“We would rather not know,” are words rarely uttered in science. Unless, of course, the question is something like, “What would happen if an asteroid hit Earth?” In the case of an asteroid big enough to cause harm, not to mention an earthquake, a tidal wave and/or the end of life as we know it, we’d very much like to avoid discovering the answer to that particular question first-hand.
Never finding out what would happen if an asteroid hits Earth is the primary goal of planetary defense, the pursuit of which involves locating, tracking and even moving asteroids out of Earth’s planetary flightpath.
Close Encounters of the Rocky Kind
The first step toward avoiding the answer to, “What would happen if an asteroid hit Earth?” is asking, “Where are the asteroids that might hit Earth?” and then, “Which of the ones headed our way do we need to worry about?”
The answer to this second question is easier to lock down. Literal tons of small rocks and interstellar dust fall onto our planet’s atmosphere every day. These smaller objects burn up in the blink of an eye. Most asteroids fall in that pebble-to-baseball-to-motorcycle size range. If the space rock in question is more metal or carbon than silica — less like a rock and more like a cannonball or a lump of coal — the remaining hunk of space debris might touch down on the ground or the ocean. More than 22,000 of these objects have been collected from the desert floor in Antarctica.
Rocky asteroids that drift up to the 100-foot (30-meter) diameter class usually aren’t a major problem, which is fortunate. Current thinking is that Earth encounters an asteroid of this size at least every five years, possibly as often as a year. On the flip side, an asteroid of this size or even smaller need not actually set foot on the planet to leave an awfully big footprint.
If the asteroid happens to be stony and several stories tall, instead of making landfall, it might break up in the atmosphere. The ensuing explosion can generate an abrupt change in air pressure, temperature and speed known more generally as a shockwave. The shockwave generated by the midair break-up of a stony 50-60 foot (17-20 meter) asteroid may have taken place outside of Chelyabinsk, Russia in 2013. The area under the shockwave was flattened by a multi-megaton energy blast moving faster than the speed of sound that injured more than a thousand people.
If an asteroid hits Earth, or even just comes close, a recent study projects that shockwaves and wind are the most concerning hazards to human life, followed by heat and tsunamis. When the asteroid is bigger than 150 meters wide, as it was during the Tunguska event in 1908, the shockwaves can have as much force as a major volcanic explosion and can be felt on the other side of the world.
Asteroids on the scale of the one that ended the Mesozoic Era — roughly the size of Manhattan — are exceedingly rare but can create enormous craters and kick up enough dust to change the planet’s climate.
We’ve learned a lot from the levels of destruction in Chelyabinsk, Tunguska and at the end of the Mesozoic Era. According to projections, an asteroid the size of a skyscraper could destroy modern cities or even small countries. On the plus side, according to the latest sky surveys, no known 140-meter asteroids will be headed our way for at least 100 years. However, that is only no known asteroids. Computer models of the population of 140-meter asteroids indicate that we’ve only discovered a fraction of such asteroids to date. As many as 60% of the 25,000 objects in that size range have yet to be pinpointed.
That’s not for lack of trying on the part of the asteroid-hunting community. They’re looking with some of the best tools on the planet — and off of it. In orbit, the NEOWISE mission has been scanning the entire sky in the infrared, looking for anything with a heat signature.
Meanwhile, two missions on Earth are responsible for 90% of the Near Earth Objects discovered recently, including asteroids, comets and interplanetary dust. The first is the Panoramic Survey Telescope and Rapid Response System (PanSTARRS) on Haleakala, Hawaii and the second is the Catalina Sky Survey on Mt. Lemmon, Arizona. Further asteroid tracking missions have been planned for 2022, with the goal of identifying up to 85% of 140-meter-diameter Earth-crossing asteroids.
Even with the best telescopic eyes and the brightest scientific minds on the problem, a cool, dark object moving in the chilly near-void of space isn’t an easy thing to find. With the current known asteroid count at more than 1.1 million, and millions more are unaccounted for, rather than waiting to be caught off-guard, the planetary defense community is researching what it would take to move a literal space mountain should one head our way.
Aiming for a Bullseye
NASA’s recently launched Double Asteroid Redirection Mission (DART) is headed for a binary asteroid system 11 million kilometers away from Earth. One asteroid in the system is 160 meters across. It orbits around a much larger 780-meter asteroid. These two objects are known collectively as the Didymos binary. The mission plan involves using the DART spacecraft as a kinetic impactor on the smaller of the two asteroids.
The aim of playing a cosmic game of darts with a binary asteroid system is to put one more quiver in the arrow of planetary defense. Hurling a spacecraft into a 160-meter wide space rock will allow us to measure how much such an impact changes the orbit of this kind of asteroid: the kind, it so happens, we’re most concerned about.
If we can deflect a Near Earth Object releasing the energy equivalent of 185 Hiroshima-sized nuclear bombs with one small, well-aimed punch moving at 15,000 mph (24,140 kph), we’re that much closer to a functional planetary defense system. And that much further away from questions we only want answered in computer simulations, physics homework assignments and science fiction.
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