On this day 245 years ago – July 1, 1770 – humanity had its closest known encounter with extinction (with the possible exception of the Cuban Missile Crisis).
Two weeks before that date the French astronomer Charles Messier had discovered a faint comet in the constellation Sagittarius, which thereafter rapidly brightened and began moving swiftly across the sky. At its peak it was naked-eye, and its coma, according to various observers, the apparent size of from 5 to 16 full moons across. Lexell’s Comet, so named after another astronomer who subsequently calculated its orbit, was then under one-and-a-half million miles from Earth, or less than six times the distance of the Moon, and thus the nearest a comet has ever approached us in recorded history. (Kronk n.d.)
It was also larger than any asteroid known to have come that close, and in fact large enough to have wrought global consequences had it impacted our planet. The comet’s nucleus is estimated to have been 5 kilometers in diameter, or approximately half that of the comet or asteroid that wiped out the dinosaurs.
It is a curious fact of history that the celestial spectacle most superstitiously associated with presaging calamity has now been given scientific legitimacy as a major threat to human existence. (Bayle 2000 & Genuth 1997) It is an ironic fact of very recent times that the celestial spectacle most popularly associated in times past with presaging calamity and now known to have this potential in fact, is today looked upon mainly as a showpiece and photo op by much of the public and amateur astronomical community and as a scientific opportunity by the professional astronomical community. I refer in both instances to the appearance of a new comet.
These are thus the best of times and the worst of times for planetary defenders against potential impactors from outer space, since, on the one hand, for the first time since the Earth came into being, some of its inhabitants have an accurate awareness of the nature of this hazard and even the technological potential to do something about it, while, on the other hand, insufficient steps are being taken to protect us from it. What is most needed, I submit therefore, is a raising of comet consciousness among both the general and expert populace, who are currently, to coin a term, cometose.
And how better to do this than to institute a Comet Day? Global recognition has just been rallied for analogous awareness with the first annual Asteroid Day. This took place yesterday, on the anniversary of the largest impact event in recorded history, which occurred on June 30, 1908, in (or over) Tunguska, Siberia. The resulting explosion of the object upon penetration of the atmosphere would have obliterated any major metropolitan area that happened to lie beneath it. Current estimates are of one million objects of this or greater size in the Earth’s vicinity, only one percent of which have been discovered and are being tracked to date. The purpose of Asteroid Day is to build a global consensus for finding all the rest as soon as possible, to give us time to devise a suitable defense against any that might be heading our way.
This is all well and good, but it is not enough because it excludes from explicit recognition the equally or possibly even more dire threat from comets. Why has this happened? The asteroids-first initiative is built on the following premises. First, there are far more asteroids than comets among objects near the Earth.
Second, the average near-Earth asteroid is much smaller than the average near-Earth comet, and hence more amenable to deflection by current and affordable technologies. Third, we are likely to have far more time to mount an effective deflection mission against an incoming asteroid than against an incoming comet.
This is because most near-Earth asteroids are in solar orbits that are roughly “parallel” to the Earth’s, so that any predicted collision would probably be many revolutions or years in the future; whereas comets, especially long-period ones, approach Earth from a great distance on highly elliptical or near-parabolic orbits, which results in a very fast closing speed with our planet that could leave mere months to attempt to prevent an impact. Put all of these premises together and the conclusion is that, since asteroids are both more likely than comets to threaten us and more likely than comets to pose a threat we can meet, it is safe to ignore comets for the time being.
But I find this argument to be both flawed and downright peculiar. It is flawed because it fallaciously infers a low probability for a type of event that I think is more accurately characterized as of unknown probability. While the incidence of cometary apparitions is in fact small compared to the number of near-Earth asteroids, this does not establish a known probability of the arrival of a new comet in the inner solar system.
Why not? Because such events are completely random. There is no clockwork mechanism that accounts for all of them. A comet may begin its long journey inward if it collides with another one, or if its orbit in the Oort Cloud is perturbed by a passing star. Then there will be possible further encounters with the gas giants. So the randomness of a comet on collision course with Earth is not even like the randomness of blindly drawing one red ball from a jar containing a given quantity of otherwise white balls, since in that kind of case the odds can be calculated. With comets the randomness is so profound that no calculation even makes sense; it would be based on pure guesswork. Thus, the next comet to be targeting Earth is just as “likely” to appear on our radar screen today as one million years from now. We just have no idea at all. (Marks 2015)
But the argument for prioritizing asteroids over comets is also peculiar, it seems to me, because, even if the assignment of low probability were accepted, it would not follow that the risk of cometary extinction is low. The reason is that risk contains two components. Probability is one of them, but what it is the probability of is the other – that is, the negative value of the feared outcome. In the cometary case that outcome is human extinction. However one goes about assigning a number to the negative value of human extinction, it is sure to be a large one. Hence it would offset the low probability of our being annihilated by a comet and still yield a large enough number to warrant a significant investment of resources to trying to prevent it. (Matheny 2007)
But current policy does not recognize this. And I call this peculiar, as opposed to simply mistaken, because the cornerstone of all planetary defense policy is precisely the formula of risk as outcome times probability. It is a mantra of planetary defense, and of the asteroid campaign in particular, that the probability of a major impact is low, but the destruction it might wreak, even if “only” of a large city, high enough to warrant a vigorous national and global commitment to trying to prevent it.
