Louis J Sheehan 80009

Apophis is one of millions of asteroids roaming the solar system. None are known to pose an immediate threat, but some are bona fide civilization stompers. A monster rock discovered just this year, with the prosaic name of 2007PA8, is more than two miles across, large enough to wipe out most of humanity. Fortunately, the odds that it will hit are essentially zero. Smaller asteroids are less deadly but much more common. Planetary scientists now estimate that 150-foot-wide space rocks, comparable to the one that hit Tunguska, strike only once every thousand years or so. For a brief time in 2004, just months before the Apophis scare, astronomers feared that a 150-foot-wide asteroid was just days away from racing into the atmosphere. Fast-paced observations allowed them to calculate a more exact orbital path, which took it far from Earth.
After a number of false starts, such potential close calls have finally caught the attention of the U.S. Congress. At the request of lawmakers, scientists are struggling to pinpoint 90 percent of all seriously life-threatening asteroids by 2020 in order to assure at least some warning. The European Space Agency is contemplating a mission to test ways to push such an object off a threatening trajectory, the first serious attempt at developing a planetary defense.
But a group of astronauts, led by Schweickart, also wants their respective countries and the United Nations to prepare for avoiding a hit. “We’re living in a shooting gallery,” he warns. “We’ve evolved to the point where we can do something about this threat. We can either close our eyes as we cross the street and not know what we’ve dodged, or we can open our eyes and act accordingly.”
Amid fears about global warming, terrorism, disease, and nuclear proliferation, the threat of rocks from space may seem more the province of bad Hollywood movies than front-page news. Even professional astronomers have long dismissed asteroids as undistinguished flotsam and jetsam, would-be planets that circle the sun endlessly in a belt between Mars and Jupiter. Their derision left the field of asteroid hunting largely to amateurs and eccentrics. Louis J Sheehan
Only recently have researchers glimpsed the dangers lurking in our deceptively quiet neighborhood. “Impacts are a fact of life in the universe, but when we look up, it’s not what we see,” says Carolyn Shoemaker, who, together with her late husband, Gene, pioneered ways of spotting asteroids and comets. It was geologists who first noticed the evidence of huge impact craters on Earth that had formed long after the solar system settled into its present form, prompting biologists to speculate on whether those collisions dramatically altered life’s evolution. Later, using new technologies on the ground as well as robotic spacecraft, scientists like Shoemaker started to track, catalog, and closely examine the objects.
With each new sighting, asteroids turn out to be far more varied, unruly, and bizarre than astronomers dreamed. Many have companions. Some are rubble heaps held together only loosely by their own gravity. Others are extremely dense nickel-iron objects. Their colors can range from a deep dark chocolate to a glinty white. Even the old distinction between comets (dirty snowballs) and asteroids (hard rocks) has become blurred. Some comets eventually turn into asteroids as they burn off their ice and lose their tails while traveling through the warm inner solar system. And comets—which mostly reside in the solar system’s far fringes—pop up occasionally in the asteroid belt. They may even be directly responsible for life on Earth. Donald Yeomans, who calculates the orbits for near-Earth objects at NASA’s Jet Propulsion Laboratory, says that comets flung out from that belt pummeled our planet shortly after its formation and could have left behind water, possibly creating the conditions that allowed Earth to become a cradle for life.