Monday, September 04, 2006

Stones and Fish Falling from the Sky

Copyright © 2005 by Joel Marks

This paper is based on a talk presented at the University of New Haven, under the auspices of the provost's and dean of students' offices, on October 25, 2005. The title of the paper comes from the wonderful coincidence that on that very date (according to my official Congressional calendar), exactly two hundred years prior, Thomas Jefferson wrote to Andrew Ellicott that he did not believe stories of stones and fish falling from the atmosphere. We know now that stones and fish do on occasion fall from the sky in prodigious numbers. That is the subject of this paper.


In these days of catastrophe upon catastrophe -- tsunamis, hurricanes, earthquakes, the melting of the ice caps, ozone layer depletion, genocide, terrorism, war, impending pandemic ... -- one might almost believe we are approaching some prophesied "end of days." More likely we are experiencing an artifact of global communication, which makes us instantly aware of phenomena worldwide that, in one form or another, have always been a part of the human condition, heretofore separately suffered. This is analogous to the way medical students are liable to temporary hypochondria. Indeed, a recent study indicates that, contrary to our technologically enhanced awareness of, for example, the horrors of war, the number and morbidity of wars has significantly decreased in the last decade.1

To add to our tortured awareness, we not only know of dire happenings all over the earth, but of various specters from outer space as well; and so once more our knowledge may induce a sense that the end is nigh. In this case, however, I want to argue that the threat is more real. While again acknowledging our artificially induced hyperawareness of the phenomenon in question, I maintain that the extremity of this danger makes up for its relative rarity of occurrence. I am referring to the collision of our planet with a comet or an asteroid. While such an event might occur on average only once in millions of years,2 that "once" could be at any time, and when it does occur, it will be more devastating in its consequences than any other catastrophe we can accurately envision,3 save a nuclear World War III (which we have perhaps escaped with the ending of the Cold War). About the latter Jonathan Schell once famously noted that it presented the prospect of a “second death" -- the annihilation not only of billions of individuals but also of the entire future cultural memory of humanity that is our worldly equivalent of the hope of immortality.4 I see nothing less at stake in the case of a cosmic impact.


Let me now array before you some salient facts and speculations about the threat of our annihilation by a comet or an asteroid. I will first discuss some previous impacts and their effects; next, what we know or surmise about their origins; third, several recent "near misses"; and finally, what we might do to avoid future astro-catastrophes. Please note that space considerations about a vast topic oblige me to but mention or briefly describe each item; furthermore, I will for the most part leave out qualifications about our state of knowledge about each item, treating them all as if precise and established facts even in those cases where they might more modestly be said to represent a widely accepted view or estimate by authorities. Some recommended sources of this information are given in the notes, but, needless to say, countless other relevant resources, some of them extraordinary, can be found by typing key terms into Google (while of course always exercising due caution about their trustworthiness) or other helpful sites, such as National Geographic News (, Sky and Telescope (, and Wikipedia ( See also Bill Arnett’s cornucopian labor of love, Nine (now Ten!) Planets, et al. at, and the compendious Spaceguard Page of the Australian Planetary Society at

Previous impacts

Without question the most dramatic impact we know about occurred in the Yucatan 65 million years ago. A body six miles in diameter has left a crater 3000 feet deep and 112 miles in diameter centered on Chicxutub, Mexico. The resultant discharge of energy was equivalent to 196 trillion tons of TNT. (For comparison, consider that the "Little Boy" bomb dropped on Hiroshima packed the power of 13 thousand tons of TNT.) In 1980 the Nobel-prize-winning physicist Luis Alvarez and his geologist son Walter Alvarez proposed (based on the worldwide presence at the appropriate geologic stratum of iridium, a mainly extraterrestrial element) that just such an impact event led to the extinction of the dinosaurs. The subsequent scientific detective story that clinched the case for the Yucatan crater is fascinating in its own right.5 Various theories compete to explain the exact mechanism of the resultant dinosaur extinction; for example, the impact may have thrown up so much dust into the atmosphere that the sun was obscured long enough to wipe out plant life, hence plant-eaters, hence the carnivores who ate the plant-eaters. Also picture a kilometer-high tsunami (hence “fish falling from the sky”).

