Asteroid strike: should you be worried?
66 million years ago, a new star appeared in the Earth’s sky. Deceptively serene, it mingled amongst the constellations that watched over the young planet’s inhabitants as its pearly light slowly but steadily grew brighter. Not that these inhabitants, including the dinosaurs, necessarily noticed; in an idyllic paradise much warmer than today’s world, covered in forests and oceans, there were decidedly more interesting things for a dinosaur to be doing than stargazing.
But one day, hauntingly juxtaposed against the peaceful song of crickets, the deadly star slammed into our planet, unleashing the power of ten billion atomic bombs like a white-hot angel of death. We will never know what went through the last ever dinosaurs’ minds. Some may have stood, baffled, gazing at the fireball, while others ran; still others may have tried to protect or comfort their young. What we do know is that every dinosaur — be it the mighty T-Rex or the herbivorous Triceratops — as well as many other species all ceased to exist.
This will happen again; perhaps not today, or even in this millennium, but it will happen. An asteroid the size of the above — 10 km in diameter — is estimated to hit Earth only once every 100 million years, so we should be safe for another 34 million or so. Asteroids in the 30–140 meter range strike our planet once every 100 years, and countless smaller space rocks hit every year which are mostly vaporized by the atmosphere. Stephen Hawking, in his book Brief Answers to the Big Questions, describes an asteroid strike as one of the biggest threats that humanity faces; this is because while the probabilities of a significant impact are relatively low, the fact remains that it takes just one big asteroid to wipe us all out. And as of right now, there’s not much we can do about it.
Asteroids are defined as minor planets, ranging from one meter to 1000 km in diameter; there are about a million of them that we know about. They are remnants of the early solar system, and are kind of like failed planets. 4.5 billion years ago, the solar system was nothing but a disk of spinning dust and gas. Gravity caused 99% of that material to form our sun in the center, and the rest of this disk — known as a protoplanetary disk — eventually clumped and collided together to form planets. But not all of it did; Jupiter’s colossal gravity, for example, prevented this clumping from happening in its vicinity, and the material stayed dispersed in what we know today as the asteroid belt. Most asteroids in the solar system are located there, with others — called Trojan asteroids — orbiting on the same plane as a planet, such as Jupiter. These stay in gravitationally stable areas known as Lagrange points by means of gravitational interactions between the sun and Jupiter.
Asteroids are pretty much everywhere in the solar system — even near Earth. Known as Near Earth Asteroids, over 27,000 of them have been identified, while only 2,224 are large and close enough to pose something of a threat (though odds of impact are still very low). These only stay in orbit for a few million years, but new ones keep coming in thanks at least in part to Jupiter; though the giant protects our planet by redirecting objects from outside the solar system away from us, it also sends us the odd asteroid to make up for it. Asteroids are joined by meteoroids, which are defined as objects smaller than one meter — but the term can also be applied to anything creating a meteor, also known as a shooting star (the term meteorite refers to the object once it reaches Earth’s surface).
There is an inverse relationship between the size of an object and how often Earth gets bombarded by objects of that size; around 25 million meteors enter the atmosphere every day, the vast majority of which are small and fragile enough to burn up during their fall (as explained here, meteors smack into the atmosphere at thousands of kilometers per hour; at a minimum, they would be traveling at Earth’s escape velocity of 11km/s, because like walking up and down a hill, the energy involved in both leaving and entering Earth’s gravity is the same. These speeds are fine in space, but not so great in the air, as it compresses when hit by the meteor and heats up, causing the meteor to glow and eventually vaporize in the atmosphere). Meteor showers exemplify this exact effect; they often consist of icy remnants of a comet and rarely reach the Earth, giving us a spectacular light show. It is estimated that only 17 objects a day survive atmospheric entry and hit the ground; but, according to Gonzalo Tancredi, an astronomer at the University of the Republic in Montevideo, Uruguay, only around 0.13% of the world’s total surface area is occupied by humans, making the chances of these meteors being seen, let alone hitting someone, very low.
