For years, astronomers have been engaged in a concerted effort to find potentially hazardous asteroids that could hit us with a wallop big enough to seriously impact civilization.
Happily, they think they’ve found the vast majority of them. But that doesn’t mean they’ve found them all – not to mention smaller ones still big enough to seriously wreck your day – or even your entire country’s day.
We know they’re out there. In 2013, the Siberian city of Chelyabinsk was shaken by an 18-meter space rock that exploded overhead with the force of a 440-kiloton bomb, injuring more than 1,600 people and blowing out windows all over the city. In 1908 a larger object – estimated by NASA at about 40m – exploded over a different part of Siberia, Tunguska, leveling more than 2,100 square kilometers of largely uninhabited forest.
One place more such rocks might be hiding, says Quanzhi Ye of the University of Maryland, is the Taurid swarm.
The swarm is part of the Taurid complex, a dust and debris system that peppers the Earth with shooting stars every June and November. It’s not a particularly active meteor shower in terms of total numbers of shooting stars, Ye says, “but it’s particularly rich in larger particles that are good at producing fireballs or very bright meteors.”
The swarm is a dense part of this system, held together by Jupiter’s gravity. That’s important he says, because the entire complex is thought to be the product of an asteroid smashup large enough that it might have produced some sizeable pieces of rubble. It’s possible that if the Earth passes through the wrong part of the swarm, it might be hit not by one sizeable chunk but by “a coherent impact” – i.e., shotgun blast – of multiple big rocks, all at once.
To assess the danger, Ye’s team used a 1.2-meter wide-field telescope on Mt. Palomar at a time when the densest part of the swarm was relatively close to the Earth. The goal was determine how many 100-meter-class asteroids might be there.
The telescope couldn’t see the entire swarm, but in the part it could see, Ye reported today at a meeting of the American Astronomical Society’s Division for Planetary Sciences (DPS), the count was zero. That doesn’t mean there are none at all in the rest of the swarm, but it does make it likely that there aren’t a lot.
Unfortunately, his team wasn’t able to look for smaller, Tunguska- or Chelyabinsk-sized asteroids. “Smaller asteroids are harder to find because they’re fainter,” he says. “And when they’re bright enough, that means they are closer to the Earth [and] will move much faster [across the sky].” But he says, the swarm will be close to Earth again next year and in 2026, giving his team two opportunities to figure out how to solve this problem.
Another scientist looking for ways to assess the risk from smaller asteroids is Elizabeth Silber of Sandia National Laboratory in Albuquerque, New Mexico. But instead of using telescopes, her team is listening for them with microphones. Specifically, infrasound sensors, designed to detect bursts of sound well below the limits of human hearing.
Ultrasound bursts, she said at the DPS meeting, can be used not just to count fireballs plunging into the Earth’s atmosphere, but to determine the power of their impacts. And because ultrasound travels a long way, it’s possible to do this across the entire globe, even in remote regions or in bad weather. “Understanding the influx rates of objects ranging from meters to tens of meters in size is crucial for planetary defense,” she says.
Also useful, she says, is that infrasound can be used to understand how the shockwaves from such impacts propagate, important because some spread out along the path of the incoming asteroid, while others are more focused, right in front of it. “We are interested in how that shock is distributed,” Silber says. She didn’t mention it, but that information is of more than scientific interest; it could also be useful to emergency crews mobilizing to respond after a Chelyabinsk-type event.
Ye adds that it’s also becoming increasingly possible for telescopes to see imminent impactors before they hit – even if they are moving so fast across the image that they are nothing but blurry smears. (Especially, he might have added, if astronomers know where the most likely ones might be coming from, as might be the case with the Taurid swarm.)
That makes it possible to issue warnings – even if only to advise people to keep away from windows. “That’s the number one reason why Chelyabinsk caused so many injuries,” he says. “Not because [people] were hit by asteroid fragments. It’s because they were right next to windows.”
