By Joel Marks
Published in the Institute for Ethics and Emerging Technologies blog, July 1, 2015
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. (http://www.asteroidday.org/) 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.
References
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,
http://cometography.com/pcomets/1770l1.html.
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).
