Even before its landing, NASA's Mars Science Laboratory has sent back its first scientific results — shedding light on the radiation exposure that astronauts would face during a future mission to the Red Planet.

The good news is that the spacecraft's protective shell, which is similar to the shielding being developed for NASA's Orion deep-space capsule, reduces the exposure inside the spacecraft to about a hundredth of what it was outside the spacecraft. That's according to Don Hassler, the principal investigator for the Radiation Assessment Detector that's installed on the MSL Curiosity rover.

The bad news is that the internal exposure levels measured during MSL's 8-month-plus cruise to Mars would still constitute "a significant contribution to an astronaut's career limit," Hassler said.

Hassler discussed the findings in general terms today during a pre-landing briefing conducted at NASA's Jet Propulsion Laboratory in Pasadena, Calif. The Mars Science Laboratory is due to plunge through Mars' atmosphere and lower Curiosity to the surface at 10:31 p.m. PT Sunday (1:31 a.m. ET Monday).

Mission managers said the probe was almost precisely on target for the landing in Gale Crater, within sight of a mountain that towers 3 miles (5 kilometers) above the crater floor. "I just can't wait," said Michael Meyer, the lead scientist at NASA Headquarters for the Mars exploration program.

Scientists say the layers that make up the mountain, known as Aeolis Mons or Mount Sharp, record billions of years of geological history — including the era when Mars was warmer, wetter and more conducive to life. The Curiosity rover's main task is to seek out the chemical evidence that could tell scientists whether Mars was ever potentially habitable. "What you've got here is really kind of a geologist's paradise," said Caltech geologist John Grotzinger, project scientist for Curiosity.

Preparing the way for humans
Although the habitability question is the $2.5 billion mission's top issue, Mars Science Laboratory will also gather data to help NASA prepare for eventual human missions to Mars. The early readings from the Radiation Assessment Detector, known as RAD, are part of that investigation.

NASA

An artist's conception shows the Mars Science Laboratory during its cruise phase, heading from Earth toward Mars. The spacecraft's protective shell is similar to the hull of a crewed spaceship.

RAD's readings are still being analyzed in advance of submission to a scientific journal, Hassler told me, but the exposure equaled "a few tens of percent" of NASA's career limit. And that's just for a one-way trip. Astronauts would face additional exposure during their work on Mars and on the return trip.

NASA's career limit for radiation exposure is in the range of 1 to 4 sieverts, depending on the age and gender of the astronaut. The space agency associates that level with a 3 percent excess risk of cancer. 

Space radiation has long been recognized as one of the biggest space hazards humans would face on long-duration trips beyond Earth's protective magnetic field, but the RAD results are expected to represent the best effort to quantify that hazard for Mars trips. RAD will continue to monitor radiation exposure on the Martian surface throughout Curiosity's two-year primary mission.

A manned mission to Mars and back would take two to three years, based on the scenarios currently studied by NASA. The space agency's exploration roadmap calls for astronauts to journey to Mars and its moons starting in the 2030s.

"Our observations are already being used in planning for human missions," Hassler, a researcher from the Southwest Research Institute in Boulder, Colo., told journalists.

Solar storms recorded
Most of the exposure recorded by RAD came in the form of five spikes in solar radiation levels during the cruise phase, including a monster spike in March. To determine how much protection the MSL spacecraft's outer shell provided, Hassler and his colleagues compared the internal readings from RAD with levels in the space environment that were recorded by a satellite known as the Advanced Composition Explorer. The researchers found that the highest peaks of solar activity were reduced by two orders of magnitude, or roughly a factor of 100.

"The lesson here is that these events happen very quickly, and they can be very intense, but with proper shielding we can help prepare for future human missions to Mars," Hassler said.

Experts are looking into potential ways to beef up the radiation shielding for deep-space journeys — for example, by distributing water and other supplies in a particular way around the outside of the capsule, or by creating a more heavily shielded "safe haven" for use during solar storms. In addition to solar storms, galactic cosmic rays contribute to radiation exposure levels in space.

After the landing, RAD will be able to compare the space exposure levels to the exposure levels on Mars itself. The Martian surface is thought to be pummeled by radiation to a far greater degree than Earth, primarily due to the fact that the Red Planet has a much thinner atmosphere and no global magnetic shield to divert electrically charged particles. Hassler said the RAD experiment could lead to much better models to predict the effects of space radiation on future deep-space astronauts.

