Why planetary defense efforts are looking for ‘city killer’ asteroids

This week, astronomers announced the discovery of three previously unknown near-Earth asteroids that had been hiding in the glare of the Sun — and one of them is a giant. At nearly a mile (1.5 kilometers) across, it is big enough to cause planet-wide destruction if it hit the Earth, though it won’t come anywhere close enough to be a threat for a hundred years or so. 

The search for potentially hazardous asteroids like this one has ramped up in recent years, with multiple ongoing efforts to identify and catalog large space rocks passing close by. From twilight observations from the ground to space-based telescopes peering into hard-to-see regions of the sky, these programs are focused on identifying potentially dangerous asteroids as early as possible. The goal is to make sure that we have the chance to avoid catastrophe by sending an intervention like the recent DART mission, which managed to change the orbit of an asteroid for the first time. But most of these initiatives aren’t expecting to find many more mile-wide asteroids. Instead, experts say that, currently, the biggest unknown threat comes from medium size asteroids, which they call “city killers.”

That’s because there are probably only a few really big near-Earth asteroids that remain undiscovered. “In total, there are about 1,000 asteroids one kilometer or larger in size near the Earth, of which 95 percent have been found,” lead author of the paper detailing the discovery, Scott Sheppard of the Carnegie Institution for Science, explains to The Verge. These large asteroids are known as planet killers because of the complete destruction they could wreak if one impacted Earth. There are likely only between 10 and 30 more of these to find, most of which will stay near the Sun.

“The likelihood of a big asteroid hitting us is very, very low,” Sheppard says. 

The issue is the slightly smaller asteroids, between 140 meters and one kilometer in size, which, if they impacted the planet, wouldn’t disrupt the entire Earth but could cause widespread devastation. “What we call the city killers are the ones which are most dangerous now,” Sheppard says. 

Even though they aren’t so huge, an impact from one of these could be regionally catastrophic. “An object that’s about in that 100- to 200-meter size range is really capable of inflicting a lot of damage,” Amy Mainzer, leader of the upcoming NEO Surveyor project that will search for near-Earth objects, tells The Verge. “That is equivalent to somewhere around a couple hundred megatons of TNT. And that’s quite larger than the largest nuclear weapon that has ever been detonated and hopefully ever will.” The effects of such an impact would depend on where it hits, particularly if it impacts on land or in the sea or in a rural area versus a densely populated metropolis. But in the worst case, such an impact could wipe out a region the size of the greater Los Angeles area.

These smaller asteroids are also harder to observe because they’re fainter. They typically orbit the Sun every three to five years, so most of the time, they are too far away to be visible, and we only get occasional opportunities to view them. 

Asteroids often have highly elliptical orbits, meaning they are sometimes closer to the Sun and sometimes much further away. So to work out the orbit of an asteroid, it isn’t enough to observe it just once — you need three observations, each spaced at least 24 hours apart, to be able to predict the path it will take. Astronomers will often call up colleagues working with other telescopes to help get these crucial observations before an asteroid becomes too faint to see.

A recently discovered trio of asteroids was spotted between the orbits of Earth and Venus, where asteroids are particularly difficult to observe due to the immense glare of the Sun. As well as the large asteroid, named 2022 HP7, the researchers also spotted asteroids 2021 LJ4 and 2021 PH27, which will stay safely away from Earth’s orbit. Typically, asteroid hunters look away from the Sun, but to look for these inner solar system asteroids, the researchers have to make use of a 10-minute window at twilight that allows them to point their telescopes toward the Sun. There’s a brighter background, which makes it harder to spot objects of the same size than when looking away from the Sun.

“It’s like when you go out at night and there’s no Moon,” Sheppard says. You can see many more stars when there’s no Moon because when the Moon is out, its glow obscures the fainter stars. Something similar happens when trying to spot faint asteroids in the glare of the Sun, which is why these big asteroids have remained hidden until now.

There are also space-based missions looking for near-Earth asteroids like the NEOWISE project and the upcoming NEO Surveyor. These have the advantage of not having to deal with interference from Earth’s atmosphere and being able to detect fainter objects, but they are also massively more expensive than using ground-based telescopes. 

NEO Surveyor, tentatively set for launch in 2027, will look in infrared wavelengths to spot asteroids by their heat signatures instead of relying on visible light observations. That’s important because around one-third of asteroids have low reflectivity and a dark color that makes them difficult to see in visible light, but dark and light asteroids are equally visible in infrared. It will also be able to observe objects in different areas of the sky from ground-based telescopes, looking at regions nearer the Sun that would be obscured by the Earth’s atmosphere from the surface. 

This mission is expected to find around 300,000 near-Earth asteroids of all different sizes over its five-year mission. “We think it’ll expand what we know about the population by about a factor of 10,” says Mainzer. As for whether any of these will be potentially threatening, Mainzer says, “We might find nothing, which would be great! That would be the best possible case, that we find absolutely nothing heading our way.” In that case, the mission could still help us learn a great deal of scientific information about the solar system and its formation.

Other ground-based projects like the upcoming Vera C. Rubin Observatory will also offer a huge boost in our ability to hunt for these midsize asteroids by performing a survey that covers the whole southern sky every few nights with a much more powerful telescope than what is available now.

Sheppard isn’t worried about the Earth being destroyed anytime soon, but he does think we ought to be ready in case an asteroid could one day pose a danger. “I don’t think it’s an immediate threat, but you want to be prepared,” he said. “Right now, there’s nothing known that’s even a slight danger to the Earth. We’ve found most of the planet killers, so now, it’s looking for the city killers.”