Tag Archives: Comets

Contextualising Cosmic Danger


We all delight in some measure of fright. There is some allure in taunting the reaper, something endearing about the dark, deserted and blood-stained that we can’t quite explain. But it’s there. This weekend, fittingly around Halloween, our appetite for horror was indulged by yet another close encounter with a giant mass of rock hurtling through space. If it were to collide with earth, the impact of a meteorite this size would quite literally threaten our species with extinction. However, is such a scenario probable? To what degree has that appetite for horror caused us to distort the degree of danger posed by close encounters of the cosmic kind?

The object in question this time is called Asteroid 2015 TB145, although that may prove to be a misnomer. Some within the scientific community have begun to postulate that TB145 is in fact a comet. As a point of clarity: asteroids are the rocky remnants of planetary formation and orbit the sun, primarily in a belt between Mars and Jupiter. Comets, on the other hand, formed beyond the frost line, far from the warm embrace of our star. They are hence composed of rocky nuclei, blanketed in layers of ice which, upon approaching the sun, birth brilliant tails of dust and plasma. TB145 conspicuously lacks this characteristic coma, pointing to the conclusion that if our visitor is indeed a comet, the sun has eaten into its envelope of ice. The traveller from exile at the fringes of our solar system has, this time around, arrived as a corpse.

At closest approach, TB145, nicknamed Spooky, will pass a mere 480 000 km from our home planet. This ‘close shave’  is what has motivated the recurring narrative of earth narrowly escaping global devastation. Granted, on a scale of Astronomical Units, light-years and parsecs, the distance is indeed relatively miniscule, in much the same way that Alpha Centauri is a relatively close star. However, in assessing the danger, it is imperative to remember that on this astronomical scale, Earth is indeed a speck. To illustrate: suppose that Spooky’s orbit crossed into the space between its actual closest approach and the earth. The odds that it would, at random, collide with the earth can be conceptualised by imagining a ‘dartboard’ of radius 480 000 km, with earth as the bull’s eye. The probability is therefore approximated by taking the ratio of the area of the earth to that of the cosmic ‘dartboard’. From figure 1.1, it is apparent that this probability lies far below 1%. The danger, it seems, has been gravely overstated.

Figure 1.1

Figure 1.1

Admittedly, this model is simplistic and fails to account for the intricacies of celestial mechanics. The probability yielded then, is merely an order of magnitude approximation. It aims to illustrate the tremendous vastness  and pervading emptiness of space – the truth that, in the cosmos, the void reigns supreme. However, even when accounting for these shortcomings, it is evident that the chances of a collision within the closest approach are overwhelmingly slim. Moreover, Spooky – and scores of others like it – were on trajectories which we knew would never result in an impact with our home planet. Declaring that each one of these ‘nearly hit’ us is therefore akin to standing a hundred meters from the edge of a motorway and yelling that you narrowly avoided an accident each time a car whizzed by. When placed in proper context, the sensationalism sounds somewhat like the panicked ramblings of a paranoid mad man. Nonetheless, we cannot afford to be lulled into a false sense of security.

Much of the reason why events of this kind are met with alarm is not because the probability of impact is high, but rather because the consequences of such an improbable impact are so extraordinarily dire. Aware of the fate of those great reptilian beasts who roamed and ruled this planet before us, we know too well that an impact may lead to our extinction – an abrupt end to the anthropocene. Efforts, informed by data from objects like TB145, are underway to craft protocols that deal with the possibility of an asteroid impact. From nuclear detonation to a more understated, trajectory-altering nudge, plans are being developed to save earth from catastrophe. For the mad man, in all his crazed rambling, speaks in the spirit of a threat that, while improbable, is real: we simply cannot afford to find ourselves in the headlights of a car that has veered off the road.

These close encounters then, are a reminder from the cosmos that we cannot afford to neglect our space program, if we wish for our species to survive. They nudge us forward into a space-faring age, in which the repository of human potential will not be concentrated on a single planet. Perhaps most importantly, they provide us with a potent reminder of the vulnerability of our terrestrial home and allow us to contemplate our perilous existence upon it. As each asteroid streaks past earth, the message sent to us is clear: our cosmic neighbourhood, while alluring, is hostile to human life. With each near miss, our resolve to preserve this planet should be bolstered. We must realise that these threats of an existential kind needn’t come from the heavens.

Mesmerised by profit, we continue to deplete earth’s resources. Recklessly, we fuel a runaway greenhouse effect that drives climate change and global warming. Nuclear and biological weapons continue to sit in our stockpiles, straining for release. For as long as these characteristics of the human age continue to exist, the destruction of our species is in no need of any help from a rogue asteroid. So the next time the excessive panic machine begins to rumble, remember that after the threat has disappeared into the void, a danger lingers to our continued existence on this planet. And it springs directly from the sins of our species.