Tuesday, November 19, 2013

More from CRaTER on radiation health hazards

CRaTER, in a fifth year in lunar orbit aboard LRO is providing a solid data set on the real health hazards, and possible mitigation, of high-energy radiation in deep space. Using proven technologies cosmic, not solar, radiation, is a significant block to long-term human spaceflight beyond the Moon [NASA/GSFC/UNH/SwRI].
David Sims
University of New Hampshire
Institute for the Sutdy of Earth, Oceans and Space

Scientists from the University of New Hampshire and colleagues have published comprehensive findings on space-based radiation as measured by a UNH-led detector aboard NASA's Lunar Reconnaissance Orbiter (LRO). The data provide critical information on the radiation hazards that will be faced by astronauts on extended missions to deep space such as those to Mars.

The papers in a special issue of the journal Space Weather document and quantify measurements made since 2009 by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) radiation detector.

"These data are a fundamental reference for the radiation hazards in near Earth 'geospace' out to Mars and other regions of our sun's vast heliosphere," says CRaTER principal investigator Nathan Schwadron of the UNH Institute for the Study of Earth, Oceans, and Space (EOS).

The space environment poses significant risks to both humans and satellites due to harmful radiation from galactic cosmic rays and solar energetic particles that can easily penetrate typical shielding and damage electronics. When this radiation impacts biological cells, it can cause an increased risk of cancer.

Standard spacecraft shielding, integrated into hull design,
is strong protection from most solar radiation, but defeats
this purpose with high-energy cosmic rays it simply
splits into deadly showers of secondary particles
Before CRaTER's long-term radiation measurements were derived using a material called "tissue-equivalent plastic" - a stand-in for human muscle capable of gauging radiation dosage - those hazards were not sufficiently well characterized to determine if long missions outside low-Earth orbit can be accomplished with acceptable risk.

CRaTER's seminal measurements now provide quantified, radiation hazard data from lunar orbit and can be used to calculate radiation dosage from deep space down to airline altitudes.

The data will be crucial in developing techniques for shielding against space-based radiation dosage. The measurements have also played a vital role in UNH space scientists' efforts to develop both the first Web-based tool for predicting and forecasting the radiation environment in near-Earth, lunar, and Martian space environments and a space radiation detector that possesses unprecedented performance capabilities.

The near real-time prediction/forecasting tool known as PREDICCS integrates for the first time numerical models of space radiation and a host of real-time measurements being made by satellites currently in space. It provides updates of the radiation environment on an hourly basis and archives the data weekly, monthly, and yearly - an historical record that provides a clear picture of when a safe radiation dose limit is reached for skin or blood-forming organs, for example.

CRaTER offers an opportunity to test the capability of PREDICCS to accurately describe the lunar radiation environment. The Space Weather special issue provides comparisons between dose rates produced by PREDICCS with those measured by CRaTER during three major solar energetic particle events that occurred in 2012.

The detector developed at UNH, known as DoSEN, short for Dose Spectra from Energetic Particles and Neutrons, measures and calculates the absorbed dose in matter and tissue resulting from the exposure to indirect and direct ionizing radiation, which can change cells at the atomic level and lead to irreparable damage. Schwadron is lead scientist for both the PREDICCS and the DoSEN project.

"DoSEN is an innovative concept that will lead to a new generation of radiation detectors, or dosimeters, to aid in understanding the hazards posed by the radiation environment of space," says Schwadron. "The ability to accurately understand these hazards will be critical to protect astronauts sent beyond low-Earth orbit on extended space missions."

DoSEN combines two advanced, complementary radiation detection concepts that present fundamental advantages over traditional dosimetry. The dosimeter measures both the energy and the charge distribution of energetic particles that affect human and robotic health in a way not presently possible with current technology. Protons, heavy ions, and neutrons all contribute significantly to the radiation hazard.

"Understanding how different particles such as neutrons and heavy ions pose hazards will be extremely important in completely characterizing the types of environments we will operate in," Schwadron says. "For example, on the Moon, there are additional hazards from neutrons that are created by high-energy radiation interacting in the lunar soil and radiating outward from the surface."

That 'backsplash" of protons, which was discovered by CRaTER and is known as the Moon's radiation "albedo," is caused by the partial reflection of galactic cosmic rays off the moon's surface. This creates a surprising one-two punch of deadly radiation and can also be used to peer below the lunar surface like a geological probe.

Says Harlan Spence, CRaTER deputy lead scientist and director of EOS, "Until now, people have not had the 'eyes' necessary to see this particular population of particles. With CRaTER, we just happen to have the right focus to make these discoveries."

UNH team members on the CRaTER instrument and co-authors on the Space Weather papers include Schwadron, Spence, Sonya Smith, Mike Golightly, Jody Wilson and Colin Joyce, Jason Legere, and Cary Zeitlin of the Southwest Research Institute Earth, Oceans, and Space Department at UNH. Coauthors from the UNH Space Science Center on the DoSEN project include James Ryan, Peter Bloser, and Chris Bancroft.
Figure 1. Wilson, et al, "First Albedo Proton Map of the Moon," Diagram of CRaTER instrument showing cross-sectional cutaway view of the stack of six detectors (D1–D6) and pieces of tissue equivalent plastic (TEP). Example particle trajectories are shown for a high-energy galactic cosmic ray from the zenith passing completely through the instrument (red line) and for an albedo proton (blue line) coming up from the lunar surface and passing through four detectors before being stopped in one of the blocks of TEP. (Adapted from Spence et al. [2010].)

