On Thursday afternoon, while you were fretting about how well President Joe Biden might perform in his debate with Donald Trump, a “planet-killer” asteroid roughly the size of Mount Everest passed within five million miles of Planet Earth. In cosmic terms, that’s really close—about 17 times the distance to the moon. Two days later, on June 29, a “city-killer” asteroid roughly the size of the Great Pyramid of Giza will pass within 180,000 miles of our planet. That’s actually closer than the moon.
I don’t mean to downgrade other threats to the planet, such as climate change, nuclear annihilation, overpopulation, and environmental degradation. We should do everything in our power to address these ills. But it never fails to amaze me how little the government does to address this pesky asteroid problem. NASA’s Planetary Defense Coordination Office, the lead agency for tracking asteroids and comets, wasn’t even created until 2016. It’s budgeted at a paltry $138 million, even less than we spend on the (underfunded) National Endowment for the Arts. I can only repeat what I wrote 18 years ago in Slate: “However many guys we have working on this problem, it can’t possibly be enough.” (I meant women too.)
This solar system of ours is a dangerous place! You want to know how dangerous, look at the moon. Kinda pocky, no? The moon has more than 100,000 craters, all of them created when some asteroid or comet smashed into it. The moon has no atmosphere—therefore no weather—and no large bodies of water. Also, it registers virtually no geologic activity. Its terrain is therefore unchanging (excepting 500,000 pounds of human trash left there since 1969). To look at the moon is to look at its entire history, and that history consists entirely of stuff slamming into it. A mere 239,000 miles away, Planet Earth too dwells inside an interstellar shooting gallery.
Granted, our planet has some advantages the moon does not. Tiny objects that bombard us daily burn up in the atmosphere. Smallish asteroids three feet across strike the earth several times a year; these get burned up too. For this, let’s be thankful.
But how often does bigger stuff penetrate the atmosphere and cause trouble? Frequently enough that most of us remember the asteroid the size of a sperm whale that exploded 19 miles over the Russian city of Chelyabinsk in 2013. Nobody saw it coming. The resulting shock wave smashed windows all over Chelyabinsk and five neighboring cities and injured about 1,500 people. Episodes like this are said to happen every 60 years or so. If you want to calculate the likely impact of an asteroid hitting the Earth, based on its size, density, and speed, click here.
Planet killers are the biggest worry, obviously, followed by city killers. One way you can tell the science of asteroid risk is young is that it has not yet developed bureaucratic-sounding euphemisms. Planet killers are exactly what they sound like. Sixty-five million years ago, a planet-killer asteroid a little bigger than Mount Everest crashed into the Gulf of Mexico near Mexico’s Yucatán peninsula, creating the Chicxulub crater. Seventy percent of all living species (plants and animals) were wiped out, including the dinosaurs. The impact on human beings was zero because we had the good sense not to exist (and would not until 300,000 years ago). Within the past million years, according to James Garvin, chief scientist at NASA’s Goddard Space Flight Center, four half-mile objects slammed into Nicaragua, Bolivia, Ghana, and Kazakhstan. We don’t know the consequences, exactly, but Garvin said, “It would be in the range of serious crap happening.” Oscar Fuentes-Muñoz from the University of Colorado, Boulder, says we’re probably good for 1,000 years, based on NASA’s cataloging of the 1,000 planet killers that we know are out there. Scientists believe these represent 95 percent of all the planet killers in the general vicinity. Thursday’s planet-killer near-miss, for instance, has been anticipated since 2011.
City killers are more of a problem. There are people still living (not a lot) who were alive at the time of the last one on June 30, 1908, when an asteroid the size of a 13-story building exploded six miles above the Podkamennaya Tunguska River in a remote corner of Siberia. The force of the explosion knocked over or set fire to trees for miles around and killed scores of reindeer. Fortunately, only about 30 people (mostly reindeer-herders) lived near the blast, of whom a reported three were killed. Word of the collision traveled slowly; our information about the event is sketchy because two decades passed before scientists appeared on the scene. What’s certain is that, had this been a populated area—as many more places on earth are, 116 years later—the death toll would have been catastrophic.
Scientists have been able to locate only about 40 percent of the 30,000 or so city-killer asteroids they believe to reside in earth’s vicinity. The European Space Agency keeps a list of 1,624 city killers that pose a “non-zero” risk of colliding with the Earth over the next century. But keep in mind that city killers often take astrophysicists by surprise; the one passing near Earth June 29 was first spotted only last year.
The good news is that, in 2022, NASA slammed a spacecraft into a 581-foot asteroid called Dimorphos and managed to change its orbital pattern. Dimorphos was big enough to be a city killer, but it wasn’t on a trajectory to collide with Earth. The mission was a test to see whether NASA could alter a more dangerous asteroid’s trajectory. It turns out that it can. But again, NASA can do this only if it knows there’s a city killer out there, and 60 percent of the time it doesn’t.
To spread global awareness of the asteroid threat, the United Nations has declared June 30 to be International Asteroid Day, in memory of the Tunguska incident. Please take a moment on Sunday to reflect that it is not crazy to urge the U.S. government, and other governments around the world, to give asteroids more attention—before they give us more attention.