Aside from the velocity and size of the object, its density and makeup — whether it is rocky and porous or metallic and dense, for example — and its angle entering the atmosphere determine its fate and the energy it releases. A stony, porous asteroid, being less dense, allows for more of the hot air to fill its cracks and break it apart from the inside out; this explosion releases an airburst, which creates shockwaves and intense heat. In 2013, an asteroid 20 meters in diameter entered the atmosphere over Chelyabinsk, Russia, creating an airburst 30 times as powerful as the Hiroshima bomb; 7,200 buildings were damaged by the shockwave, and many people were injured by the shattering of windows. Something like that happens about every 60 years. The largest impact event in recorded history — although the object disintegrated in the air — was the 1908 Tunguska event, which involved a meteor up to 60 meters in diameter. The air burst was 60 times more powerful than Chelyabinsk, flattened 80 million trees, and killed three people in rural Siberia; if it had hit a large city, it would have wiped it off the map completely.
If that’s harrowing, remember that this asteroid was tiny by comparison; it didn’t even reach the ground in one piece. But as Gerrit L. Verschuur, an astrophysicist and radio astronomer at Rhodes College, explains, if an object over one kilometer in size hurtles our way, we’re in trouble; if it is bigger than five, we’re done for. Such an asteroid would survive reentry and strike the surface of the Earth, killing anyone close enough to see it. Ejecta — materials flung into the air by the impact — would scatter across the world and rain back down in a shower of fire; the sky would quite literally be falling. This would then set everything else aflame. Resulting smoke and ashes would envelop the Earth in a blanket of shadow, blocking out sunlight for up to a year depending on the size of the asteroid. Lack of sunlight and dropping temperatures leads to the death of plants and later many animals. Chemical reactions between sulfur from the asteroid and nitrogen oxides created by the heat make acid rain that will kill any poor soul that somehow managed to survive all of this. To finish it all off, the debris, chemicals, and carbon dioxide arising from the impact mess up the ozone so badly that it won’t be the same for centuries to come.
It’s safe to say that the end of civilization as we know it is something to try and avoid, and a good first step is knowing what we’re dealing with. Many programs surveying the skies for asteroids exist; for example, NASA’s Near Earth Program sponsors projects and telescopes around to world specifically for tracking Near Earth Objects (NEOs), some of which have been around since 1942. And while the US currently is responsible for discovering 98% of the 1,500 new NEOs found yearly, China has plans for further tracking as well as planetary defense against asteroids.
Planetary defense could, someday, be what literally saves the world. With enough forewarning, most NEOs could be intercepted by a spacecraft that interferes with it in some way and stops it from hitting Earth; however, asteroids are less dangerous and more predictable than comets, which are erratic and could leave us with only a few months of preparation. The importance and urgency of planetary defense is clear, but as of now, no tried and tested method of asteroid deflection exists, although that’s about to change.
On September 26th, NASA’s DART mission (Double Asteroid Redirection Test) will crash itself into the 170-meter moonlet Dimorphus, which orbits the asteroid Didymus. The expected change in velocity will only be 0.4mm/s, but will result in a larger cumulative effect over time, changing its trajectory. Though categorized as potentially hazardous, this asteroid system poses no actual risk to Earth; the mission’s objectives are merely to test if such a deflection technique would work.
Another way to defend the planet would be to nuke the asteroid, like in the movie Armageddon; here, a nuclear weapon would be placed in the asteroid, in the hopes that the fragments either miss Earth completely, or end up small enough to burn up in the atmosphere. However, this method may pose problems as it is hard to predict the behavior of the fragments, and whether their gravity will cause them to re-clump and happily continue on their trip to wipe out life on Earth.
Yet another method is known as a gravity tractor: a spacecraft that influences the gravitational field around the object, either by hovering near it or orbiting it. Though the gravitational effects will be minimal, they could be just enough for the asteroid to change its trajectory slightly. If executed from far enough away, such a tiny change in trajectory could be all that is needed to avoid Earth.
