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Asteroid Hunter Gives an Update on the Threat of Near-Earth Objects

The views expressed are those of the author and are not necessarily those of Scientific American.


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Two very different asteroids. Photo courtesy of NASA

An Earth impact by a large comet or asteroid could knock out human civilization with a single blow, as most people are now aware thanks to recent Hollywood movies and public outreach by planetary scientists. Since 1998, when NASA initiated its Spaceguard program to find comets and asteroids 1 km in diameter and larger, researchers have made some crucial inventories of the risky space rocks with orbits that come into close proximity of Earth. For instance, there are almost 1,000 of these so-called near-Earth objects with diameters of 1-kilometer or more.

However disconcerting this might seem, we can rest assured that none will make it here in our lifetimes. “We can say with a very good deal of certainty that no asteroid or comet large enough to threaten life as we know it will hit Earth in the next 100 years,” says Donald Yeomans. At NASA’s Jet Propulsion Laboratory, Yeomans is a senior research scientist and manager of the Near-Earth Object Program Office. He has spent his career studying the physical and dynamical modeling of near-Earth objects, as well as tracking them down.

Yeomans works with an international network of professional and amateur astronomers who find and monitor asteroids and comets with orbits that come within approximately 0.33 AU, which is equivalent to 150 million kilometers. The team has identified 8,800 near-Earth objects as of early 2012, he noted during a talk at the American Museum of Natural History in New York on January 14 on his new book Near-Earth Objects, Finding Them Before They Find Us. The book gives readers an inside account of the latest efforts to find, track and study life-threatening asteroids and comets.

There are literally millions of asteroids and comets in the solar system, ranging in size from the microscopic to hundreds of kilometers in diameter. Both are made of rocky, metallic materials that failed to aggregate into planets during the early days of the solar system. Yeomans says the only real difference between asteroids and comets is that a comet actively loses its dust and ice when near the sun, causing a highly visible tail to form behind it.

Scientists have made exponential progress in identifying and tracking near-Earth objects in the past decade. NASA-sponsored near-Earth object surveys have found 90 percent of all asteroids and comets larger than a kilometer in diameter and projected their orbits at least 100 years into the future. Yeomans says the challenge now is finding all asteroids larger than 35 meters across, the size where one would pose a threat to a town or city, rather than all life on Earth.

Historically, Earth impacts by large asteroids and comets are rare. In addition, there is no clear record of a person being killed by one. Yeomans says that while Earth impacts by large asteroid and meteors are very low probability events, they are of very high consequence.

A prime example is just outside Winslow, Ariz., where a large crater was blasted into the Earth 50,000 years ago by a nearly 30-meter asteroid. Despite its relatively small size, the asteroid generated around 10 megatons of energy upon collision. By comparison, the atomic bomb dropped on Hiroshima during World War II generated around 0.02 megatons.

The asteroid that killed the dinosaurs 65 million years ago was much larger—a chunk of rock 10 to 15 kilometers across. The crater that formed when it struck near what is now the Yucatán Peninsula is 150 kilometers in diameter. The impact caused an immense explosion that deposited a layer of debris 10 meters deep as far as several hundred kilometers away from the impact and rained burning ash down on all corners of the globe. Most animals on the surface of the Earth died, and debris in the upper atmosphere launched the planet into a global winter. Many of the life forms that survived were either in the ocean or underground.

Today, if a survey detected a giant NEO headed for Earth, Yeomans says, humanity would have more than 50 years to prepare for it. He says a spacecraft could theoretically be used to divert such an asteroid off its Earth-colliding trajectory and out into space, and put in his plug for his employer, or at least organizations that support human ingenuity. “We have conceptual plans on how this could be done,” he says. “The reason the dinosaurs went extinct is because they didn’t have a space program.”

About the Author: Will Ferguson is an online intern at Scientific American. Follow on Twitter @wferguson1111.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. suitti 2:42 pm 01/22/2013

    50 years for a NEO that we know about. An estimate is that 10% are not yet known. Further, many long period comets are coming at us for the first time. These are currently all unknown. IMO, we’re not out of the woods yet. The odds have improved. It’s encouraging that Comet Halley can be imaged out by Neptune.