50,000 years ago an object 50 meters in diameter impacted near Winslow, Arizona, with the force of 10-20 million tons of TNT, leaving what is perhaps the earth's most perfectly preserved impact crater. Known as the Barringer, or formerly Canyon Diablo, Crater, it is 570 feet deep and 4000 feet in diameter. The planetary geologist Eugene Shoemaker proved the crater's extraterrestrial origin, which had been in dispute because of the numerous volcanic calderas in the same region.6 Eugene Shoemaker's life is itself a great story. His dream was to go to the moon, but Addison's disease precluded his being chosen as an astronaut. He took all of the Apollo astronauts headed for the Moon's surface on field trips to the Barringer Crater to train them about what to look for, since the same question of volcanic versus impact origin existed for the Moon's countless craters. I will continue Shoemaker's story throughout this essay.

On June 30, 1908, a meteoroid 30 meters in diameter exploded in the atmosphere above Kunguska, Siberia, flattening 60 million trees over a diameter of 50 kilometers. The power of the blast was equivalent to the Barringer's, i.e., 10-20 million tons of TNT. "Meteoroid" is a convenient term for an asteroid smaller than 50 meters, this being the approximate threshold size for such an object to withstand passage through the Earth's atmosphere and impact the surface intact. As the Kunguska event demonstrates, however, an air blast can be equally or more devastating than an impact.7

From July 16 to 22, 1994, an unprecedented opportunity presented itself to earth- and space-based observers. A comet that had been discovered by Eugene Shoemaker, his wife Carolyn, and their colleague David Levy -- called Shoemaker-Levy 9 -- split into 21 fragments of kilometer-size and then crashed, one by one, into the planet Jupiter. The resultant impact "smudges" were each approximately the size of the Earth's diameter and were easily visible even through amateur telescopes. The largest fragment -- "G" -- impacted with the force of 6 trillion tons of TNT or 750 times the world's nuclear arsenal.


The reservoir of asteroids is a belt of such objects orbiting the sun between the orbits of Mars and Jupiter, at a distance from the sun of roughly 2 to 4 AU or astronomical units. (1 AU is the mean distance of the Earth from the sun, roughly 93 million miles.) They represent perhaps a failed planet, which was prevented from coalescing by the overwhelming gravitational influence of Jupiter. Occasionally asteroids collide, altering their orbits such that some of them fall closer to the sun and into earth's neighborhood. A Congressionally mandated (in 1998) 10-year effort is now well along to inventory at least 90 percent of the asteroids of kilometer or larger size whose orbits cross the earth's orbit -- known as large NEAs or near-Earth asteroids. These are the ones with whom a collision would have global consequences. As of mid-2005 about 800 out of an estimated 1200 had been discovered.

The reservoir of short-term comets, i.e., comets that orbit the sun in under 200 years, is the Kuiper Belt, which begins beyond Neptune, whose distance is 30 AU. Halley's Comet is a paradigm example, orbiting the sun every 75 years and reaching almost to Pluto's orbit at aphelion (furthest point in its orbit from the sun). Halley's nucleus is 16x8x8 kilometers. As with asteroids, collisions can send some of these comets into the inner solar system, where they could threaten the Earth. By the way, there is now reason to believe that Pluto itself is perhaps best conceived not as a planet but as a Kuiper Belt object. Pluto is smaller than earth’s moon. Several other Pluto-sized Kuiper Belt objects have been discovered just in the past year, and some astronomers expect to find hundreds more ... perhaps even Earth-sized!

