Asteroid anxiety: Astronomy’s 300-year quest to predict cosmic collisions

On February 18, NASA announced the recently discovered asteroid, 2024 YR4, has a 3.1% chance of striking Earth in 2032. We can sleep a little easier, though, because the following day this was revised down to 1.5%. And the odds were then slashed again down to a reassuring 0.004%, though the chances of the asteroid hitting the Moon in 2032 stand at 1.7%, per NASA’s statement on February 24.

However, fears of celestial collisions — and calculations of their likelihood — are far from new. They go back to the very origins of modern science itself. In fact, in many ways, they provided early proof of the vast promise of the scientific method, in an age before it was clear scientists really could predict the future.

The birth of cosmic risk calculations

Back in the early 1700s, science was in its early infancy. The word “scientist” didn’t yet even exist. Scientific inquiry hadn’t yet undeniably proved its power to accurately predict, let alone to reinvent and transform, our world. To many, it instead seemed a trifling pursuit: fiddling with microscopes; little better than alchemy. 

Reflecting this, in 1726’s Gulliver’s Travels, the Irish litterateur Jonathan Swift satirized early scientists as buffoons. In particular, he lampooned them for pompously proclaiming they had predicted the paths of “ninety-three different Comets” with “great Exactness.” Swift depicted them as hapless neurotics: too distracted from daily jobs and joys by abstract theories, too quick to spook themselves with flimsy projections, having become confident the world would be destroyed by a cometary collision “one-and-thirty years hence,” which would reduce Earth “to ashes.” 

Swift was taking aim at the scientists clustered around London’s Royal Society, the first scientific society of its kind, which included Newton among its ranks. In Swift’s crosshairs was Newton’s colleague and friend, Edmond Halley. In 1705, Halley decided to apply Newton’s new-fangled mathematical method of calculus to compute comets’ paths. Having passed close by Earth in 1682, he forecast that the one that now bears his name — Halley’s comet — would return in 1758. That is, precisely “one-and-thirty” years after Swift’s sarcastic attack in 1726’s Gulliver’s Travels.

This was the first prediction of its kind. Back then, this was a bold application of an unproven method. When the comet did return, exactly as predicted, Halley was proven right, and Swift was proven wrong. This was an early triumph: proving science isn’t idle tinkering, nor fearmongering cozenage because it can accurately forecast the future at cosmic scales.

Among themselves, Royal Society members had already made murmurs about the possibility of comets interfering with Earth. Halley himself calculated that had his comet arrived 31 days later, it would have spelled doom. The idea soon spread. Seven years before Halley’s comet returned, the French polymath Pierre Louis Maupertuis was likewise warning a direct collision could decimate our planet “from top to bottom.”

When, in 1770, another comet was observed very close to Earth — which remains, to this day, the closest encounter on record — astronomers felt the need to begin calculating the probability of a deadly collision. Like Newton’s calculus, probability theory was then a method of mathematics in its relative youth, experiencing spurts of innovation and expansions of application.

The Finnish-Swedish mathematician who computed the comet of 1770’s path, Anders Johann Lexell, himself reassuringly stated that “the probability of such an event is almost infinitely small.” Others, like the French philosopher Jean-Baptiste-Claude Delisle de Sales, concurred, writing (more poetically) that in “the calculation” of such probabilities, “terror, by dint of spreading thin, disappears.”

But the first to put actual numbers on the odds was the French astronomer Joseph Jérôme Lefrançois de Lalande. In a small pamphlet, written in 1773, he considered the possibility of a direct “shock,” before similarly dismissing it as “infinitely” unlikely. Assuming a comet’s path intersects Earth’s orbit at some arbitrary point, he calculated a 1/76,000 chance of a direct collision, given the relative smallness of both objects compared to the size of their orbit.

But Lalande saw a more probable threat: that a comet passes near enough to exert gravitational influence on Earth’s oceans, raising juggernaut tidal waves — several thousand meters tall — sweeping the circumference of the planet with destruction and death. For this, given similar assumptions of intersecting orbits, Lalande saw a 1/8,000 chance of disaster. But to reassure, he nodded to “mortality tables” on Earth. Even though thousands of people, he claimed, die every hour, we do not worry hourly of our own death, because there are so many people.

So too with, in Lalande’s words, “the enormous masses which roll over our heads”: there are fathoms more orbits possible than those directly intersecting our own. Accordingly, Lalande computed the probability of such close encounters is yet lower. Given available knowledge, guesstimating there are 300 roaming rocks in our Solar System, he proposed a 1/64,000 annual chance of a flyby proximate enough to raise deadly tidal waves.

It is interesting that, back then, Lalande thought this risk tolerable: not enough to enter “the moral order of hopes or fears.” But then again, in the 1700s scientists didn’t yet grasp how old the Earth really was or how long it might yet host life. Many still believed Earth’s biography could be bookended by a span of thousands, or tens of thousands, of years.

Lalande’s technique was rudimentary, but it was an important start. It is very likely the first time anyone properly applied probability theory to the question of existential risk or threats of human extinction. Unfortunately, he was misunderstood. Rumors percolated, eventually snowballing into panic in Paris, claiming Lalande had declared that a comet would definitely strike Earth within a fortnight. Remote possibility was mistaken for impending inevitability. Lalande was forced to clarify his position in national newspapers. 

