cascades of particles and photons. These secondary rays peak at about 60,000 feet, a region known as the Pfotzer maximum.[6]

RaD-X was launched from Fort Sumner, New Mexico, on September 25, 2015, and it collected more than eighteen hours of radiation data using four different types of dosimeters at altitudes from 26,000 to over 120,000 feet.[5-6] Companion measurements were conducted on NASA's ER-2 aircraft and commercial aircraft to assess the potential of aircraft to monitor the aircraft radiation environment.[5] Says Chris Mertens, principal investigator of the RaD-X mission at NASA's Langley Research Center (Hampton, Virginia),

"The measurements, for the first time, were taken at seven different altitudes, where the physics of dosimetry is very different... By having the measurements at these seven altitudes we're really able to test how well our models capture the physics of cosmic radiation."[6]

NASA RaD-X radiation survey balloon and spacecraft

Left, a RaD-X balloon launch at Fort Sumner, New Mexico. The RaD-X instrument packages is shown on the right. (Left, NASA image by Christopher Mertens. Right image by NASA.)


Data from the RaD-X program will be used in development of NASA's Automated Radiation Measurements for Aerospace Safety program that will use instruments flown aboard commercial aircraft to do real-time monitoring at high altitudes.[6] Since the standard instrument for such monitoring, the Tissue Equivalent Proportional Counter (TEPC), is too large and expensive, the RaD-X mission is testing smaller, solid state instruments for this purpose.[6] Radiation data will be used to improve the Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS) space weather model, whose predictions allow rerouting of aircraft when necessary.[8-9]

References:

  1. Loren Grush, "Riding to space in style," The Verge, January 25, 2017.
  2. F. A. Cucinotta, F. K. Manuel, J. Jones, G. Iszard, J. Murrey, B. Djojonegro, and M. Wear, "Space Radiation and Cataracts in Astronauts," Radiation Research, vol. 156, no. 5 (November, 2001), pp. 460-466.
  3. Blinding Flashes, NASA Web Site, October 22, 2004.
  4. Francis A. Cucinotta, "Space Radiation Risks for Exploration," National Academies of Sciences Web Site, May 30, 2013.
  5. Christopher J. Mertens, "Overview of the Radiation Dosimetry Experiment (RaD-X) flight mission," Space Weather, vol. 14, No. 11 (November, 2016), pp. 921-934, DOI: 10.1002/2016SW001399.
  6. Mara Johnson-Groh, "NASA Studies Cosmic Radiation to Protect High-Altitude Travelers," NASA Web Site, January 27, 2017.
  7. NASA RaD-X mission Web Site.
  8. Video: Radiation dose rates of NAIRAS model for November 14, 2012, 20:00-21:00 GMT, NASA/NAIRAS.
  9. Nowcast of Atmospheric Ionizing Radiation System.

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