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 [NAS].

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:

CRaTER shows lighted materials may mitigate cosmic ray risks (June 12, 2013)

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

CRaTER on LRO shows lighter materials may better mitigate cosmic ray health risks

As CRaTER, flying with LRO, closes out a fourth year in lunar orbit, long duration exposure to cosmic rays while traveling within and beyond Earth's magnetic field shows materials lighter than traditional aluminum and titanium alloyed hulls may reduce the probability of Radiation Exposure Induced Death (REID) [NASA/GSFC/UHN/SwRI].

University of New Hampshire - Durham –- Space scientists from the University of New Hampshire (UNH) and the Southwest Research Institute (SwRI) report that data gathered by NASA’s Lunar Reconnaissance Orbiter (LRO) show lighter materials like plastics provide effective shielding against the radiation hazards faced by astronauts during extended space travel. The finding could help reduce health risks to humans on future missions into deep space.

Aluminum has always been the primary material in spacecraft construction, but it provides relatively little protection against high-energy cosmic rays and can add so much mass to spacecraft that they become cost-prohibitive to launch.

The scientists have published their findings online in the American Geophysical Union journal Space Weather. Titled “Measurements of Galactic Cosmic Ray Shielding with the CRaTER Instrument,” the work is based on observations made by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on board the LRO spacecraft. Lead author of the paper is Cary Zeitlin (zeitlin@boulder.swri.edu) of the SwRI Earth, Oceans, and Space Department at UNH. Co-author Nathan Schwadron of the UNH Institute for the Study of Earth, Oceans, and Space is the principal investigator for CRaTER.

“This is the first study using observations from space to confirm what has been thought for some time—that plastics and other lightweight materials are pound-for-pound more effective for shielding against cosmic radiation than aluminum," Zeitlin said. "Shielding can’t entirely solve the radiation exposure problem in deep space, but there are clear differences in effectiveness of different materials.”

The plastic-aluminum comparison was made in earlier ground-based tests using beams of heavy particles to simulate cosmic rays. “The shielding effectiveness of the plastic in space is very much in line with what we discovered from the beam experiments, so we’ve gained a lot of confidence in the conclusions we drew from that work,” says Zeitlin. “Anything with high hydrogen content, including water, would work well.”

The space-based results were a product of CRaTER’s ability to accurately gauge the radiation dose of cosmic rays after passing through a material known as “tissue-equivalent plastic,” which simulates human muscle tissue. 

Prior to CRaTER and recent measurements by the Radiation Assessment Detector (RAD) on the Mars rover Curiosity, the effects of thick shielding on cosmic rays had only been simulated in computer models and in particle accelerators, with little observational data from deep space.

The CRaTER observations have validated the models and the ground-based measurements, meaning that lightweight shielding materials could safely be used for long missions, provided their structural properties can be made adequate to withstand the rigors of spaceflight.

Since LRO’s launch in June 2009, the CRaTER instrument has been measuring energetic charged particles— often very heavy and spectacularly energetic particles traveling at nearly the speed of light and cause detrimental health effects—from galactic cosmic rays and solar particle events (SPE's). 

Fortunately, Earth’s thick atmosphere and strong magnetic field provide adequate shielding against these dangerous high-energy particles.

To view the Space Weather article (behind academic pay wall), visit http://onlinelibrary.wiley.com/doi/10.1002/swe.20043/abstract

For more on the CRaTER instrument, visit http://crater.sr.unh.edu/ and for the LRO mission visit http://lunar.gsfc.nasa.gov/mission.html.

CRaTER team at UNH

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)
LUNAR-TEX radiation blanket: Skeptical (May 11, 2009)
NASA cataract detection down to Earth (January 18, 2009)
NASA and Congress sacrifice radiation shielding flexibility
by 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

Cosmic ray threat to manned spaceflight tested on MSL

The MSL cruise phase as unmanned proxy for Orion, testing the deep space radiation environment [NASA].