Then, as now, communicating risk had its difficulties. After Lalande’s lead, another French astronomer provided more detailed calculations, arriving at a likelihood of 1/752,730 per year. These calculations impressed Pierre-Simon Laplace, another pioneer of probability theory and one of France’s most prominent scientists, who — reflecting on a direct collision — remarked that “the small probability of this circumstance may, by accumulating during a long succession of ages, become very great.” Luridly, Laplace depicted planet-spanning desecration, imagining civilization collapsing as a result. 

Quoting Laplace, journalists — proving Swift’s sarcastic suspicion had yet to be fully overcome — facetiously quipped that “the probability of such a disaster is daily increasing.” “Here, then, is a very rational end of the world,” they smirked: one prophesied by elliptics and perihelions, not raptures or revelations. Many still dismissed the scientists.

By 1810, the German astronomer Wilhelm Olbers — who theorized comets were the fragments of an exploded planet, once occupying the yawning Mars-Jupiter gap — decided to put a number on Laplace’s “long succession.” Again deploying probability, he output 4,000,000 years between close approaches and 220,000,000 between collisions. Magazines — again facetiously — declared: “fortunately, we have time to prepare.” 

The year after Olbers’s forecast, a new comet appeared in night skies. Olbers predicted it would become extremely bright. He was proven right. It was visible for months, trailing a luminous tail 1,000,000,000 miles long.

In the 1830s, trepidation set in again when it was realized Biela’s comet — set to hurtle by in 1832 — does intersect with Earth’s orbit. Olbers again put probability to work, calculating a very “close approach” — perhaps even encounter — will occur roughly every 2,500 years. When it did pass, all was fine, but John Herschel, Britain’s leading astronomer, remarked it had been a close shave. Had the object arrived days earlier, he claimed, it could have struck us.

Writing a few years afterward, the Russian prince and poet Vladimir Odoevsky penned a short story imagining a future encounter between Biela’s comet and Earth. Notably, he cast it in the year 4338 AD. At the date of writing, this was 2,500 years hence. Clearly, Odoevsky was inspired by Olbers. He depicted a future utopian Earth — replete with imaginative and world-transforming innovations — threatened by incoming impact. 

Having already deployed geoengineering to tame the planet’s weather, humankind is confident it can act together to repel the risk. Odoevsky imagined scientists meticulously modelling the comet’s trajectory and speeds, and governments rallying to launch “projectiles” to deflect the incoming intruder. Unfortunately, Odoevsky never finished writing his story, so we don’t know if he imagined humanity succeeding. Given the triumphant scientific tone of his fragmentary tale, it appears likely he foresaw our species prevailing.

Clearly, by now — that is, 1835 — science had done enough to prove itself in the eyes of the litterateurs. From the steam engine to the development of fizzy drinks, things had come a long way in the century since Jonathon Swift — that is, from a time when authors eyeballed scientific inquiry as a hobbyhorsical punchline, to one where they lauded it as a potential world-saver.

In the decades since, various other panics came and went, as our Solar System’s roaming rocks returned and departed to and from Earth’s skies. But, by the opening of the 1900s, new discoveries and developments, such as the invention of radiometric dating, proved our Earth was, indeed, billions of years old and had likely hosted life — uninterrupted — for that period. This made many scientists confident that large catastrophes hadn’t befallen our planet in the past, and, thus, were also unlikely to occur in the future.

As the geologist Kirtley F. Mather put it in 1940: The “lurid pictures of a sudden catastrophic debacle resulting from collision” with some “comet” are “products of a vivid imagination wholly without foundation in astronomic fact or theory.”

Although appetites for visions of collision and calamity remained healthy beyond, the prospect of cometary catastrophe became derided in the annals of serious science. The prospect became actively derided after 1950, with the publication of Immanuel Velikovsky’s Worlds in Collision. Luridly, Velikovsky conjured and confected various ancient impacts, conscripting them as explanations for world history’s major movements. It became wildly popular, provoking venom from the scientific establishment. With some notable exceptions, collisions were no longer a serious consideration.

The asteroid wake-up call

This quickly changed in the 1980s when evidence began building — before eventually becoming undeniable — that the dinosaurs had been toppled, 65 million years ago, by a planet-quaking impact. Catastrophe, abruptly, was back on the menu.

Then, in 1994, Comet Shoemaker-Levy slammed into Jupiter. This garnered the first direct witness of a collision of bodies within our Solar System. Providing dramatic visuals, the timing couldn’t have been better. Combined with prior discoveries about the dinosaurs’ fate, it provoked people to do something. Having been founded two years previously, NASA’s Spaceguard initiative, and other international efforts, gained momentum and funding. 

Odoevsky’s hope had come true, two millennia earlier than he had dared dream. Shifting from satire in the 1700s to science fiction in the 1800s, using scientific prediction for planetary defense became reality at the end of the 1900s.

Since then, as of April 2024, scientists — using the techniques Newton and Halley pioneered over three centuries ago — have tracked around 37,000 near-Earth objects, including Comet 2024 YR4.  Today’s efforts to survey our skies, scouring the cosmic neighborhood for roaming risks, remain a monument to science’s early and enduring success: as a means to predict the far future and mitigate the deadly risks it contains. 

It is an undeniable triumph. Through the cooperative action and clever calculus of humans, our planet — hurtling through space — has developed something like an immune system. Perhaps, moving forward, we might develop antibodies for the other perils jeopardizing our shared world.