More about Curiosity and space radiation:

• What will we see from Mars, and when will we see it?
• Even ordinary microbes may survive Martian radiation
• How a long mission to Mars could kill you
• Is there a virtual Mars in our future?

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Athit Perawongmetha / Getty Images Contributor

A man undergoes a radiation test at a screening center in Kiriyama in Japan's Fukushima Prefecture, Japan.

Radiation experts are painting a sobering picture of the Fukushima nuclear disaster's long-term impact on Japan in a series of reports published today by the journal Nature. At best, the country faces more than a decade of expensive cleanup, including the decommissioning of the reactor complex and the disposal of contaminated debris. At worst, wide areas of land around the complex will have to be abandoned, as they were in Ukraine after Chernobyl.

"On the basis of the Fukushima data so far, it seems likely that in some areas, food restrictions could hold for decades, particularly for wild foodstuffs such as mushrooms, berries and freshwater fish," the University of Portsmouth's Jim Smith, co-editor and lead author of "Chernobyl: Catastrophe and Consequences," wrote in a Nature commentary.

Smith says the levels of radioactive cesium-137, with a radioactive half-life of 30 years, "will determine the long-term impact on the contaminated region and its residents."

"The extent of cesium-137 contamination at Fukushima is not yet clear, but available data indicate very high levels in some areas," he wrote. Last week, the International Atomic Energy Agency sounded the alarm about high radiation readings in the village of Iitate, 25 miles (40 kilometers) northwest of the Fukushima Dai-ichi complex. The readings ranged as high as 3.7 megabecquerels per square meter. Such readings led the IAEA to suggest an expansion of the current 12-mile-radius (20-kilometer) evacuation zone.

Since then, the reported readings outside the evacuation zone have not been as high. But Smith said that if large areas are contaminated with 0.5 megabecquerels per square meter or more, "evacuation could be for the long term."

One long-term strategy could be to bring in "liquidators" to decontaminate the towns and villages, remove topsoil and resurface roads, "although this approach met with varying success at Chernobyl," Smith wrote.

He said "the long-term response to Fukushima will have to be pragmatic." Radiation exposure limits for the general public might have to be relaxed, for example, going from 1 millisievert per year to 5 to 10 millisieverts per year. Smith noted that millions of people living in areas of high natural radioactivity are exposed to more than 10 millisieverts per year.

"A turning point in my understanding of Chernobyl's impacts came while studying lakes in Belarus during the mid-1990s," Smith wrote. "In an evacuated area, lake fish contained tens of thousands of becquerrels per kilogram. A couple in their early 70s lived near the lake, eating the fish and growing vegetables. They were living off contaminated land, but leading the life they had chosen to lead. This wouldn't by any means be the right choice for everybody, but I am convinced they had made the right decision for them: They were Chernobyl survivors, not victims."

Other reports in Nature's roundup hint at the uncertainties still hanging in the air three and a half weeks after the earthquake and tsunami that led to the Fukushima crisis:

• David Brenner of Columbia University's Center for Radiological Research explains why experts "really don't know" that much about the long-term health consequences of Fukushima's radiation releases. He said authorities should start assessing "whether it would be reasonable to undertake large-scale population studies among the exposed populations in Japan." Meanwhile, researchers should focus more study on the basic mechanisms by which low doses of radiation cause cancer.
• The Japan-based Radiation Effects Research Foundation is calling on authorities to start collecting baseline data for a study of Fukushima's effects as soon as possible. Some of the basic measurements are already being collected, but the effort is "scattered and uncoordinated," researchers at the foundation say. In a separate report, Nature says researchers "are finding that making any sense of the data is proving very difficult."
• Japanese authorities have been monitoring the effects of radioactive iodine-131 on the thyroids of children in the most contaminated areas around Fukushima, and Nature says the first results show minimal thyroid doses in 946 children living in the areas northwest of the plant. The results "seem reassuring that not much iodine-131 has got into children," Richard Wakeford, an epidemiologist at the University of Manchester's Dalton Nuclear Institute, told the journal.

Are you reassured? Feel free to weigh in with your own thoughts about the long-term impact of the Fukushima crisis.

More about Japan's nuclear crisis:

• Radiation in ocean could hurt Japan fish industry
• Japan nuke firm offers 'condolence money'
• Cosmic Log archive on Japan's disaster
• Full coverage of the disaster on msnbc.com 

Nature is presenting a live Q&A with the University of Portsmouth's Jim Smith and Nature's Geoff Brumfiel at 11 a.m. ET (4 p.m. London time) on Wednesday.