Employing present, proven technology manned space travel to Mars exceeds NASA’s own limits on astronaut radiation exposure. That limit is calculated in terms of risk of “Radiation Exposure Induced Death,” or “REID,” over an individual astronaut’s life expectancy.

Ironically, as astronauts age their risk of eventually dying from causes unrelated to radiation exposure steadily increase. It’s the kind of risk coldly calculated by insurance providers. Though dying of undiagnosed heart disease is fed into the calculus, such other threats to the older astronaut's long-term survival overshadow their cumulative risk of REID.

None of this is news. This fly in the ointment in need of being overcome before humans can safely experience long-duration spaceflight beyond Earth’s magnetic field was starkly spelled out in the influential “ (2007),” a report put together by the National Academy of Science before the Constellation program was cancelled. The hard numbers have been gathered from the opening of the Space Age, from Explorer 1 through Apollo, from the Voyagers through the International Space Station.

Now these projections have been verified again by an instrument that traveled to Mars with Curiosity.

The lead investigators for these sensors announced their results during a NASA audio press conference Thursday. Dr. Cary Zeitlin, a principal scientist in the Southwest Research Institute’s (SwRI) Space Science and Engineering Division discussed detailed measurements of energetic and highly-ionizing particle radiation gathered during the 253 day, 560 million km journey to deliver the Mars Science Laboratory (MSL) “Curiosity” rover to the floor of Gail crater on Mars.

The 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.

NASA/JPL/SwRI

"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."

Titanium alloy in the hull of a manned spacecraft is a good shield
against most solar particle events, but counter-productive against
the heaviest cosmic rays. These heavy nucleons split and shower
damage into human tissue.

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.

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.

"Scientists need to validate theories and models with actual measurements, which RAD is now providing. These measurements will be used to better understand how radiation travels through deep space and how it is affected and changed by the spacecraft structure itself," says Donald M. Hassler, a program director at Southwest Research Institute and principal investigator of the RAD investigation. "The spacecraft protects somewhat against lower energy particles, but others can propagate through the structure unchanged or break down into secondary particles."

Only about 5 percent of the radiation dose was associated with solar particles, both because it was a relatively quiet period in the solar cycle and due to shielding provided by the spacecraft. Crew exposures during a human mission back and forth to Mars would depend on the habitat shielding and the unpredictable nature of large SEP events. Even so, the results are representative of a trip to Mars under conditions of low to moderate solar activity.

"This issue will have to be addressed, one way or another, before humans can go into deep space for months or years at a time," said Zeitlin.

SwRI, together with Christian Albrechts University in Kiel, Germany, built RAD with funding from the NASA Human Exploration and Operations Mission Directorate and Germany's national aerospace research center, DLR.

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)
LUNAR-TEX radiation blanket: Skeptical (May 11, 2009)
NASA cataract detection down to Earth (January 18, 2009)
NASA and Congress sacrifice radiation shielding flexibility
by 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

Scientific Context for the Exploration of the Moon (2007)
Space Studies Board
National Research Council

Cosmic ray flux effects lunar ice

Space scientists from the University of New Hampshire and colleagues report they have quantified levels of radiation on the Moon's surface from galactic cosmic ray (GCR) bombardment that over time causes chemical changes in water ice and can create complex carbon chains similar to those that help form the foundations of life.

The radiation process causes the lunar regolith to optically mature (OMAT), or darken, over time; important in understanding the geologic history of the Moon.

Scientists present their findings online in the American Geophysical Union's Journal of Geophysical Research. "Lunar Radiation Environment and Space Weathering from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER)," is based on measurements made by the CRaTER instrument on-board NASA's Lunar Reconnaissance Orbiter (LRO). 

The paper's lead author is Nathan Schwadron, an associate professor of physics at the UNH Space Science Center within the Institute for the Study of Earth, Oceans, and Space (EOS). Co-author Harlan Spence is the director of EOS and lead scientist for the CRaTER instrument.