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  2. 2. And Then What? 2:57 pm 01/22/2013

    “Surveys have found 90 percent of NEOs larger than a kilometer in diameter” While I commend those who work diligently to track these objects, doesn’t this statement imply that One would have to definitively know exactly how many NEOs there are, and if this were so, then what would be the point in stating that you had found 90% of something that you knew 100% of?? This is like me saying I had found 90% of all the Fishes in the ocean that weigh more than 1 Kilogram.

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  3. 3. mipakeli 4:36 pm 01/22/2013

    The story of cosmic billiards will continue to play out as mankind searches for all the balls in play. The trouble with finding them all is that the game is ACTIVE and many billiard balls will smack into each other as well as the big planets like Earth–and that can change their orbits without us knowing about it until it’s too late. So finding them all and forgetting to track them continuously is a harbinger of doom.

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  4. 4. moss boss 7:21 pm 01/22/2013

    Although no one has been struck in the head with a rock from space, there have been a couple of close calls; two potentially deadly meteorites have fallen in the same town within the timeframe of just over a decade.

    http://peabody.yale.edu/collections/meteorites-and-planetary-science/wethersfield-meteorite

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  5. 5. DaniEder 5:33 pm 01/23/2013

    @And Then What? – You can estimate the total asteroid population from the discovery rate and how big your telescopes are and how much time they have spent searching. As you can see from this graph, the discovery rate has fallen off, implying we have found most of them. The 90% number comes from careful analysis of the data for the ~900 found so far:

    http://neo.jpl.nasa.gov/stats/images/site_km.png

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  6. 6. Prairie Dog 9:43 am 01/25/2013

    @suitti – “It’s encouraging that Comet Halley can be imaged out by Neptune.” Not really. Halley’s orbit is well known. It won’t endanger Earth for several hundred thousand years at least.

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  7. 7. Prairie Dog 9:49 am 01/25/2013

    “…within approximately 0.33 AU, which is equivalent to 150 million kilometers.” This is vague and misleading. Is .33 AU equivalent to 150 million kilometers, or is the AU itself 150 million kilometers? Clearer would have been something like “… 0.33 AU; one AU is approximately 150 million kilometers,” or “… 0.33 AU, or approximately 50 million kilometers.”

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  8. 8. michaelguinn 7:46 am 01/28/2013

    .33 au is equal to 49,367297.3 kilometers, not 150 million kilometers

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  9. 9. michaelguinn 7:47 am 01/28/2013

    correction
    .33 au is equal to 49,367,297.3 kilometers, not 150 million kilometers

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  10. 10. kforinash 1:13 pm 01/30/2013

    What, exactly, is ‘megatons of energy’?

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  11. 11. 2008RealityCheck 4:44 pm 01/30/2013

    What’s the probability that two asteroids will collide and change the course of each?

    It’s always that last ten percent, isn’t it?

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  12. 12. 2008RealityCheck 4:46 pm 01/30/2013

    Kforinash – A megaton is force equal to a million tons of TNT.

    Or 4,184,000 gigajoules of energy.

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  13. 13. myron 9:06 am 02/1/2013

    The sooner they start mining these asteroids the better !

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  14. 14. WizeHowl 6:34 am 02/2/2013

    With only a 90% certainty rate how can he make the statement “We can say with a very good deal of certainty that no asteroid or comet large enough to threaten life as we know it will hit Earth in the next 100 years,” just think of it if just 0.1% was to be missed of all the hundreds of thousands of *possible* candidates out there were to start heading our way there is no certainty.

    After all they missed the one that went passed Earth back about 10 years ago until it was actually passing us, and if had hit us it could have wiped out a large proportion of any continent, it lit up the night sky here in Queensland and cast shadows it was that large and that close to us, I was outside at the time of its passing and saw it.

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  15. 15. JanetColdwell 12:31 pm 06/14/2013

    maybe the huge nuclear arsenal that the super powers refuse to get rid of is secretly planned for asteroid defense purposes. dianabol

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