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about my book on Pluto and the search for planets, check out the website for "The Case for Pluto." 

Sukree Sukplang / Reuters

An official from Thailand's Food and Drug Administration takes a sample from a shipment of frozen fish imported from Japan to test for possible radiation contamination at a customs station in Bangkok today.

Experts say that fish and other marine species shouldn't be as affected by Japan's nuclear crisis as species on land, in part because of differences in the ways radiation is dispersed.

But that doesn't mean authorities can ease up on monitoring the sea and its bounty for contamination. To the contrary: Inspectors around the world are keeping a close eye on food imports from Japan, and some countries have ordered special inspections or even outright bans on fish coming from areas near the plant.

Twenty days after Japan's earthquake and tsunami touched off a breakdown and partial meltdown at the Fukushima Dai-ichi nuclear complex, some radiation experts are still struggling to get an accurate read on the situation.

"My basic feeling is that they're going to come to grips with this, and at the end of the day, it's not going to be as bad as people fear," said Florida State University oceanographer William Burnett, an expert on the environmental effects of radioactivity. "Having said that, trying to follow this story has been difficult. I see almost no real data."

The most reliable measurements have been coming from the International Atomic Energy's daily updates on the situation, said Andrew Maidment, a professor of radiology and chief of the physics section at the Hospital of the University of Pennsylvania. So get ready for some real data.

The latest fish radiation readings are above background levels, but still nowhere near the safety limits. The highest radiation reading for fish from the Japanese port of Choshi was 3 becquerels per kilogram of cesium-137 — far below Japan's limit of 500 becquerels per kilogram, or Bq/kg.

"This confirms what scientists including myself have been saying: First of all, the water will dilute this, and the uptake will therefore be lower than it would be for a terrestrial animal," Maidment told me. "The greater the volume of water, the higher the dilution, and the lower the impact."

When radioactive fallout is dispersed on land, it collects on what is essentially a two-dimensional carpet of vegetation, to be ingested later by livestock or humans. But when the fallout reaches the sea, it's dispersed in a much deeper three-dimensional space.

Maidment said this phenomenon was seen clearly in the wake of the 1986 Chernobyl disaster, which still ranks as more of an environmental catastrophe than Fukushima: Wild boars that were tested within a 30-kilometer radius of the Chernobyl site registered 470,000 Bq/kg of cesium-137, while freshwater perch showed concentrations of 4,000 Bq/kg. Eight years later, the levels were reduced to 5,000 Bq/kg for the boars, and 200 Bq/kg for the perch. Even those levels are unpalatably high, but they illustrate Maidment's main points: Marine life tends to absorb less radiation, and contamination levels go down over time.

Yardsticks for radiation
Let's take a moment to talk about the radiation standards: When we're talking about the absorbed dose for humans, that tends to be expressed in terms of millisieverts. For example, the typical annual radiation dose from natural sources amounts to roughly 3 millisieverts.

But when we're talking about the radioactivity contained in various substances, the standard measure is becquerels per kilogram or per liter. The safety standards vary according to what type of radioisotope we're talking about, the type of substance we're talking about, and the type of person who might come in contact with that substance. That's because there's a wide range of variation in the uptake of radioisotopes and their effects on the body.

Here are the safety limits set by Japan's Food Safety Commission and reported by Bloomberg:

• Iodine-131: 300 Bq/kg for drinking water, milk and dairy products. 2,000 Bq/kg for vegetables except for root vegetables and tubers.
• Radioactive cesium: 200 Bq/kg for drinking water, milk and dairy products. 500 Bq/kg for vegetables, grains, meat, eggs and fish.
• Uranium: 20 Bq/kg for infant foods, drinking water, milk and dairy products, 100 Bq/kg for vegetables, grains, meat, eggs and fish.
• Alpha-emitting nuclides of plutonium and transuranic elements: 1 Bq/kg for infant foods, drinking water, milk and dairy products. 10 Bq/kg for vegetables, grains, meat, eggs and fish.

Materials exceeding 100 Bq/kg should not be used as the basis for powdered infant milk formula.

Close to the Fukushima plant, the radiation levels are alarming: The IAEA said samples of seawater collected 330 meters east of the nuclear complex's discharge point showed iodine-131 concentrations of 74,000 becquerels per liter, roughly equivalent to Bq/kg. The cesium levels were 12,000 Bq/kg for cesium-137 and another 12,000 Bq/kg for cesium-134. But those levels drop sharply with distance, due to the dispersion factor.

