Sky Unseen
The Threat of Near-Earth Asteroids
Nothing, it seems, is more vast and empty than the endless expanse of space. Our nearest celestial neighbor, the Moon, resides a distance away that would take more than 5 months to traverse at highway speeds driving non-stop, and it’s on our front doorstep compared to the distance of the other planets just in our Solar System. And between the iconic planets and moons of the Solar System? Nothing, but a blanket of darkness… or so it would seem. In reality, empty space is not so empty. Though most of the Solar System’s matter has spiraled into the powerful gravity wells of its proverbial cosmic monsters, an incomprehensible number of minor objects remain unscathed in the space between them — pristine remnants of the violent past of the early Solar System. Most are small lumps of rock, metal, ice, or a combination of the three, orbiting the Sun in a region known as the asteroid belt. Unlike the planets, however, their paths around the sun are wild and erratic, constantly being tweaked and warped by the presence of their massive neighbors. As a result, the Earth is constantly careening through an interplanetary dust storm, only the grains of dust are as large as buildings, and their velocities are clocked in kilometers per second.
In 1908, a single event redefined the way that we see empty space when a meteor smaller than the inside of a high school track flared through the atmosphere and exploded over a sparsely populated region in central Russia. The air burst flattened 80 million trees across an area 3 times the current size of New York City, devastating all wildlife within the blast radius in a fury of wind, fire, and flying debris. Astonishingly, only a few accounts of human death were reported in the impact that would become known as the Tunguska Event. A similar event would happen in Russia over a century later in the city of Chelyabinsk — this time, an air burst that was powerful enough to shatter windows, topple brick walls, and knock people off their feet. Remarkably, though thousands reported injuries, no one was killed in this event. Asteroids such as either of these are statistically only expected to impact Earth on the order of a few times per millennium, but statistics isn’t meant to predict the future, only to tabulate the past. What, then, is the true threat of near-Earth asteroids today?
A near-Earth asteroid is defined as an asteroid that passes within 1.3 AU of the Sun, or at most, 30% farther than the Earth orbits the Sun. These objects pose the greatest threat to our planet because they have the highest likelihood of having orbits which may already, or may eventually, cross ours, given gravitational perturbations. In total, it is estimated that there are around half-a-billion near-Earth asteroids larger than a meter in diameter; about the size of a kitchen table. But rocks such as these pose little threat to life on Earth. Asteroids smaller than a meter across hit the Earth on the order of thousands of times per day, and produce nothing more than streaks of light across a night sky — a far cry from a sign of danger. But around 30 meters or so (a little larger than the Chelyabinsk meteor, or about the size of a 10-story apartment building), a falling meteor makes the transition from a spectacle, to a threat. Rocks such as these are capable of dangerous air bursts that could flatten entire towns or city centers in the blink of an eye. When the impactor obtains a diameter of 140 meters, it is granted enough momentum to tear all the way through Earth’s protective atmosphere, relinquishing its energy not into the air, but directly into the surface of the Earth itself: a crater.
NASA’s Center for Near-Earth Object Studies (CNEOS) only classifies objects greater than 140 meters, large enough to cause direct impacts, as “potentially hazardous”. Using this metric, NASA has done a fair job detecting and tracking potentially hazardous asteroids, plotting the orbits of over 9,000 of the estimated 40,000 or so near-Earth objects which meet this size threshold. But as was just previously brought to light, objects such as the ones that caused the Tunguska and Chelyabinsk air bursts were much smaller than this. On the small, Chelyabinsk-sized end of things, there could nearly a million potentially hazardous near-Earth asteroids, with CNEOS having mapped less than 18,000 of them. Though CNEOS have determined the orbits of those 18,000 asteroids well enough to know that none are likely to impact within the next century, there are still likely 50 times more similar asteroids that haven’t even been observed. Concluding that an asteroid impact isn’t likely because none of the 2% of asteroids we’ve discovered are on an impact course would be like reaching into a 2 ounce bag of Skittles, pulling out one red Skittle, and assuming the entire package must be red Skittles.
Despite an asteroid as large as a 10-story building sounding impossible to miss, the truth is that finding and tracking these illusive “city killers” is no cake walk. Asteroids and other minor solar system bodies can’t have their diameters measured by direct observation because they are too small and distant from the Earth. Instead, their sizes must be determined by how much light they reflect, known as their absolute magnitude. Down to about 1 kilometer, most asteroids can be identified and continuously tracked without a problem. But asteroids smaller than this reflect so little light many are never found, and even those that are have a chance of being lost before their orbits can be determined. In the last century alone, the Earth has had close encounters with 10 Chelyabinsk-or-larger sized meteors which passed within the orbit of the Moon that our instruments never even detected until after they passed. Half of these were large enough to cause an impact crater, while one, an asteroid measuring over a kilometer in diameter, would have impacted Earth with the explosive energy of 10,000 Tunguska events. It silently drifted past the Earth in 1918, just 0.9 Lunar distances away.
Something which makes asteroid detection even more difficult is the presence of the Sun, which can saturate a detection telescope’s camera beyond the point of seeing minor Solar System bodies. Because of this effect, all but a tiny handful of near-Earth asteroids have been discovered on the “night side” of Earth and, in fact, 87% of asteroids are discovered in a 45 degree cone facing away from the Sun. While most asteroids have trajectories that take them outside of Earth’s orbit for us to detect, a known group of asteroids dubbed Atiras orbit the Sun entirely within Earth’s orbit. Though Atiras do not currently cross Earth’s path, such asteroids may yet be of consequential threat to Earth due to gravitational encounters from Venus or Mercury. To date, we have only discovered 22 Atira asteroids, with no estimate as to how many more there could be in total. However, since such asteroids are always in the direction of the Sun, we may only be able to find and track exceptionally bright ones, while smaller, dimmer Atiras could slip past the radar and sneak up completely undetected.
NASA uses the Torino scale, which ranks asteroids from 0 to 10, to classify possible impact threats. Currently, all known asteroids are a level 0 on the Torino scale, but this wasn’t always the case. In 2004, the 370-meter asteroid 99942 Apophis made headlines when it briefly obtained a Torino scale rating of 4, boasting an impact probability of 2.7%. Its ranking was subsequently downgraded to 0 after its trajectory was better tabulated, but this event reminded people that impact threats remain an ever-present facet of our lives on Earth. Despite this scare and others however, neither NASA nor any national government or space agency has a working contingency plan for what happens if a true threat is realized. In the meantime, our vast network of asteroid hunting satellites continue to locate and catalog new potential threats, each discovery only adding a drop in the bucket compared to the hundreds of thousands of cosmic bullets possibly taking aim at our planet.
Two weeks ago, an enormous fireball soared over rural western Australia, captured on camera by several bystanders who watched in awe as the space rock ignited a trail of bright green fire in its wake. Just like the Chelyabinsk meteor, the rock went undetected by our dozens of asteroid hunting telescopes and instruments before impact. Though this meteor posed no threat to life, it just goes to show how little of the sky we really see, even with our eyes wide open. Let this serve as a humbling reminder of how much we are at the mercy of nature, and how little we still know about the final frontier. Though statistics has been generous to our species since our inception, humanity needs to better educate itself on the silent, often forgotten threats from above: asteroids.
Happy #AsteroidDay! And if you ever see a meteor or bolide like the one above, report it on the American Meteor Society’s website. Scientists use that data to extract knowledge on the number and type of meteors that hit Earth every year!
