Sky Lights

The heavens seem peaceful—but there is danger in the darkness

By Bob Berman|Tuesday, October 1, 2002

Once a source of morbid myth, the dust-cloaked Pleiades now helps astronomers study stellar evolution.
Photograph by David Malin/Anglo-Australian Observatory/Royal Observatory, Edinburgh.

Long before medieval festivals merged with early Christian rituals to create Allhallows' Eve—later shortened to Halloween—the sky staged its own version of trick or treat. Inexplicable celestial events brought beauty but also fueled so much anxiety that the word disaster originally meant "bad star." Auroras seemed eerie, meteor showers were startling, and solar eclipses shot off the anxiety scale. Scientists have since stripped the ominous mystery from these events, yet the latest studies have repeatedly affirmed that the sky really is a source of danger.

The greatest risks are often invisible. Muons, subatomic particles 200 times heavier than electrons, continuously crash through our bodies. Created when cosmic rays smash into the nuclei of atmospheric atoms some 40 miles up, muons occasionally strike critical bits of genetic material in the chromosomes of our cells, possibly causing mutations. Equally damaging are high-energy ultraviolet rays from the sun, responsible for savage melanoma skin cancers. Fortunately, the worst ultraviolet rays are blocked by a good sunscreen.

But no sunblock could guard against the short-wavelength radiation that would pummel Earth if a supernova exploded nearby, within 30 light-years or so. Such a proximate event occurs just once every few tens of millions of years, but when it does, it could trigger additional mutations. Here again there is a treat, however. Such mutations could have helped accelerate the evolutionary changes that eventually led to the arrival of humans.

Sometimes the old sinister connotations melt away in the face of modern scholarship. The Pleiades star cluster, located in the constellation Taurus, has been linked with death in many cultures. Astronomy authors Robert Burnham and William Olcott believe some of the dark connections may derive from the Pleiades' conspicuous position in the sky at the time of the 1450 B.C. eruption of the volcano on Santorini, Greece. That calamity is believed to have contributed to the downfall of the Minoan civilization of Crete—but it had nothing to do with the Pleiades. To modern eyes, the cluster is an unadulterated joy sparkling in the eastern autumn sky like a swarm of distant fireflies.

In other instances, ancient superstitions return in a fresh guise. Comets, once regarded as bad omens, again seem threatening as researchers have come to appreciate that comet and asteroid impacts can cause mass extinctions. Last year a team led by Kunio Kaiho of Tohoku University in Japan uncovered convincing evidence that one such collision triggered the greatest extinction of all, the Permian-Triassic event 250 million years ago. During a cataclysm dwarfing the one that snuffed out the dinosaurs, roughly 95 percent of all species perished. Earlier studies in the United States had found molecules whose composition resembles the gases in a meteorite trapped in sediments from the time of the Permian-Triassic transition. Kaiho bolstered the case by uncovering evidence that an incredibly energetic impact vaporized solid sulfur from terrestrial rocks, unleashing a torrent of sulfur-rich gas that turned the surface of the oceans as acidic as lemon juice.

Just four months ago, a 300-foot-wide asteroid called 2002MN passed 75,000 miles from Earth, less than a third of the distance to the moon. It was by far the largest asteroid ever seen to approach that close to us. Had it hit, it would have unleashed as much devastating energy as the most powerful H-bomb.

John S. Lewis of the University of Arizona in Tucson calculates that there is a 1-in-5,500 chance of an impact that would cause more than a million fatalities in any given year. So when you look up, you have to wonder: What will it be tonight, trick or treat?

Details about the Pleiades can be found at
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