The telescope provides the fundamental measurements needed to test our understanding of the lunar radiation environment and shows that "space weathering" of the lunar surface by energetic radiation is an important agent for chemical alteration. CRaTER measures material interactions of GCRs and solar energetic particles (SEPs), both of which present formidable hazards for human exploration and spacecraft operations. CRaTER characterizes the global lunar radiation environment and its biological impacts by measuring radiation behind a "human tissue-equivalent" plastic.

Serendipitously, the LRO mission made measurements during a period when GCR fluxes remained at the highest levels ever observed in the space age due to the Sun's abnormally extended quiet cycle. During this quiescent period, the diminished power, pressure, flux and magnetic flux of the solar wind allowed GCRs and SEPs to more readily interact with objects they encountered -- particularly bodies such as our Moon, which has no atmosphere to shield the blow.

The arrival of cosmic rays at Earth, far more of a threat to survival than solar radiation, more than doubles on average when the Sun is relatively quiet. The peak in neutron flux since 1958 (at right) occurred during the unusually long solar minima 2009-2010, "serendipitously" coincident to the beginning of LRO's mission and the CRaTER instrument on-board [Moscow Neutron Monitor].

"This has provided us with a unique opportunity because we've never made these types of measurements before over an extended period of time, which means we've never been able to validate our models," notes Schwadron. "Now we can put this whole modeling field on more solid footing and project GCR dose rates from the present period back through time when different interplanetary conditions prevailed." This projection will provide a clearer picture of the effects of GCRs on airless bodies through the history of the solar system.

Moreover, CRaTER's recent findings also provide further insight into radiation as a double-edge sword. That is, while cosmic radiation does pose risks to astronauts and even spacecraft, it may have been a fundamental agent of change on celestial bodies by irradiating water ice and causing chemical alterations. Specifically, the process releases oxygen atoms from water ice, which are then free to bind with carbon to form large molecules that are "prebiotic" organic molecules.

In addition to being able to accurately gauge the radiation environment of the past, the now more robust models can also be used more effectively to predict potential radiation hazards spawned by GCRs and SEPs.

Says Schwadron, "Our validated models will be able to answer the question of how hazardous the space environment is and could be during these high-energy radiation events, and the ability to do this is absolutely necessary for any manned space exploration beyond low-Earth orbit."

Indeed, current models were in agreement with radiation dose rates measured by CRaTER, which together demonstrates the accuracy of the Earth-Moon-Mars Radiation Environment Module (EMMREM) being developed at UNH. EMMREM integrates a variety of models describing radiation effects in the Earth-Moon-Mars and interplanetary space environments and has now been validated to show its suitability for real-time space weather prediction.

NASA to irradiate squirrel monkeys to research long-term exposure in Deep Space

Tom Chivers
Telegraph.UK

"There's a long-standing commitment on the part of NASA to deep space travel and with that commitment comes a need for knowing what kinds of adverse effects deep space travel might have, what are the risks to astronauts. That's not been well assessed.

"The beauty of this is that we can assess at different time points after exposure, so not only do we get a sense of rather immediate effects, but then we can look again at longer time points.

"That kind of information just hasn't been available."

Read the story, HERE.

Where will the Sun's magnetic field hit bottom?

At the moment the Sun's magnetic field, "the interplanetary magnetic field," is bottoming out with a long solar minimum, a low point in solar activity between the slow dying of Cycle 23 and the unexpectedly slow start to Cycle 24.

The Sun's relative quiet presents an opportunity to further determine any natural floor, what the "absolute" bottom background might be, to the interplanetary magnetic field, without the background noise of its eleven-year swing between often dramatic and chaotic peak activity.

This may not seem as immediately important to the earthbound as the health of Earth's magnetic field (though Earth's magnetic field's shielding energy is, in some measure, determined by our orbital vector perpendicular through the Sun's particle streams and magnetic field lines) but for machines and humans traveling outside Earth's magnetic field an improved understanding if the interplanetary magnetic field is essential.