The reservoir of long-period comets is hypothesized to be the Oort Cloud, consisting of trillions of fragments orbiting at 50,000 AU from the sun, which is somewhat under 1 light-year from the sun or almost 1/4 the distance to our nearest stellar neighbor, Alpha Centauri. Besides the collision mechanism, a novel explanation for journeys into our neck of the woods from the Oort Cloud was proposed in 1984 by astronomers Richard B. Strothers and Michael R. Rampino of NASA. They noted that there is paleontological evidence, going back 225 millions years, for a mass extinction of species on this planet every 26 million years or so (the dinosaur extinction being two cycles ago). Meanwhile, the solar system orbits the center of the Milky Way Galaxy over a period of hundreds of millions of years. They speculated that the extinctions could therefore be due to perturbation of the Oort Cloud from coming into some periodic proximity to other stars or matter (e.g., dust clouds) in the Milky Way, whose gravitational influence could send a storm of comets hurtling into the inner solar system, some of which would inevitably strike the Earth with terrifying effect. However, the logic of that proposal is questionable, and there are competing explanations of the periodic extinctions, none of which has much evidence in its favor.

Recent Near Misses

Asteroid 1996 JA1 came to within almost the distance of the Moon to the Earth (1/4 million miles) on May 18, 1996. This asteroid is 1/3-mile in diameter. It was discovered only four days prior to its closest approach!

Asteroid 2002 MN came to within 75,000 miles of the Earth -- less than 8 Earth diameters -- on June 14, 2002. It is 100 meters in size (three times the size of the Tunguska meteoroid) and was only discovered three day after its closest approach!!

Asteroid 2004 FH -- 30 meters or the size of the Kunguska meteoroid -- came to within 26,500 miles of the Earth -- just over three Earth diameters -- on March 18, 2004!!! It was discovered just three days before, which, if prior to an impact, would have allowed the kind of early-warning evacuation of the target area analogous to that for a hurricane.

Asteroid 2004 MN4 or "Apophis" (the Greek god of destruction) will come within 18,000 miles of the Earth -- just over two Earth diameters -- on April 13, 2029!!!! It will be visible to the naked eye (from Europe and Africa) moving at 42 degrees per hour through the constellation Cancer. (For comparison: The sun appears to move in the sky at 15 degrees per hour on average.) It is 1000 feet long and, if striking the earth, would have the force of 850 million tons of TNT (again, the Hiroshima bomb was 13 thousand tons). There is an as-of-yet-undetermined chance of its striking the Earth seven years later.

What to do

There are various proposals for averting close encounters of the catastrophic kind. They begin with conducting exhaustive surveys of near-earth asteroids. As we have seen, even a 30-meter object can have extreme local consequences. There are estimated to be 4 million NEOs (near-earth objects) of that size or larger. Several programs have begun to inventory them.8

Then of course there would need to be a combination of preparedness for disasters (e.g., expanding FEMA's purview -- God help us!) and efforts to avert them. Proposals for the latter are not lacking. However, they all of necessity call for feats of engineering and thus would require testing. Testing takes time. Hence the need for the exhaustive survey of NEOs to enable us to predict which are threats as early as possible.

A typical proposal is to alter the orbit of the offending asteroid or comet. The essential point to realize about such a task is that the more lead time, the less energy required to do the job. An asteroid about to hit the earth would have to be pushed perhaps almost the diameter of the earth to protect us, which might be technologically or economically or politically prohibitive, whereas an asteroid not due to strike us for, say, ten years -- or a comet like Halley's if intercepted far enough out in its orbit -- might require a comparative nudge, which would then be compounded automatically over the ensuing decade, thereby removing the Earth from harm's way.

One group of concerned scientists, engineers, and military experts, chaired by former astronaut Rusty Schweickart, has therefore urged the controlled altering of an asteroid orbit by the year 2015.9 This would then allow sufficient time to engineer the altering of the orbit of Asteroid 2004 MN4, should it turn out to be headed for Earth impact in the year 2036. (It has also been pointed out that as the inventorying of NEOs proceeds, we might discover even more certain and more imminent threats in need of deflecting than 2004 MN4.)

