"Saving Earth from a Killer Asteroid" By Jessica Metzger
Asteroids have posed a real existential threat as long as life has existed on Earth. Malchus’ Chronicle of Paros names a “thunderstone” that fell in Crete in 1478 B.C.E., and 14th century Chinese records describe a fatal “iron rain.” We’ve become more aware of this threat in recent decades, when in the 1980s the 200-km crater from the K-T impactor—the one that wiped out the dinosaurs—was discovered, and then in the early 90s we witnessed the comet Shoemaker-Levy 9 hit Jupiter. The background hysteria grew, and by 1998, Armageddon and Deep Impact were both released and Congress gave a mandate (but, sadly, no funds) to discover 90% of the biggest Near Earth Objects (NEOs). It’s terrifying to know that we – a species making enormous strides in fields like artificial intelligence, genetic engineering, and theoretical physics – may be helpless against these random events. So, naturally, there has been recent development in techniques to divert a killer asteroid. But which techniques are the most effective, and in which cases?
NEOs are thought to originate mostly from the asteroid belt, where interactions with Jupiter inject them into the inner solar system. Observations constrain that asteroid impact frequency should be inversely related to size, so that the most devastating impacts are the least common. For reference, impacts like the Chelyabinsk event, a ~20-m asteroid airburst captured on Russian dashcams that left no fatalities, happen about every 200 years. ~50-m asteroids like the Tunguska impact, which leveled 2000 km2 of Siberian forest in 1908 and will leave fatalities if over a populated area, occur about every 2000 years. Species-ending K-T impactor scenarios (~10-km across, 65 million years ago) happen every 100 million years. Many recent efforts have been focused on tracking NEOs, e.g. the optical Catalina Sky Survey which discovers and tracks asteroids, and the radar Arecibo & Goldstone observatories, which determine asteroids’ physical properties. Future promising efforts include the highly-sensitive Large Synoptic Survey Telescope (LSST), and hopefully a space-based telescope.
So, let’s suppose we’ve determined that an asteroid will impact Earth within the next few years or decades. How do we mitigate the damage? The National Academies report puts strategies into four main categories: civil defense, slow push/pull, kinetic impact, and nuclear detonation. The civil defense strategy (e.g. evacuation and sheltering), although it won’t save us from the worst events, is the one we are currently best equipped for. Not only is it the most cost-effective option for even a Tunguska-like event, it may be the best strategy for worse events which we haven’t detected in time—the Chelyabinsk meteor came straight from the direction of the sun and wasn’t previously observable; this may happen for a larger asteroid.
The most controllable deflection strategy is the “push/pull” strategy, where a small acceleration is imparted onto the asteroid for a long period of time to cause gradual changes to its orbit. Examples include gathering sunlight to the surface to increase vaporization of surface materials, leading to a conservation-of-momentum shift away from the sun; painting the surface white to increase the pressure from solar photons; and sending a “gravitational tractor” spacecraft to pull it using the mass of the spacecraft (or, sending an actual tractor, to physically pull it). However, these strategies require both decades of advance knowledge of the threat and a previous close approach. They also only apply to asteroids up to about 100-m in diameter.
Defense strategies for bigger threats include kinetic impacts – hitting it with high-velocity spacecraft – and nuclear detonations. The kinetic impact method has actually been tested before, in NASA’s 2006 Deep Impact mission (although in Deep Impact, the movie, they used the nuclear strategy). It’s relatively simple – scientists would send a spacecraft as massive as possible at as high a velocity as possible for a head-on collision with the asteroid, resulting in a conservation-of-momentum velocity change. Effective for NEOs up to about 1 km (and already tested), this strategy may be possible with as little as 1-2 years of warning time and is thus one of our current best options for moderately-sized NEOs.
The final type of strategy, the nuclear strategy, is both the only option for the most massive NEOs and last-minute scenarios, and the most mass efficient in general. Rather than vaporizing the entire body as the movies depict, the best use of this method would be causing an explosion some distance from the surface of the body, which would heat up and accelerate away some of the object’s surface material, imparting a conservation-of-momentum change in velocity. While nuclear detonations themselves have been extensively studied and simulated, it would take a while to design the necessary payload, and this is one of the main aspects of asteroid defense that needs to be worked on. In addition, using nuclear detonations to divert 10 km or larger NEOs requires energies that haven’t been tested, and thus should be researched.
We almost had to start putting these methods to the test when the ~340-m asteroid 99942 Apophis (like the Egyptian god of chaos) was found to potentially collide with earth in 2029, 2036, or later. While recent radar observations have ruled out the possibility of a collision for a while; due to uncertainties in Apophis’ future orbital evolution, there’s no ruling out collisions after, say, 2060. In particular, when it makes a close approach in 2029 (closer than the moon), it may enter a “gravitational keyhole,” perturbing its orbit in an uncertain way that may lead to a future collision. What will we do if this happens?
Since Apophis is greater than 340 m in diameter, slow push/pull methods likely won’t be effective enough, and we’ll have to use a kinetic impact and/or a nuclear detonation. A recent study found that using a kinetic impactor to deflect an Apophis-size asteroid would require almost a decade of warning time, which may not be the case. However, as is usually the case, the researchers found that a nuclear detonation would be effective with only a few years’ warning time, and thus would be optimal for deflecting Apophis.
Our potential ability to deflect a killer asteroid, like all our major technological strides, is a testament to our abilities to change the world (well, the solar system) for better or for worse. What if, as pointed out by Carl Sagan in Pale Blue Dot, one were to use one of these strategies not to divert an asteroid, but to send it towards Earth? One of the main takeaways from this research is the enormous effort, planning, and funds that must be poured into diverting an asteroid, especially one large enough to decimate the entire world. It would truly be a global endeavor. An organization with the capacity to use this power maliciously could much more easily achieve their goals through other means. And, while the average yearly fatalities due to asteroids is much lower than those due to, say, HIV/AIDS or tobacco, a species-ending K-T impactor-like event is almost certain to happen sometime in the future, and it’s important to at least have a plan to prevent it.
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 National Academy of Sciences. 2010. “Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies: Final Report.” Washington, DC: The National Academies Press.
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