Radioactive iodine-131 is a huge concern for people living around the Fukushima plant, because that can be quickly taken up into the thyroid and pose a significant cancer risk. What's more, iodine is taken up readily by seaweed, which is a popular food item in Japan. Elevated (but still safe) levels of radioactive iodine have been detected in seaweed as far away as Vancouver, and in milk samples from Spokane, Wash. (The reading for the milk was 0.8 picocuries per liter, or roughly 0.03 Bq/kg.)

The flip side is that iodine-131 has a relatively short half-life of eight days, so as time passes, the iodine risk should drop significantly for fish as well as for people.

Cesium-137 has a longer half-life (30 years), so it poses a longer-lived threat. The fish tests suggest that the cesium radiation levels are just one-tenth of the iodine levels, Maidment said.

Stay focused on the fish
Authorities will have to be extra-vigilant about watching radiation levels from Fukushima for a long time — on the ground, in the air and at sea, said Edwin Lyman, a senior staff scientist at the Union of Concerned Scientists, an activist group.

"Even dilute levels of contamination can be enhanced in certain marine life, you know, just like mercury concentrates in large fish, like tuna," he told reporters during a briefing on Monday. "Also, plants like seaweed are known to concentrate certain isotopes, and so are certain types of shellfish. But I would think certainly in the fishing industry in the region, they're most likely going to have to take measures to inspect their catches, and I guess the primary responsibility for that will have to be with the Japanese to inspect and interdict any contaminated seafood."

Maidment agrees that more monitoring will be needed. He also suspects that shellfish living on the seabed around Fukushima might face more contamination than the fish that just happen to migrate through the seas near the stricken plant. But he says "it's too early to draw conclusions," and he emphasizes that the general public needs to put the radiation issue in perspective.

For example, suppose that your drinking water contained 100 becquerels per liter, which is basically 100 Bq/kg. "If that water constituted 10 percent of your dietary intake of food, by weight, and you consumed that exclusively per year, you would increase your background radiation by about 20 percent," Maidment said. "I can double my background radiation just by moving from Philadelphia to Denver. So these are levels of radiation that most of us are not aware of."

I'm betting that you're more aware of the radiation issue than you were 20 days ago, and that you have some thoughts you'd like to share. Feel free to weigh in with your comments below.

Update for 12:55 a.m. ET March 31: One of my Facebook friends, Lynda Williams (the Physics Chanteuse), points out that I have not actually defined what a becquerel is. One becquerel is a unit of radioactivity that's equal to one nuclear decay per second. Here's the way she put it: "It is one atom of a radioactive isotope decaying and emitting radioactive decay. So 5,000 Bq/kg means that 10,000 atoms are decaying per second and shooting off a particle per second in one kilogram. So if the pig has a mass of 100 kilograms, we are talking about a whole body exposure of — do the math, 500,000 particles shooting through its body every second. The 'data' means more if you explain what it really means." Thanks, Lynda!

More angles on radiation:

• Seattle physicists detect radioactivity, fear it'll wreck experiment
• Japanese evacuees' new woe: Radiation prejudice
• World Blog: Tokyo fishmonger fears more radiation leaks
• Cosmic Log recap on the disaster in Japan
• Msnbc.com's special report on the disaster

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about my book on Pluto and the search for planets, check out the website for "The Case for Pluto." 

NHK via Reuters

Smoke rises from the nuclear reactors of Japan's Fukushima Dai-ichi nuclear power plant in an NHK video image from Thursday.

The battle to stabilize Japan's stricken Fukushima Dai-ichi nuclear complex just got tougher, due to the radioactive water that is apparently leaking from the reactors. The leak doesn’t change the battle strategy — to get water into the reactor buildings to stabilize "hot" fuel rods — but it does raise more uncertainties about how to get that done.

"The operators are having to do a lot of improvisation to figure out what best to do to keep the amount of radiation being released into the atmosphere to a minimum," nuclear engineering expert Elmer Lewis, a professor emeritus at Northwestern University, told me today.

The uncertainties currently focus on the leak, which exposed three workers wading through the water to so much radioactivity that they had to be hospitalized for radiation burns.

Some reports suggested that the skin radiation exposure amounted to 2,000 to 6,000 millisieverts. Exposure to that much full-body gamma radiation over the course of an hour would be deadly. However, in this case the burns were due to shorter-range beta radiation. The gamma radiation exposure was estimated at 170 to 180 millisieverts.

