How to Defend the Planet
A six-mile-wide asteroid ended the dinosaurs, and the same could happen to us one day. But NASA is at work on ways to keep us safe.
Asteroids, supervolcanoes, nuclear war, climate change, engineered viruses, artificial intelligence, and even aliens — the end may be closer than you think. For the next two weeks, OneZero will be featuring essays drawn from editor Bryan Walsh’s forthcoming book End Times: A Brief Guide to the End of the World, which hits shelves on August 27 and is available for pre-order now, as well as pieces by other experts in the burgeoning field of existential risk. But we’re not helpless. It’s up to us to postpone the apocalypse.
News that a “city-killer”-scale asteroid came within 45,000 miles of the Earth on July 25 — and that astronomers didn’t see it until the object had already passed by the planet — is a timely reminder that the universe can be a hostile place. But unlike a number of other natural existential risks, we have the ability to protect ourselves from such near-Earth objects (NEOs) — at least, theoretically.
The way to stop an NEO is to deflect it, though that word is deceptive. Rather than trying to knock an asteroid to the side, we would try to either slow down or accelerate the asteroid along its given orbital path. An NEO impact occurs when an asteroid and the Earth intersect while traveling along their separate orbital paths. Asteroid experts compare the process to cars merging on a highway. To avoid a collision, one driver has to speed up or slow down. There’s no speeding up or slowing down the Earth, so we have to alter the velocity of the asteroid, ensuring that it arrives either too late or too early for its appointment with our planet.
One option is to take advantage of gravity. Here’s a quick high school physics refresher: all objects with mass or energy — planets, asteroids, even light itself — are attracted toward each other through the force of gravity. If a large object — called a gravity tractor in this case — could be placed in space near an incoming asteroid, its gravitational attraction could be just enough to slightly tug the NEO’s orbital path away from an intersection with the Earth.
If the fate of the Earth is at stake, we’d have to opt for a more direct application of Newtonian physics.
We could also use what is known as the Yarkovsky effect. Just like the Earth, asteroids rotate as they journey along their orbits, which means each half of an asteroid has a day and a night that alternate as the object spins. When the warmer daylight side of the asteroid rotates to face away from the sun, it releases infrared photons that carry a bit of momentum from the asteroid, acting like a minuscule rocket thrust. That’s the Yarkovsky effect.
As anyone who has worn a black T-shirt on a sunny day knows, dark colors absorb light. Paler colors, by contrast, reflect it. By painting one side of the asteroid — perhaps by using paintballs made of dry powder with an electrical charge, which could theoretically survive the vacuum of space — we could use the Yarkovsky effect to tweak the asteroid’s speed. A similar method would involve employing a laser to burn away one surface of the asteroid; as it ejected the vaporized rock and metal, the asteroid would be pushed ever so slightly in the opposite direction.
Though each of these methods would create only tiny changes in the orbital path of an NEO, if we act decades before it is predicted to hit the Earth, those tiny adjustments could accumulate over the years to ensure that the asteroid would miss our planet. But we may not have decades of warning, and if the fate of the Earth is at stake, we’d have to opt for a more direct application of Newtonian physics.
An object in motion — like an asteroid — stays in motion with the same speed and the same direction unless acted upon by an unbalanced force. We could bring that unbalanced force to bear on an asteroid by ramming an unmanned spacecraft called an impactor into it. Newton’s second law — force equals mass times acceleration — would do the rest, slowing down or speeding up the NEO. If we know how large an asteroid is and how fast it is traveling, we should be able to figure out how large and how fast our impactor needs to be.
NASA knows this method can work, because they’ve tried it — though not exactly on purpose. In 2005, the Deep Impact spacecraft rendezvoused with the comet Tempel 1, some 266 million miles from Earth. Upon arrival, Deep Impact — which, for the record, was not named after the 1998 Elijah Wood comet disaster film — released an 820-pound impactor that rammed into the comet at about 23,000 mph, delivering a jolt of force equivalent to 4.8 tons of TNT. Given that the comet was nearly four miles across while the impactor was the size of a washing machine, there was no measurable deflection to speak of — that’s Newtonian physics for you — but the collision did leave a measurable crater, and gave NASA at least an outline of how a kinetic impactor could work on a smaller asteroid, or with a bigger impactor.