At the moment, only a few asteroids are known to have even a slight chance of impacting Earth, including 99942 Apophis and Bennu. The 340-meter-wide Apophis scared the living daylights out of everyone in 2004 when it was thought it had a 2.7% chance of impact in 2029. Now, though, it is estimated to pass us at a distance of about 32,000 km — that’s closer than some satellites. Bennu measures in at a little bigger — half a kilometer in diameter — and has a one-in-2,700 chance of impact between now and 2300.
Those odds are comforting; what’s a little less comforting is how many more of these could be lurking around out there that we don’t know about.
In 2005, Congress ordered NASA to identify at least 90% of the estimated 25,000 NEOs bigger than 140 meters, and, as of September 2022, the job is less than halfway done. To help mediate this, the Near Earth Object Surveyor telescope, which was proposed in 2005 and was to be launched in 2026, would look for asteroids hidden in the sun’s glare. Without the telescope, this task would take another thirty years; with it, ten. But when it came to hashing out NASA’s budget for 2023, the space agency itself requested ‘just $39.9 million compared to the $174.2 million NASA projected last year would be needed in FY2023’, leading to a delayed launch. Congress, however, had none of it, finally providing NASA with $94.9 million and telling them to get their act together for a sooner launch.
The mission had been turned down for launch a previous five times to date in favor of other projects; this time, as detailed here, NASA’s reasoning is vague at best, citing growing costs and other priorities for the dramatic slashing of the budget. It is a strange move by NASA; unlike most of their missions (which, while scientifically vital and otherwise valuable on many levels, do not make their benefits to humanity so obviously clear), a plan to track potential civilization-enders should be a no-brainer when it comes to securing the generally self-serving congressional funding. And in most cases, NASA asks for more money than they get, not the other way around. One theory is that they were hoping the money gets allocated somewhere else, but even experts find the move baffling.
But this should not really come as a surprise. The coping mechanism of kicking something down the road to deal with later is something we see in everyone from ourselves to lawmakers; look no further than climate change for proof. Right now, apart from the preliminary DART test, the planet has no physical means of deflecting an asteroid — none. Our planet’s defense strategy consists of monitoring the skies and praying nothing comes our way, like sitting ducks. And while that is a problem in itself, the most glaring issue is that even that plan is now being delayed. In order to deflect something, you have to see it coming; in a way, this out of sight, out of mind approach saves officials from having to deal with a pesky doomsday scenario that actually requires action.
The NEO Surveyor, which arguably has no real profitability whatsoever (apart from, you know, detecting deadly asteroids), is the epitome of boring, preventative action. Its best-case scenario would be detecting nothing newsworthy. Consider the endless amounts of capital involved in the Artemis program and its world-famous flightless rocket; nobody seems to have a problem spending money on that, and it’s just decorating the launch pad. But Artemis, a PR spectacle, is there for moneymaking, and it works. What else could motivate a space agency that put men on the moon in the 1960s, with the world’s foremost technology at its fingertips, to cease its groundbreaking developments in human spaceflight and asteroid detection?
It may eventually come to a point where we discover a deadly asteroid when it’s too late; one could already be on its way. But if asteroids are recognized as a significant threat to life itself, technology to redirect them can be developed and they become a problem easily mitigated with early action; history has shown us that necessity and a touch of desperation often drives innovation (like how a political rivalry landed humanity on the moon), and the thought of our entire civilization being wiped out may just bring that about. Though the scenario seems abstract and bizarre, one day, it will be very, very real. Future generations will depend on today’s innovations in the field of planetary defense for not just their own survival, but the survival of our legacy too.
Impact events have driven the world to where it is today. The moon was formed when a Mars-sized planet collided with Earth; the dinosaurs going extinct led to the rise of mammals and, eventually, us; there is even a theory that the very spark of life was delivered to Earth by means of a comet. But just because impacts have made our world what it is does not mean they will stop. We may even be just another era, another chapter in the book of the history of our planet, like the dinosaurs. Our extinction might lead to the rise of another more advanced civilization doing things we can’t even imagine. Millennia from now, successors may dig up our bones and piece together what we looked like, how we may have lived and died. But we are not helpless. If an asteroid were to threaten us, one thing differentiates us from the dinosaurs: the ability to do something about it.
Originally published at https://notrocketscience.substack.com on September 25, 2022.