So where is that water coming from? "The data we're seeing is contradictory," said former nuclear engineer David Lochbaum, director of the Union of Concerned Scientists' nuclear safety project. It could be coming from the spent fuel rods stored near Fukushima Dai-ichi's Unit 3 reactor, or from the reactor vessel itself. The workers who came in contact with the water were working in the turbine room connected to Unit 3.

NBC News' Robert Bazell quoted outside nuclear engineers as saying that the water contained radioactive iodine-131, which could come only from the rods in the reactor's pressure vessel. That led some experts to say that the vessel was breached, but Lochbaum said that didn't mesh with the pressure readings being taken inside and outside the vessel.

"Either the pressure data we're seeing is inaccurate, or the breach isn't as serious" as some have claimed, Lochbaum told me.

If the water is leaking from the reactor vessel, it should still have been contained within the primary containment chamber that surrounds the vessel. However, there might be some sort of leak in the plumbing between the vessel and the Unit 3 turbine room. That's the scenario favored by Tom Crimmins, president at Executive & Nuclear Consulting. "There's a larger-than-expected leak somewhere in the system that's releasing this radioactive water," he said on MSNBC.

Whatever the cause, the leak is bad news for workers at the nuclear complex, "primarily because it makes working conditions there more difficult or potentially impossible," Princeton physicist Frank von Hippel said on MSNBC.

James Acton, a nuclear expert at the Carnegie Endowment for International Peace, said workers might have to pull back from the turbine rooms — not only at Unit 3, but at Units 1 and 2 as well, where elevated radiation levels have reportedly been detected. "Work in those areas is clearly going to be massively hampered, if not stopped entirely," he said, "but if radiation levels on the site as a whole don't rise, then work across the site can presumably continue."

Unit 3 is of particular concern, because that's the only reactor at the Fukushima site that uses a mixed uranium-plutonium fuel, also known as a mixed-oxide fuel or "MOX." But Lewis said some commentators are making too much of that distinction.

"Whenever you're burning uranium, you're always producing some plutonium," Lewis told me. "It's a matter of degree. The plutonium, like the uranium itself, is a ceramic element, and it's not very volatile at all. So I doubt seriously whether the water in the turbine room contains much if any plutonium. It tends not to be released except at very high temperatures. I think they're seeing fission-product radiation." (That is, radioactivity from elements such as iodine or cesium.)

Virtually all the experts are dismayed that the plant still hasn't been brought under control. "There's still too much energy coming out of that fuel to walk away," von Hippel said. "They still have to keep trying to cool it. The problem is that we're now two weeks after the accident started, and they don't have a handle on the situation yet."

The strategy remains the same as it has been for the past week: Make sure the fuel rods in the reactors as well as in the spent-fuel pools are covered with water and wait for the radioactivity in those rods to cool down.

Because Fukushima lacks electrical power to get water circulating through the reactors and the pools, workers have had to pump seawater into the works — and that could be creating a fresh round of problems, including corrosion in the plumbing and releases of radioactive steam. The prime objective right now is to get the standard water-cooling system working again.

"My own criterion for when I'll breathe easier is when they don't need seawater any more," Lewis said. So that means workers — or perhaps robots — will have to find a way to continue with the job of spraying water on the reactors, hooking up electrical power and regaining control of the nuclear battlefield.

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto."

Michael Penn / Juneau Empire / AP

Gus van Vliet of the Air Quality Division of the Alaska Department of Environmental Conservation works on a radiation detection monitor that is on the roof of the Floyd Dryden Middle School in Juneau, Alaska.

Last updated 9:30 p.m. ET:

Americans are being exposed to almost twice as much radiation as they used to get — but not because of fallout from nuclear accidents in Japan or elsewhere. Medical tests, not nuclear accidents, account for the dramatic rise in our radiation exposure. Based on today's readings, the radiation coming from the troubled Fukushima Dai-ichi nuclear complex rates barely a blip.

The Environmental Protection Agency said the readings from its nationwide network of atmospheric air-sniffing sensors showed "typical fluctuations in background radiation levels" that were "far below levels of concern." (You can check the updates on this Web page.) The initial U.N. radiation counts from California were "about a billion times beneath levels that would be health-threatening," one diplomat told The Associated Press.

In a later statement, the EPA and the U.S. Department of Energy said none of their detectors picked up "any radiation levels of concern."