The more force we can deliver to an NEO, the more we can alter its orbit — and for better or for worse, there is nothing in the human arsenal more forceful than a nuclear weapon. If we needed to deflect a large asteroid, or one that was already close to Earth — so the change in the NEO’s orbit would need to be more extreme — nukes would likely be our only alternative. Erika Nesvold, an astrophysicist formerly with the Carnegie Institution for Science, devised an algorithm called Deflector Selector that simulated 18 million attempts to prevent an asteroid impact. She concluded that a nuclear option was the right call for as many as half of the simulations. “It’s not all that surprising,” she told me. “This is a physics problem, and nukes have the most energy.”
What we wouldn’t do is simply fire a bunch of intercontinental ballistic missiles at the asteroid and hope to blow it to smithereens, as if we were playing a game of Missile Command with existential stakes. It may sound counterintuitive, but you don’t want to blow up an asteroid if you’re trying to defend the Earth, as there’s no telling where the resulting debris might hit. One 2019 computer model study even found that if an impactor did break up an asteroid on a collision course, the space rock would eventually pull itself back together. As with other deflection techniques, the aim is to speed up or slow down the asteroid along its orbital path. Nuclear weapons just provide extra oomph.
One method would be to explode a nuclear device several hundred feet away from the asteroid. Space is a vacuum, so there is no air to carry the destructive force of a blast shockwave as on Earth, but the high-energy gamma rays, X-rays, and neutrons released by the detonation would hit the nearby surface of the asteroid and vaporize part of it, creating plasma that ejects particles back into space and so thrusts the asteroid in the opposite direction.
Detonating a nuclear warhead below the surface of an asteroid, rather than on it, could increase the explosive power by as much as twentyfold.
Hopefully nothing gets blown up — especially the Earth. “This isn’t about sending Bruce Willis to the asteroid with a bomb,” Ian Carnelli, a program manager at the European Space Agency who works on asteroid surveillance and defense, told me.
About that. You can’t discuss asteroid defense — even among PhD-holding astrophysicists — without Bruce Willis and Armageddon coming up sooner or later. On the one hand Armageddon — and the somewhat more scientifically sound Deep Impact — introduced audiences to the existential threats posed by NEOs in cinematic fashion, and proved that we weren’t helpless to stop them. On the other hand, certain licenses were taken with the science.
In Armageddon, the killer asteroid is described as being “the size of Texas,” reportedly because Michael Bay didn’t think that audiences would believe than an NEO six or seven miles across would possibly be big enough to wipe out the human race. (It would be.) A group of scientists at the University of Leicester in Britain calculated that the bomb Willis and his crew planted after drilling into the asteroid would have needed at least 50 billion megatons of kinetic energy in order to blow apart a Texas-sized NEO. For the sake of comparison, that’s a billion times more powerful than the biggest nuclear bomb ever built, the Soviet Union’s Tsar Bomba.
NASA also wouldn’t send astronauts — let alone a team of untrained oil rig drillers — to intercept an incoming asteroid. Any mission would be unmanned. But it is possible, as a last-ditch effort should a large NEO be discovered with little time to spare, that NASA might take a page from the Michael Bay playbook and try to plant a nuclear bomb inside an asteroid. The agency has studied using a Hypervelocity Asteroid Intercept Vehicle, a theoretical spacecraft that would crash into an oncoming asteroid at high speeds, burrowing several feet deep into the object, before setting off a nuclear device. Detonating a nuclear warhead below the surface of an asteroid, rather than on it, could increase the explosive power by as much as twentyfold. It’s worth noting that one of the reasons the U.S. National Nuclear Safety Administration gave for not dismantling America’s largest atomic warheads after the Cold War was the possibility that they might be required for planetary defense, which would bring a new meaning to the term “nuclear umbrella.”