TCS Daily


The Real Threat to the Planet

By Sallie Baliunas - April 9, 2002 12:00 AM

"...[G]lobal warming [is] the most serious threat that we have ever faced..." (Al Gore, Earth in the Balance, p. 40)

"...[A large asteroid striking the earth is a] serious and surprising danger posed to our global civilization from outer space." (Carl Sagan)


Two global threats - one supposedly human-made, the other natural. How much worry should be parceled to each? Calamitous global warming from human activities like fossil-fuel burning finds little support in reliable measurements of the temperature of the earth - either at the surface or in the sensitive lower atmosphere. On the other hand, an eventual asteroid strike is a dead certainty.

Just discovered is the threat of asteroid 1950 DA, which might endanger the earth in 878 years. The risk of comet or asteroid strike is not unusual - it is ever present. However, for the first time in the history of life on earth, science may allow Homo sapiens to see and deflect a cosmic destroyer.

Natural catastrophes have always pounded the earth and its precious life. As the plates of the crust of the earth ride the underlying molten flow of magma, land masses smash together only to tear apart. Crumpled ridges jut into high mountain ranges, and crushes of volcanoes emit choking dust and gases, plus waves of lava that blanket continent-sized regions.

Today the lever of science helps to measure, classify and comprehend past calamities. But the action of continental drift, winds, seas and volcanoes blurs the evidence, making scientists' detective work hard. Still, geologists gaze, for example, at the Himalayas and see the 1,800-mile long wrinkling of land produced by the enormous force of the collision between the roughly 75-mile thick Eurasian and Indian tectonic plates around 50 million years ago.

Through the 4.5 billion years of the earth's existence, internal actions have jostled, jolted, smoothed and havocked the earth, with life and its Darwinian process forced to respond.

And then there is the moon.

Thoughts of the moon run first to its strangeness compared to the earth. A verdant, sapphire-blue, cloud-dappled and dynamic planet brimming with life, orbited by a satellite starched in gradations of unrelenting gray, arid, airless, lifeless - and anciently cratered. The craters attest to the early phase of intense bombardment as the planets nearer the sun grew out of the amalgamation of rocky bits built in turn from the sticky collisions of dust motes over eons. How can two bodies so seemingly disparate end up orbiting each other?

By accident! About 4.5 billion years ago, two planets that had formed -- the earth and a planet about the size of Mars - collided. The debris of the disintegrated smaller body collected in a nearby gaseous ring whose particles collided, stuck together and grew to form another planetesimal - this time the moon in orbit around the earth.

Just north of Los Angeles sits Mount Wilson Observatory with its deliberately time-frozen exhibits in the Astronomical Museum. Built in 1936, the Museum displays splendid backlit glass photographs of astronomical objects, many taken with the Observatory's 100-inch Hooker Telescope, then the largest in the world. The early captions describe the 1930's state of astronomical knowledge. Accompanying a glorious close-up of the pocked lunarscape is the best science of that period: "Lunar craters are probably of volcanic origin, although some of the smaller ones may have been caused by impacts from meteors."

Thanks to science, that notion has been revised. Lunar craters reveal the chaos to which both the earth and the moon must have been subjected in the inner solar system during the first billion years or so. Lacking the great erosive forces of wind, ocean, vulcanism and plate tectonics, the moon's surface preserves a record of early solar system history. On earth, that record has been all but obliterated.

Fossil evidence on earth from about 3.5 billion years ago suggests that life persisted and flourished soon after the early pummeling by asteroids and comets ceased. On the arid, airless moon, life must not have even begun - there is no hint of evidence favoring lunar life.

The scarred lunar regolith warns of exo-terrestrial hazards that have endangered the earth and its life. While the early chaos has long since calmed, asteroids and comets continue to strike the planets, including the earth. One line of evidence is the remnants of recent craters on the earth, like Barringer Meteor Crater in Arizona, an awful sight near Interstate 40. There, an exo-terrestrial projectile about 150 feet across exploded and disintegrated upon impact with the ground about 50,000 years ago, leaving a crater three-fourths of a mile across.

More evidence of strikes comes from recent impact sightings, some historically described. NASA researcher John S. Lewis speculates in Rain of Fire and Ice that the Bible's Joshua (Chapter 10) may tell of a deadly asteroid strike ca. 1400 B.C.E., when a rain of "great stones out of the heavens" killed Joshua's battlefield enemies. Just a few verses later sits the astonishing line, "and the sun standeth in the midst of the heavens, and hath not hasted to go in - as a perfect day." Rather than the rotation of the earth temporarily stalling, the exploded debris of an asteroid or comet that decimated the men must also have lit the sky with long-burning glows that would have seemed like sunlight and full moonlight persisting long beyond sunset. To the ancient mind, bright, nighttime light from the debris' disintegration equaled the sun's standing still - and the timing and consequence of the asteroid strike would have seemed a miracle.

Television history was made with the broadcast of the 1994 collision of Jupiter with the Comet Shoemaker-Levy. Because Jupiter is the largest planet in the solar system, its strong gravitational attraction will tend to draw asteroids toward it. However, the Sun is also tugging at them, and some near-earth objects (NEO's) may end up on trajectories intercepting earth's orbit. The evidence is irrefutable: exo-terrestrial objects have struck the earth, and will do so in the future. The laws of celestial mechanics say that earth is not immune to exo-catastrophe.

The impact of an NEO larger than about one-half mile in size could produce severe environmental damage. The only way to avoid the destruction and death from a large NEO is to detect and then deflect it. Several groups around the world, and especially in the United States and Britain, search the skies for NEO's, and then determine their orbits. That process allows the probability of a future collision with earth to be estimated. To date, about half of approximately 1,000 destructive asteroids that may cross the earth's orbit have been catalogued, and their orbits give assurance that there would be years of warning before a strike. But the unknown objects foment anxiety: there would be no warning that an undetected NEO is on a collision course with the earth, because the existence of that NEO would not yet be suspected.

The NASA Spaceguard Survey will improve the odds by attempting to detect about 90% of the NEO's by 2008. The idea is to locate, track and deflect these true exo-terrestrial hazards. On rough average, an asteroid a bit larger than a mile across strikes the earth every million years or so, and would present a major global catastrophe.

The detection and tracking technology is relatively straightforward and inexpensive, but requires constant vigilance. However, deflection capability is still in its infancy. Rockets fast enough to reach an asteroid on intercept course with earth would have to be deployed, with energetic means of shoving the asteroid onto a non-perilous orbit. That might require advanced nuclear devices that are detonated beside the asteroid, to shove it into another orbit, while avoiding shattering the asteroid into uncontrollable pieces.

Deflection technology remains rudimentary, and no one yet knows how quickly it may be needed. But it will eventually be needed. As an aside, if we assume that the astronomers' estimates of an enormous number of habitable planets in the universe are correct, the cosmos may harbor trillions of intelligent species. On more than one of those planets, odds are that a putative intelligent species or two was cratered to oblivion - perhaps just before its technology dawned to avert a low-probability but certain exo-catastrophe.

The cosmic environment streams with hazards for earth and earthlings. Asteroid and comet strikes truly endanger life on earth. Until the science says otherwise, human-made global warming winds up low on the list of anxieties.
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