The agencies also provided more details on the U.N. count, which was detected by a radiation-sniffing station in Sacramento and fed into the Comprehensive Nuclear Test Ban Treaty Organization's monitoring network. For the geeks out there, the level was 0.0002 disintegrations per second per cubic meter of air, with radioactive isotopes of iodine, tellurium and cesium represented in the mix.

A similar reading was reported by the Department of Energy in Washington state: 0.1 disintegrations per second per cubic meter of air, attributed to xenon-133.

"The doses received by people per day from natural sources of radiation — such as rocks, bricks, the sun and other background sources — are 100,000 times the dose rates from the particles and gas detected in California or Washington state," the agencies said in their joint statement.

That would imply that the fallout packed a punch on the order of 0.00003 millisieverts a year. In comparison, a dental X-ray amounts to 0.01 millisieverts, and a full-body CT scan can deliver 10 millisieverts. It used to be that our average exposure was 3 millisieverts a year from natural sources and 0.6 millisieverts from extra sources (such as X-rays). Today, the average is more like 3 plus 3 or more.

Researchers have found that increased cancer risk is associated with extra radiation exposures ranging from 10 to 100 millisieverts, depending on how spread out the doses are and who's conducting the study. When you're exposed to a 1,000-millisievert dose over a short period of time, you're likely to experience the symptoms of radiation sickness. And 10,000 millisieverts is lethal.

At one point this week, the readings at Fukushima rose to 400 millisieverts per hour. Two and a half hours of that would make you sick. A day would kill you.

All this makes 0.00003 millisieverts sound pretty puny. It's true that these are merely the first U.S. readings to be announced, and if significantly more radiation is released in Japan, those numbers might go up. But they won't go up by a factor of a million or a billion — which is why even those who have sounded grave warnings about the radiation threat say that U.S. residents needn't fear the winds coming from the west.

"I don't think the people in California need to be overly concerned with it, other than the fact that the people in Japan are facing disaster," said David Lochbaum, director of the Nuclear Safety Program at the Union of Concerned Scientists, a watchdog group.

For the Japanese, the situation could get more dire. Here are some of the factoids from the UCS briefing, as well as from my talks with other experts:

• If the nuclear fuel rods stored at the Fukushima complex were to break down and catch fire, it would take just hours for a cloud containing radioactive fallout to rise from the site, Lochbaum said. "Cesium would be the worst, but there's an awful lot of other radioisotopes that would follow along. You have krypton. There's just a whole litany of things that are in that spent fuel that are posing the risk," he said.
• Aerial readings suggest that the worst hazardous contamination has not spread beyond the 19-mile-radius (30-kilometer-radius) zone established by the Japanese government. A fuel-rod fire would spread significant fallout farther, but not all the way to America, said Edwin Lyman, a senior staff scientist at UCS. "It's still my judgment that most of the fallout would be within several hundred miles of the site," he said. "There would be hot spots, you know, potentially further away, like we did see in Chernobyl, but still, the dilution over the course of thousands of miles would be significant."
• Even with today's upgrade in Fukushima's danger level, Chernobyl still ranks as the world's worst nuclear accident. But in terms of its fallout effect for the United States, the nuclear weapons tests from a half-century ago loom even larger. "What most people don't realize is that only 6 percent of the cesium floating around out there is from Chernobyl," said Fred Mettler, a professor emeritus of radiology at the University of New Mexico who has studied the effects of the 1986 disaster. More than 90 percent of the cesium contamination has been traced to weapons testing, he said.

Update for 8:10 p.m. ET: Researchers from the University of Maryland are drawing up computerized projections showing how radiation from the Fukushima nuclear site should be transported through the atmosphere. Check out this Web page for the latest projections, and this news release for an explanation of the projections.

Update for 9:30 p.m. ET: The figures from the EPA and the Department of Energy have been updated to reflect the latest radiation readings.

More on the disaster in Japan:

• Robot scouts en route to Japan
• Radiation risk can be hard to assess
• Is it time for the Chernobyl option?
• U.S. military detects more radiation
• What is Japan doing to fix reactors?
• If there's a meltdown, then what?
• Q&A: Clearing up nuclear questions
• Cosmic Log archive on the Japan crisis
• Special report on the disaster in Japan

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto."

U.S. Air Force

The Air Force's WC-135W Constant Phoenix aircraft collects samples from the atmosphere for radiation analysis.

U.S. officials have told NBC News that they're seeing a disparity between Japanese radiation readings and the readings they've been getting from military monitors.