I cannot help but surmise, therefore, that extraneous factors must be at work to account for the current neglect of comets among planetary defense policy makers. And one candidate for such a factor leaps immediately to mind, namely, nukes. When the impact threat first became a scientific possibility, a mere 35 years ago upon discovering that the cause of the dinosaurs’ demise was likely extraterrestrial (Alvarez et al. 1980), nukes were immediately put on the table as the only viable way to keep us from following in the dinosaurs’ pawprints. (Mellor 2007) But with the end of the Cold War, the original raison-d’être for having a nuclear arsenal became obsolete. Here, then, is another supreme irony of planetary defense, that just when a wholly legitimate and truly salutary use of nuclear bombs became known, retaining them became politically incorrect. (Birch 2013)
Thus for this reason, it seems to me reasonable to suggest, attention was diverted to non-nuclear means of defending against potential impactors. But this meant that only city-killer-size and slow-moving objects could be targeted – in a word, asteroids. Voilà: a textbook case of the tail wagging the dog.
This is my explanation for what otherwise strikes me as utterly inexplicable. Not, mind you, the rise in estimation of the peril from asteroids, which I view as a genuine scientific discovery and certainly worth addressing with a sense of urgency. Rather, a corresponding diminution to almost zero of the attention being paid to the peril posed by comets.
Case in point: At a conference on planetary defense held in April in Frascati, Italy, which I attended, not a single talk on the program featured the cometary threat. I will qualify that by noting that there was certainly a general awareness of the sorts of issues I am herein raising. Furthermore, nukes were surprisingly prominent in several discussions. But there was no systematic focus on comets and the special challenges they present. The use of nukes was always in terms of a “last resort” in cases where there was very little notice of an impending impact by a medium-size or small object, not a dinosaur-killer. And the assumption seemed to be that improved tracking of objects orbiting the Sun near the Earth would soon obviate the need even for this.
The nuke hypothesis also explains why the Chelyabinsk meteor explosion was universally recognized as a “wake-up call” re (small) asteroids, and yet who but a handful of aficionados recognized the discovery the month before – of a comet thought to be huge and heading for a collision with Mars—as a wake-up call re (large) comets? Comet Siding Spring turned out to be “only” under half a mile in diameter, and it narrowly missed hitting Mars, but so what? It could have been larger, it could have hit Mars, or it could have hit Earth.
Just as the Chelyabinsk meteor could have been a little larger and killed one million people. Why was the latter prospect more salient than the former? And why, a mere 21 years this month after the fact, is the relevance of Comet Shoemaker-Levy 9 to planetary defense but a distant memory? This comet split up before striking Jupiter and made 21 separate impacts, the largest releasing energy hundreds of times greater than the force of Earth’s entire nuclear arsenal, and all of them visible to anyone with even a small telescope, including yours truly. Again, the only explanation I can come up with has nothing to do with astronomical reality and everything to do with political reality.
Thus is my case for instituting an annual Comet Day, which could be the day after Asteroid Day (thereby pairing Lexell’s Comet with the Tunguska Asteroid), as a way to inform and arouse the world’s populace to sufficient awareness and action on behalf of governmental investment in a comprehensive planetary defense, as if our species’ survival depended on it.
Joel Marks is Professor Emeritus of Philosophy at the University of New Haven and a Bioethics Center Scholar at Yale University. He is deeply grateful to Harold Reitsema, Michael A’Hearn, Ed Lu, Dan Mazanek, Ted Roupas, Felicity Mellor, Jason Matheny, Donald Yeomans, Toby Ord, Andrew Rivkin, and AMMAD/D for helpful communications, and also wishes to absolve them of any responsibility for errors committed or opinions expressed herein.
Alvarez, L.W., Alvarez, W., Asaro, F. & Michel, H.V. 1980. “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” Science 208 (1095-1108).
Bayle, Pierre. 2000. Various Thoughts on the Occasion of a Comet. Tr. Robert C. Bartlett. Albany: SUNY Press.
Birch, Douglas. 2013, “Hunting Rogue Asteroids Could Be a New Use for Nuclear Weapons.” Huffington Post, 16 October, viewed 16 June 2015, http://www.huffingtonpost.com/2013/10/16/nuclear-weapons-asteroids_n_4107236.html.
Genuth, Sara Schechner. 1997. Comets, Popular Culture, and the Birth of Modern Cosmology Princeton: Princeton University Press.
Kronk, Gary W. n.d. “D/1770 L1 (Lexell)”, viewed 16 June 2015,
Marks, Joel. 2015. “Heaven Can’t Wait: A Critique of Current Planetary Defence Policy” in Commercial Space Exploration: Ethics, Policy and Governance, edited by Jai Galliott. Farnham UK: Ashgate.
Matheny, Jason G. 2007. “Reducing the Risk of Human Extinction.” Risk Analysis 27:5 (1335-44)
Mellor, Felicity. 2007. “Colliding Worlds: Asteroid Research and the Legitimization of War in Space.” Social Studies of Science 37:4 (499–531).