Researchers calculate radiation exposure associated with journey to Mars

The RAD instrument measures radiation dose using silicon detector and plastic scintillator technology.

The latter has a composition somewhat similar to tissue and is more sensitive to neutrons than are the silicon detectors.

This illustration of RAD shows the silicon detectors (A, B & C) that measure charged particles and the plastic detectors (D, E & F) that measure both charged and neutral particles.

Credit: Hassler et al., 2012. Space Science Reviews, 170, 503.

On November 26, 2011, the Mars Science Laboratory began a 253-day, 560-million-kilometer journey to deliver the Curiosity rover to the Red Planet.

Radiation Assessment Detector

En route, the Southwest Research Institute (SwRI) Radiation Assessment Detector (RAD) made detailed measurements of the energetic particle radiation environment inside the spacecraft, providing important insights for future human missions to Mars.

Cary Zeitlin

"In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," said Dr. Cary Zeitlin, a principal scientist in SwRI's Space Science and Engineering Division and lead author of Measurements of Energetic Particle Radiation in Transit to Mars on the Mars Science Laboratory, scheduled for publication in the journal Science on May 31.

"Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep space destinations is critical for planning future crewed missions," Zeitlin said.

"Based on RAD measurements, unless propulsion systems advance rapidly, a large share of mission radiation exposure will be during outbound and return travel, when the spacecraft and its inhabitants will be exposed to the radiation environment in interplanetary space, shielded only by the spacecraft itself."

Two forms of radiation pose potential health risks to astronauts in deep space: a chronic low dose of galactic cosmic rays (GCRs) and the possibility of short-term exposures to the solar energetic particles (SEPs) associated with solar flares and coronal mass ejections.

Radiation dose is measured in units of Sievert (Sv) or milliSievert (1/1000 Sv). Long-term population studies have shown that exposure to radiation increases a person's lifetime cancer risk; exposure to a dose of 1 Sv is associated with a 5 percent increase in fatal cancer risk.

GCRs tend to be highly energetic, highly penetrating particles that are not stopped by the modest shielding provided by a typical spacecraft.

These high-energy particles include a small percentage of so-called heavy ions, which are atomic nuclei without their usual complement of electrons.

Heavy ions are known to cause more biological damage than other types of particles.

Energetic protons constitute about 85 percent of the primary galactic cosmic ray flux and easily traverse even the most shielded paths (reds) inside the MSL spacecraft.

Heavy ions tend to break up into lighter ions in thick shielding, but can survive traversal of thin shielding (blues) intact.

The solar particles of concern for astronaut safety are typically protons with kinetic energies up to a few hundred MeV (one MeV is a million electron volts).

Solar events typically produce very large fluxes of these particles, as well as helium and heavier ions, but rarely produce higher-energy fluxes similar to GCRs.

The comparatively low energy of typical SEPs means that spacecraft shielding is much more effective against SEPs than GCRs.

"A vehicle carrying humans into deep space would likely have a 'storm shelter' to protect against solar particles. But the GCRs are harder to stop and, even an aluminum hull a foot thick wouldn't change the dose very much," said Zeitlin.

"The RAD data show an average GCR dose equivalent rate of 1.8 milliSieverts per day in cruise. The total during just the transit phases of a Mars mission would be approximately .66 Sv for a round trip with current propulsion systems," said Zeitlin.

Time spent on the surface of Mars might add considerably to the total dose equivalent, depending on shielding conditions and the duration of the stay.

Exposure values that ensure crews will not exceed the various space agencies standards are less than 1 Sv.

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Increasing BPA levels in urine associated with worsening male sexual function

Increasing BPA levels in urine associated with worsening male sexual function

Increasing Bisphenol-A levels in urine are associated with worsening male sexual function, according to a Kaiser Permanente study appearing online in the Journal of Andrology.

Increasing urine BPA level is associated with decreased sexual desire, more difficulty having an erection, lower ejaculation strength and lower level of overall satisfaction with sex life, researchers said. The five-year study examined 427 workers in factories in China, comparing workers in BPA manufacturing facilities with a control group of workers in factories where no BPA was present.

BPA is an ingredient in manufacturing polycarbonate plastic and epoxy resins and is now contained in a wide variety of consumer products from baby bottles, plastic containers, and the resin lining of cans for food and beverages, to dental sealants. People can be exposed to BPA by using BPA-containing products.

In a previous related study, Kaiser Permanente researchers measured BPA exposure based on work history and environmental BPA exposure in the workplace (http://xnet.kp.org/newscenter/pressreleases/nat/2009/111109bpa.html). This new study measured urine BPA among participants and examined the correlation between their urine BPA level and their reported problems of sexual dysfunction.

"This is the first human study to show that high urine BPA is associated with lower male sexual function," said study lead author De-Kun Li, MD, PhD, a reproductive and perinatal epidemiologist at Kaiser Permanente's Division of Research in Oakland, Calif. "Also, even among men exposed to BPA from only environmental sources (no occupational exposure and with average BPA level lower than the average observed in the American population), there were indications of an increased risk of sexual dysfunction."

He explained that although the estimates in the environmentally exposed group were not statistically significant due to small sample size, this finding may enhance the understanding of the BPA effect in human populations with low-dose environmental exposure and have important public health implications.

The researchers observed a dose-response association between increasing urine BPA level and declining male sexual function. The observed negative association was consistent across all categories measuring male sexual dysfunction.

This study is the second part of Kaiser Permanente's ongoing research to look at BPA's reproductive effect in humans. Both studies are the first to look at BPA's reproductive effect in humans. The adverse effect of BPA on the male reproductive system previously had been examined in animal studies only.