Additional Background from NASA Goddard Space Flight Center:

Paul Gabrielsen
Radiation in deep space comes from cosmic rays, from the solar wind and from solar energetic particles emanated during a solar storm. Particles from these sources rocket through space. Many can pass right through matter, such as our bodies. So-called ionizing radiation knocks electrons off of atoms within our bodies, creating highly reactive ions. Within Earth's protective atmosphere and magnetic field, we receive low doses of background radiation every day. The radiation hazards astronauts face are serious, yet manageable thanks to research endeavors such as the CRaTER instrument.

CRaTER measures realistic human radiation doses at the moon using a unique material called tissue-equivalent plastic (TEP). Two pieces of this plastic, roughly 2 inches and 1 inch thick, respectively, are separated by silicon radiation detectors. The TEP-detector combo measures how much radiation may actually reach human organs, which may be less than the amount that reaches the spacecraft.

"Tissue-equivalent plastic gives us an idea of the self-shielding of the body," said Larry Townsend, of the University of Tennessee, Knoxville. "The radiation spectrum at the organs is not going to be the same as the radiation spectrum that's outside the spacecraft."

Townsend notes that CRaTER's observations have come at a time when solar activity, and hence the solar wind, has been unusually quiet. The solar wind disperses some galactic cosmic rays, but in the current solar lull, more of these rays are able to bombard the Earth and moon. CRaTER, which launched aboard LRO with six other instruments in 2009, has been able to monitor the lunar environment as solar activity has declined. Further mission extensions would allow additional detailed measurements as solar activity waxes and wanes.

"They're lower-level exposures," Townsend said, of galactic cosmic rays, "but they're damaging in the sense that the particles are highly charged and heavy, and they create a lot of damage when they're going through the body."

But lab tests suggested that materials rich in hydrogen, such as some plastics, may shield against these heavy particles, said Cary Zeitlin of the Southwest Research Institute, San Antonio, Texas. "The tissue-equivalent plastic in CRaTER has fairly high hydrogen content," he said, "so it lets us test this hypothesis using data from deep space. And it turns out that plastic really is a good shield against these particles, significantly better than aluminum."

LRO's unofficial motto states that "exploration enables science, and science enables exploration." The LRO spacecraft launched as an exploration mission, a forerunner for humanity's return to the moon. But after completing its primary mission in 2010, LRO has become a powerful instrument for lunar and planetary science. CRaTER is an active participant in this scientific study, discovering a previously unmeasured source of hazardous radiation emanating from the moon itself.

This radiation comes from the partial reflection, also called an albedo, of galactic cosmic rays off the moon's surface. Galactic cosmic ray protons penetrate as much as a meter (about 3.2 feet) into the lunar surface, bombarding the material within and creating a spray of secondary radiation and a mix of high-energy particles that flies back out into space. This galactic cosmic ray albedo, which may interact differently with various chemical structures, could provide another method to remotely map the minerals present at the moon's surface.

CRaTER directly measured the proton component of the moon's radiation albedo for the first time, said Harlan Spence, deputy principal investigator at the University of New Hampshire. The TEP radiation detector measures various components of radiation separately, which enables CRaTER to, in Spence's words, "unfold" the energy spectrum of the radiation albedo. This result, he said, illustrates the value of combining exploration and science in spaceflight. "If we had been on a different science-oriented mission, we probably would've developed a different instrument," Spence said. "In fact, we probably never would have flown TEP."

Looking toward future missions, Schwadron and his colleagues are developing a next-generation radiation dose detector, drawing on CRaTER's design. The detector, called Dose Spectra from Energetic particles and Neutrons (DoSEN) builds on CRaTER's ability to break radiation down into its components and assess the doses resulting from each part of the radiation spectrum. Human exploration will benefit, Schwadron said, from this "very specific information about the spectrum of radiation we need to shield against."

Spence, who served as the instrument's principal investigator through the primary mission said he's proud of his team's foresight to equip CRaTER with the capability to accomplish its mission and continue to pursue great science.

"We had hopes and aspirations," he said, "but we didn't think we would be able to reap as much from those data as we are. Exploration now is enabling science."

Related Posts:
Cosmic Ray threat to manned spaceflight tested on MSL (May 31, 2013)
The radiation environment and its effects on human spaceflight: A Lunar Mission (January 5, 2013)
Cosmic ray flux effects lunar ice (March 19, 2012)
"A Perfect Storm of Cosmic Rays" (September 29, 2009)
Cosmic rays and manned space travel (September 16, 2009)
Cosmic ray flux highest ever recorded (September 3, 2009)
Returning to the Moon (August 9, 2009)
Skeptical: LUNAR-TEX radiation blanket (May 11, 2009)
NASA cataract detection down to Earth (January 18, 2009)
NASA and Congress sacrifice radiation shielding flexibility
removing dry landing hardware
(May 17, 2008)

Managing Space Radiation Risk in the New Era of Space Exploration (2008)
Committee on the Evaluation of Radiation Shielding for Space Exploration
National Research Council

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