Some call it the Death Star. Others say It is a mystery planet. Still others call it voodoo astronomy.

Few, however, disagree that some mysterious factor-something from Space has been periodically snuffing ort life on earth during the past 100 million years. Theorists believe that whatever this force it, it has blown animals and other substances on the planet high into the stratosphere, about once every 26 million years or so.

This idea has recently exploded into one of the hottest topics in science today and has raised a host of intriguing scientific and philosophical questions. It has also spawned intense controversy about astronomy, evolution, and the origins of life.

The launching pad for this new research focus is the new theory that, contrary is previous thought, large-scale extinction have periodically wiped out all forms of life at various stages in the earth's life. In this thinking, the extinction of the dinosaurs 65 million years ago was not an isolated case in history, but one of several waves of extinctions.

The justification for this new idea came from a series of startling geological studies in the late 1970s by two University of Chicago geologists. David Raup and John Sepkowski. By carefully analyzing fossil records of the evolution of marine animals, the two concluded that large-scale extinctions occur about every 26 million years.

The revelation touched off hot debate among scientists, and because most scholars reasoned that no known earthly phenomenon could explain the 26-million-year cycles astronomers quickly entered the picture.

"It was not just a bad weekend. Something out of the ordinary happened, " says University of Arizona paleontologist David Jablonski. "It may have taken hundreds of years, but there is no question that there have been mass extinctions which shaped life, in unpredictable ways."

While the causes of extinction of the dinosuars have led to endless speculation, a group at the University of California at Berkeley led by Luis and Walter Alvarez have proposed a theory gaining wide acceptance as one of the plausible explanations.

Analyzing clay collected from several parts of the world, they found samples containing 160 times as much iridium as is normally found in terrestrial rocks, Iridium, however, is a common component of many extraterrestrial objects, " such as comets, asteroids, and other planets.

The group then set about examining how the iridium spread throughout the world, says Helen Michael, one of the Berkeley team's principle researchers. They reasoned that iridium-rich asteroids bombarded the earth, kicking up so much terrestrial and asteroidal debris into the atmosphere that it blocked sunlight from the earth for as long as several months. As temperatures plummeted, plants and animals died, leaving the current fossil record.

The theory made a big splash, explains Michael Rampino, of the Goddard Space Institute of Space Studies in New York. "They touched off a whole new field with some deep-seated implications."

The hypothesis initially met plenty of opposition, and still does, Raup explains. For example, some believe there may have been a combination of factors leading to any or all of the extinctions, "It takes a lot of different types of stress such as climate changes to cause an extinction. It's a very muddy topic, "he says.

In addition, he states, scientists prefer "earthbound" explanations.

But the hypothesis was quickly put to the test as scientists from all over the world scrambled to test rocks for iridium, and to the surprise of many, they discovered abnormally high levels of material at layers corresponding to the most recent extinction.

The theory lost some ground when geologists failed to discover excesses of iridium corresponding to earlier extinctions.