Concerns about the release of radiation from Japan's stricken nuclear plants at the Fukushima Dai-ichi complex began with data collection on the aircraft carrier USS Ronald Reagan. On Monday, the U.S. Seventh Fleet relocated its ships and aircraft out of the downwind direction after crew members returning to the carrier were found to have picked up low levels of radioactive contamination. The personnel were scrubbed down with soap and water, then declared contamination-free.

Since then, the data on radiation releases suggest a range of outcomes, going all the way up to "dire," the officials said. They spoke with NBC on condition of anonymity because they were not authorized to discuss the findings publicly.

NBC's sources said the Japan nuclear site and its surroundings are being monitored by a variety of U.S. aircraft, including:

• U-2 spy planes. The U-2s, flying out of Okinawa, have "radiation suites" that can take readings at various altitudes.
• Global Hawk drone.The Global Hawk remote-controlled plane, now on its second run, has multispectral imaging capabilities, including thermal infrared and synthetic aperture radar. Kyodo News Service quoted Japanese government sources as saying that the Global Hawk was taking images of the inside of the reactor buildings.
• WC-135 Constant Phoenix aircraft. One radiation-sniffing WC-135, basically a converted Boeing 707 jet, is on its way from Offutt Air Force Base in Nebraska to the area around Japan, where it will take atmospheric readings.

Intelligence experts also tell NBC News that the United States has a network of ground-level stations around the world that monitor radiation and can backtrack to calculate how much has been dispersed from a specific site.

Officials said several agencies are analyzing the data, including the Department of Energy's Nuclear National Security Administration and the CIA. 

Tip o' the Log to NBC News' Robert Windrem and Courtney Kube.

More on the radiation situation in Japan:

• U.S. boosts radiation-sniffing system
• What is Japan doing to fix reactors?
• If there's a meltdown, then what?
• Q&A: Clearing up nuclear questions
• Cosmic Log archive on the Japan crisis
• Special report on the disaster in Japan

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto."

Justin Sullivan / Getty Images

A RadNet radiation monitor is seen on the roof of the Bay Area Air Quality District offices in San Francisco.

Last updated 1:50 a.m. ET March 17:

Federal agencies are beefing up their radiation-monitoring capabilities at home and abroad, even as they insist that significant amounts of fallout won’t waft from Japan onto U.S. territory.

At home, the Environmental Protection Agency said it's adding seven monitors in Alaska, Hawaii and Guam to its RadNet radiation-tracking system, which operates about 100 air-sniffing stations nationwide. Putting in those extra stations "allows us to gather data from a position closer to Japan," EPA said in an online question-and-answer guide.

Looking beyond America's borders, the U.S. Air Force is sending out a high-tech aircraft to sniff the air over Japan for radiation. The Nuclear Regulatory Commission and the Department of Energy's National Nuclear Security Administration are also sending experts to Japan to help counter the growing crisis at Japan's Fukushima Dai-ichi nuclear plant complex.

The NRC and the NNSA have teams who track how hazardous materials spread through the atmosphere, based on computer modeling and other methods. It was the NRC's revised analysis that led to today's advisory telling Americans to evacuate the area within 50 miles (80 kilometers) of the Fukushima reactors.

White House spokesman Jay Carney acknowledged that the NRC's advice goes far beyond what the Japanese government is telling its own citizens — that is, for residents to evacuate the area within a 12-mile (20-kilometer) radius of the plant, and to take shelter if they're within 19 miles (30 kilometers).

"The advice the Japanese government is giving, based on information it has, is different from the advice that we would be giving if this incident were happening in the United States of America," Carney said. "It is not about the quality of information. It is about the standards set by the Nuclear Regulatory Commission here in the United States and the kind of advice it would be giving should this incident happen in the United States."

For what it's worth, the NRC calls for protective action when projected doses exceed 10 millisieverts (1 rem) or 50 millisieverts (5 rem) to the thyroid. Radiation levels at the damaged plants rose as high as 400 millisieverts per hour.

How the calculations are made
The NRC's analysts make detailed calculations to work out what the potential radiation exposure would be at various distances.

"Usually these calculations are very specific," NRC spokeswoman Viktoria Mitlyng told me. "You have to consider the particular radioisotope, and at what concentration it's going to occur, and what distance it is going to travel, and whether it's going to travel at all toward the United States. ... The farther away you are from the radiation source, the less impact it's going to have."

Commercial sales of Geiger counters are, um, hot in the United States — but EPA's RadNet provides a much more reliable read when it comes to detecting radioactive fallout if it ever comes across the Pacific. The radiation-monitoring network not only sniffs the air, but also samples drinking water, milk and precipitation. The first elements of the system were set up back in 1959, even before the EPA was created, to monitor U.S. military nuclear testing.

