The biggest burst of gamma rays ever recorded was a mere flash in the pan by the time it reached Earth, but astronomers say even much less energetic explosions closer to home might be a different matter.

As scientific knowledge about this most energetic form of non-visible light grows, the phenomenon is starting to turn up in scenarios for mass extinction.

Researchers reported that the most powerful gamma ray burst yet recorded took place in a dusty galaxy some 12 billion light years away, with the energy bursting into our neighborhood in a brief flash just last December.

Little is known about the rays, in part because they're usually filtered out some 60 miles up in the atmosphere. That makes them only detectable on instruments carried higher than that by balloons and satellites.

Over the past several years, readings from those instruments show a steady, several-times a day bombardment of our planet by the rays, from all points in the universe and from both the far corners of the cosmos and nearby galaxies.

Only in the past year or so have new satellites and observation techniques allowed scientists to spot the exact locations in space of those gamma ray bursts. And the list of suspected causes now includes supernova explosions, pairs of neutron stars collapsing upon themselves, black holes consuming nearby stars, and starlike formations called quasars, each with different signature explosions.

But none of the bursts has yet been detected as coming from within our own Milky Way galaxy.

Even if the titanic explosion described by Columbia and California Institute of Technology scientists happened at the opposite edge of the Milky Way - some 60,000 light years away - "it would amaze people with a bright flash and maybe give them a bad sunburn, but I don't think it would wipe out life," said Stan Woosley, an astronomer at the University of California-Santa Cruz.

But the story might be different if even an average-size burst were to happen at say, 3,000 light years away, an astronomic hop-skip-and-a-jump from Earth, and a distance within which lie at least two known pairs of circling neutron stars.

According to calculations by a team of Israeli astronomers, a gamma ray burst within the Milky Way is likely to happen every 2 or 3 million years, with one within the 3,000 light year range coming about every 100 million years.

Scientists Peter Leonard and Jerry Bonnell, working on NASA's Compton Gamma Ray Observatory Project, laid out a gamma-ray burst of doom script in a recent issue of Sky and Telescope magazine. They predicted a blue flash followed shortly by a dark cloud of smog as the atmosphere's nitrogen and oxygen turned to nitric oxides, wiping out the protective ozone layer across half the world.

That would leave Earth rotating on its axis "as a chicken roasting on a spit" to endure a bath of deadly cosmic radiation that would wipe out all but the most sheltered or hardy forms of life.

"The good news is that we'd have this shower of nitrous oxide (laughing gas) coming down from the sky - we'd all die laughing," said Cal Tech researcher Shrinivas Kulkarni of that prospect.

Kenneth Brecher, an astronomer at Boston University, incorporates both gamma rays and comet impacts in his extinction theory, presented in January before the American Astronomical Society.

"The evidence of an impact in the Yucatan pretty much makes an asteroid-Earth impact the leading contender for the death of the dinosaurs," Brecher said. "But what set up the collision?"

He suggests that a nearby gamma ray burst may have jolted a host of relatively small comets out of the Oort cloud, a sort of comet nursery off in the nether regions of our solar system. "If comets are big snowballs, and you have this energy melting off a layer, it could give quite a nudge toward Earth," he said.

Brecher said that could have set up a peppering of Earth by space debris that might have gone on for many thousands or even millions of years, creating a changed climate that pushed the dinosaurs near the edge of extinction long before the final collision that apparently wiped them and 70 percent of all species out.

Two other scientists, working along similar lines, but without the gamma ray starter, report in the journal Science Friday that Earth may have passed through a cloud of space dust that could have started climate change up to a million years before the dinosaur-killing asteroid collision.

Stanley Dermott of the University of Florida and Stephen Kortentamp of the Carnegie Institute of Washington point out that millions of pounds of space dust accumulate in our sphere every year but that the amount reaching Earth seems to vary by 2 or 3 percent depending on how far our orbit takes us away from the sun.

And just a little more dust in the sky, as has been seen after volcanic eruptions, can block enough sunlight to cool the whole planet by a degree or two. But unlike volcanoes, the extra load of space dust might persist not for a year or two but for tens or hundreds of thousands of years.

According to their calculations, the orbital cycle through the dust bin is roughly every 100,000 years, which also lines up with the glacial free-thaw cycle over the past few million years, although many climate experts still believe that distance from the sun or changes in the tilt of the Earth toward the sun are responsible for ice ages.

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Additional Information

What are gamma rays?

Gamma rays are a form of light. All light travels in waves and is classified according to its wavelength, the distance between its waves. The universe produces a broad range of light, only a fraction of which is visible to our eyes. Other types of non-visible light include x-rays, ultraviolet light, infrared radiation and radio waves. Gamma rays are the most energetic.

Gamma Rays occupy the short-wavelength end of the spectrum; they can have wavelengths smaller than the nucleus of an atom.

Visible light waves are one-thousandth the width of a human hair - about a million times stronger than gamma rays.

Raiod waves, at the long-wavelength end of the spectrum, can be many meters long.

How are Gamma rays detected?

Gamma rays travel across vast distances of the universe, only to be absorbed by the Earth's atmosphere. Different wavelengths of light penetrate the Earth's atmosphere to different depths. Instruments aboard high-altitude balloons and satellites provide the only view of the gamma-ray sky.

Source: National Aeronautics and Space Administration