Between its peak strength at solar max and its weakest at solar minimum , the in-fall of sometimes very heavy and energetic cosmic rays varies by half. The stronger the interplanetary magnetic field the greater the protection against interstellar cosmic rays, and it is literally a toss up if travel beyond Low Earth Orbit is "safer" for humans and their equipment at solar max or solar minimum.

At solar max the interplanetary magnetic field is strongest, providing a statistically important shield against Galactic Cosmic Rays, but it also increases likelihood travelers will encounter a dangerous "Solar Particle Event."

Solar Flares and attendant Coronal Mass Ejections can, of course, occur at anytime,though they are more likely at solar max. Even during the present protracted solar minimum, flares have been observed, along with coronal holes allowing the hot breath of solar wind, consisting mostly of protons, to gust away from the Sun's photosphere.

It is within today's design and materials technologies and spacecraft design to greatly shield life and equipment from most solar wind. Though rarer, a heavy cosmic ray consisting of a stripped nucleon of primordal metal and traveling near the speed of light might be shattered by its encounter with an aluminum-titanium hull of sufficient thickness but a resulting shower of secondary particles actually would increase the likelihood that an astronaut would receive a wider cone of ionizing radiation.

Generating a magnetic field has been suggested as a way to change the direction of infalling cosmic rays. But the effective size and strength of such a field's strength would need to be more than hundreds of kilometers in radius, to name just one known issue with this solution.

The fact remains that reducing the probability of radiation exposure induced death to below 4 percent, over an individual astronaut's lifetime, for a trip to Mars using presently available speeds is still beyond our capability. Though this will change, reducing the likelihood of being dosed by a wide range of cosmic rays by 50 percent, possible within the interplanetary magnetic field at solar max, is not a small factor in mitigating risk in long periods traveling in deep space.

Living on the Moon immediately provides a shield from half the infall of interstellar radiation, and under 15 meters of lunar regolith doses come down to levels on Earth at sea level.

It may be possible, where it would be unthinkable on Earth, to build interplanetary transport vessels shielded by lunar concrete and propelled on trips to Mars and elsewhere using rail guns and unimpeded by atmospheric drag.

Of a more basic concern to engineers and policy makers, it may prove very unwise to travel to Mars without first building an infrastructure on Earth's Moon. This is especially likely if the Sun's present solar minimum persists.

It may be that the downslope from solar max may be driven by CMEs, carrying away kinks of the Sun's internally twisted magnetic field, up and away from the Sun and temporarily reducing the interplanetary magnetic field by as much as ten percent. A CME may be the way the Sun balances out the tensions in its magnetic field that wind up as portions of the Sun rotate at different speeds.

Because the Sun's magnetic field swaps out polarity between its hemispheres from one cycle to the next, the interplanetary magnetic field changes polarity.

On first glance it may seem finding an absolute bottom, a basic and unchanging level to the interplanetary magnetic field, would be impossible. Scientists at the Russian Academy of Sciences, however, have published an examination of the interplanetary magnetic field between 1976 and 2000. They claim there is no point when the interplanetary magnetic field reaches "zero," and this is backed by observations of the Sun over this past year's lengthy solar minimum.

In 2008-2009, the sun has produced brief outcroppings of sunspots at polarities and latitutdes that clearly mark them as a beginning to the next solar maximum, with its next expected peak in activity now expected in 2013. Over the past summer however, months after Cycle 24 officially started, sunspots from Cycle 23 briefly appeared.

In The floor in the interplanetary magnetic field (Yermolaev, et. al. 2009) predicts a floor to the interplanetary magnetic field at 4.65 ± 6.0 nT.

Today's measurement of the interplanetary magnetic field on Spaceweather.com was 4.1 nT, and their report also "agrees well" with observations over the past thirty years.