In the meantime, two related notable achievements have taken place. On February 12, 2001, the space probe NEAR-Shoemaker landed on Eros, a 33-kilometer-long asteroid. And on July 4 of this year (2005), the space probe "Deep Impact" (named after the 1998 movie) was intentionally crashed on the nucleus of Comet Tempel 1. These events also bring us to the conclusion of the extraordinary story of Eugene Shoemaker. On July 18, 1997, while on an expedition to Australia to study impact craters, Shoemaker was killed in an automobile accident (in which his comet-hunting wife Carolyn was also injured). He had also been working as the technical advisor to the movie Deep Impact. In a rather morbid display of recognition, a scene was then inserted into the movie that depicts the death of an astronomer in a car accident. As a more fitting honor, the historic space probe to Eros was named after him. But in the most wonderful memorial of all, Shoemaker's lifelong dream to go to the moon was finally realized when the Lunar Prospector was crashed onto the lunar surface on July 31, 1999, carrying Eugene Shoemaker's ashes. He is therefore the first human being to R.I.P. on another world.


What makes the cosmic catastrophe I have discussed even more compelling is that it may be eminently avoidable. The technology needed to save us is already available or readily attainable, and the monetary expense could be a tiny fraction of, say, the cost of the Iraq War ... or, in an all-out effort, certainly no more than the cost of the "space race" or the defense budget during the Cold War. It seems that we ought to be willing to spend as much to save humanity as we were willing to spend on possibly destroying it. Therefore it is not, strictly speaking, a comet or an asteroid that threatens our very existence so much as indifference based on insufficient awareness of the threat. In this spirit I have, by writing this essay for your perusal, taken up the charge of various government and non-profit groups to contribute to public education about the risk, and I also make myself available to present an illustrated version of this narration to any class or group that would care to sponsor me.10


1 University of British Columbia Human Security Centre report:
2 A table of frequencies of collisions of various magnitudes can be found in the article “Risks to the Earth from impacts
of asteroids and comets” by Harry Atkinson in Europhysics News (2001) Vol. 32 No. 4, which is on the Web at
3 For a fictional account of how much fun it would not be to survive such a collision, see the epic novel Lucifer’s Hammer by Larry Niven and Jerry Pournelle (Del Rey, 1985).
4 See the second part of his 1982 book, The Fate of the Earth.
5 See T. Rex and the Crater of Doom, by Walter Alvarez (Vintage, 1998).
6 For an account see David H. Levy’s The Quest for Comets (Oxford University Press, 1995). Meanwhile, C. Wylie Poag’s Chesapeake Invader (Princeton University Press, 1999) tells the story of another impact crater’s discovery, the largest in the U.S.
7 Another fact to note about an impacting body is that, while “size matters,” velocity matters even more. This is because kinetic energy varies directly with mass but is a function of the square of the velocity. (I thank my colleague Stephen Rocketto for pointing this out.) Yet another important factor, then, is the composition and structure of the object, since these combine with size to determine mass.
8 A list of such research programs can be found at For a daily update of close approaches, see
9 This is the B612 Foundation, named after the asteroid in Saint-Exupéry's The Little Prince. It is one of several groups to have made recommendations about the collision threat. Harry Atkinson’s government task force in the UK reported its recommendations in 2000, which were accepted the next year. Planetary scientist Michael Belton led another group, which released a report in Washington, D.C., in February 2003.
10 As a happy bonus, a classroom or other discussion of this issue could also serve as an introduction or review of astronomy (not to mention, geology, etc.), since it touches on objects in space from meteoroids to galaxies. It is hoped that students’ fascination and curiosity would then be sufficiently peaked to make them desire to know how astronomers et al. know or learn about such things.

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