Life on earth still uncertain

By Vision Reporter

Added 14th January 2002 03:00 AM

Last Tuesday, an asteroid whipped past our planet, missing it by several hundred thousand miles. Astronomers had feared for us as it faced us

THE journalists called it a “near miss”, but asteroid 2001 YB5 didn’t really come close to the Earth. It was around 600,000 km. (400,000 miles) away when it whipped past our planet last Tuesday. Given that it was not much bigger than an aircraft carrier (though a lot heavier), it was like having somebody fire a .22 bullet at you and miss by about 10 km. (6 miles). The experts did their best to whip up alarm. “Such an object could wipe out a medium-sized country if it impacted and lead to a global economic meltdown,” warned asteroid expert Benny Peiser of John Moores University in Liverpool. But the human race refused to get excited: you can safely ignore a bullet that misses you by ten km (six miles). There are an estimated 700 asteroids, give or take a couple of hundred, that are big enough to change the whole fate of life on Earth if they struck it — between one and ten kilometres across (0.6 and 6 miles across) — and in orbits that could one day lead to a collision. There are literally thousands of others, ranging down to the size of 2001 YB5, that could wipe out a country the size of France or Korea if they hit the Earth. The problem is the time-scale of the threat: ‘long-term planning’ in most human contexts is five to ten years. Yet there is no reason to despair. Given that nobody had any idea of the scale of the asteroid threat only 25 years ago, the way we have climbed the learning curve in a series of huge intellectual leaps is actually quite impressive. First, there was the whole idea of a “nuclear winter”: the hypothesis, first put forward by Carl Sagan et. al. during the 1970s, that explosions which lift large amounts of dust high into the atmosphere could have planet-wide effects, since the dust would stay up there for years, blocking sunlight, killing crops, and changing the whole climate. Sagan and his colleagues were mainly concerned about the effects of thousands of thermonuclear weapons being exploded near the ground, which seemed alarmingly likely during the Cold War. But then in the early 80s the father-and-son Alvarez team — physicist father Luis and geologist son Walter — looked at the thin layer of iridium that is found worldwide in geological strata about 65 million years old. They surmised that it could not have a terrestrial origin, and suggested that it was due to a giant asteroid strike — which also wiped out the dinosaurs by causing a prolonged nuclear winter’. The dates matched, and only a few years later the discovery of a giant crater of the right age off Mexico’s Yucatan peninsula gave the theory added credibility. Doubters pointed to the enormous volcanic eruptions of the same time that covered much of southern India with a thick layer of lava (the “Deccan Traps”) as a rival cause of global climatic disruption and the extinction of the dinosaurs, but now it seems that the asteroid strike and the intense volcanic activity may actually have been connected. There was an even larger calamity 251 million years ago: the “Great Dying”, when 70 percent of land vertebrates and 90 percent of all marine animals suddenly became extinct. Last year, scientists at the universities of Rochester and Washington published a paper in the respected journal Science identifying geological evidence of a massive asteroid or cometary strike at that time too. No crater for this strike has been identified yet, but once again it coincides with an unprecedented period of massive volcanic activity, this time in Siberia. In only 25 years, therefore, we have changed our Darwin-based ideas about evolution to include rare but massive changes caused by asteroid strikes. We have begun to suspect that these huge strikes trigger volcanic episodes that disrupt the planetary environment for long enough to cause mass extinctions worldwide. And we have identified and plotted at least a significant fraction of the most dangerous objects in the solar system. We have not yet developed the technology to divert them, but the average secondary-school graduate today is likely to understand the nature of the threat. Far more than gestures like the creation of a ‘Planetary Protection Office’ at the US National Aeronautics and Space Agency, it is this planet-wide raising of consciousness that will eventually create the political basis for a real planetary defence programme. Gwynne Dyer is a London-based independent journalist

Life on earth still uncertain

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