You can check the EPA's archived radiation readings for your own locale by clicking through an online database, or reviewing the quarterly data journals. By the way, radiation measurements for Japan are available via this Web page.

Pentagon watches radiation, too
The Department of Defense is keeping close tabs on radiation levels in the Fukushima area and beyond — not only because it has thousands of people working on the humanitarian relief effort, but also because of the potential risk to 50,000 military personnel in Japan and the impact on military installations in the Pacific.

Air-monitoring equipment on the aircraft carrier USS George Washington detected low levels of radioactivity while the ship was in port at Yokosuka in Japan, a military spokesman said Tuesday. On another carrier, the USS Ronald Reagan, 17 helicopter crew members had to be decontaminated with a soap-and-water scrubdown after returning from search-and-rescue duty. Potassium iodide pills, which can guard against the uptake of radioactive iodine, were issued to some of those crew members, the Defense Department said.

The radioactive plume from Fukushima's reactors can't be detected by satellites in orbit, but it can be tracked by the U.S. Air Force's Constant Phoenix WC-135 jets, which are designed to monitor airborne fallout from nuclear weapons tests. Constant Phoenix came into play after the 1986 Chernobyl nuclear disaster in Ukraine to sample the air over the Atlantic. "Most recently, WC-135 was used to detect seismic events associated with North Korea's claim of a nuclear test in October 2006 and again in May 2009," an Air Force spokesman, Maj. Chad Steffey, told me in an e-mail.

Steffey confirmed that a Constant Phoenix WC-135 would be sent to sample the air wafting from Japan, in response to a Japanese government request. The planes would be brought from Offutt Air Force Base in Nebraska. Steffey said he didn't yet have details about the timing of the operation. 

Speaking on condition of anonymity, a senior U.S. defense official told NBC News that Constant Phoenix's involvement was "absolutely" a significant event. "We are using it to help out a nation," the official said. "It's significant."

Extra credit:I sent the NRC's Viktoria Mitlyng some questions asking how the agency comes up with its projections for radiation exposure, and here are the answers she sent back:

Q: Are there computer models that are run to figure out how material is dispersed, or how specific radionuclides could affect residents at given distances?

A:Yes, the NRC uses a particular model for determining dispersal analysis for radionuclides from nuclear power plants.

Q: Given the distance from Japan to U.S. territories, is it a given that there will be no effect?

A:The NRC uses the limit of 1 rem [10-millisievert] dose limit to the whole body to recommend evacuation. It is highly unlikely that radiation can reach the U.S. from Japan and result in this type of exposure.

Q: What levels of emission would cause concern, based on what’s known about radioactive particulates and their dispersal?

A: The NRC recommends evacuation at 1 rem dose limit to the whole body. Models are run with varying sets of data and the results are analyzed to determine what kinds of response if any is warranted to protect public health.

Q: What sorts of resources and personnel are engaged in this sort of analysis?

A: Trained health physicists and other experts have been monitoring the situation in Japan at the NRC’s Headquarters Operations Center around the clock since the beginning of the crisis in Japan.

CTBTO via New York Times

This chart, attributed to the Comprehensive Test Ban Treaty Organization, shows the potential projected path of atmospheric plumes sent out from Japan's stricken Fukushima Dai-ichi nuclear plant. Click on the image for the full story from The New York Times.

Update for 1:50 a.m. ET March 17: The New York Times reports that the Comprehensive Test Ban Treaty Organization, a U.N. agency based in Vienna, has drawn up a simulation showing the progress of Fukushima's radioactive plume across the Pacific. Assuming that the plume began to rise on Saturday, and assuming that the radiation levels were detectable, the readings might be picked up in Alaska's Aleutian Islands today (Thursday) and in Southern California late Friday, the Times reported.

However, this projection is based merely on a reading of the weather patterns between Japan and the United States, and how those patterns might disperse material in the plume. Officials at the test ban agency made clear that this was not a prediction that radiation would be detected at any particular level. Rather, the projection was meant as guidance for atmospheric monitoring stations. Over the next few days, air-sniffing authorities should have a better fix on Fukushima's radioactive releases.   

More on the disaster in Japan:

• If there's a meltdown, then what?
• Q&A: Clearing up nuclear questions
• Cosmic Log archive on the Japan crisis
• Special report on the disaster in Japan

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