The chances of an asteroid hitting Earth has people on edge, but they now sit close to zero. An astrophysicist says even when impact occurs, it’s not necessarily catastrophic –– and we now have ways to avoid it entirely.
In recent weeks, the chances of a so-called city-killer asteroid, 2024 YR4, hitting Earth in 2032 seemed to be more and more likely. However, NASA, the European Space Agency and others have since reduced those odds to less than 1%.
However, the chance of impact is not yet 0%, and given how much fluctuation there was in the estimates from various space agencies, it raises the question: Are we safe? Or, do you still need to worry about YR4?
Jacqueline McCleary, an assistant professor of physics at Northeastern University, says it’s probably safe to breathe a sigh of relief. Agencies like NASA are constantly tracking and predicting the path of asteroids, and the rise in YR4’s impact probability, which at one point reached a 1 in 31 chance of striking Earth, is a normal part of the data collection process.
Asteroids are dark objects that don’t reflect a lot of light, making it difficult to track them until they’re relatively close. However, once they are within range, scientists can start collecting data, tracing their orbits and predicting their probability of intersecting Earth’s orbit. They can also start trying to account for the many small, unexpected factors that might impact an asteroid’s orbit, like another asteroid, or even the gravitational forces of Jupiter.
It’s an incredibly complicated and, thus, incredibly approximate calculation,” McCleary says. “It’s not quite like licking your finger and sticking it into the wind –– it’s more scientific than that –– but obviously the more data you get, the more observations you get, the better path can be traced out and thus the better the estimate becomes.”
Estimated to be between 130 and 300 feet across, YR4 is not particularly large as far as asteroids go. However, as various space agencies started to track its path, it became clear that it would be coming quite close to Earth. If it struck Earth at the right angle and in the right location, it would prove catastrophic.
“The reason that people care about is that if you take the large end of the size and a reasonable guess for the density and if it fell straight down and impacted a city, it would detonate with the equivalent of 400 Hiroshimas and Nagasakis,” McCleary says.
Even if YR4 had a higher chance of impacting Earth, McCleary says not every asteroid impact is the kind of cataclysmic event that occurred with Chicxulub, the asteroid connected to the mass extinction of the dinosaurs.
Asteroids don’t always hit the planet straight on with the force of hundreds of nuclear explosions. Sometimes, asteroids arrive on what McCleary calls a “grazing trajectory,” breaking up or even exploding in Earth’s atmosphere.
In 1908, an asteroid of about the same size as YR4 exploded over an East Siberian forest in what is now called the Tunguska event. There was no crater, just a massive explosion that flattened about 830 square miles of forest. More recently, in 2013, an asteroid about half the size of YR4 exploded over Chelyabinsk Oblast in Russia. It resulted in 112 people being hospitalized, mostly from glass that was shattered by the shockwave.
McCleary says YR4’s earlier estimated trajectory was more in line with these events, except it was more likely to cross over an ocean, minimizing the potential damage. Should the odds of it impacting Earth change again, she says it’s unlikely to have a catastrophic impact.
“If it were me, I would not be super worried about it because the odds of it actually flattening a city, which it could if it were behaving like a bomb, are significantly lower than the odds of it exploding in midair and breaking up over the ocean,” McCleary says.
YR4 might not be the next asteroid to hit Earth, but NASA and other space agencies are preparing a means to defend the planet from unwanted interstellar travelers. In 2022, NASA tested its Double Asteroid Redirection Test, a first-of-its-kind method that involved shooting a spacecraft at a two-asteroid system that would impact and alter their orbit.
DART was a success and now provides a model for how to avoid near-Earth orbiting asteroids that could pose a serious threat to the planet. Should YR4’s orbit change for any number of reasons, a forced redirection like DART “would probably be our best option,” McCleary says.
“People sometimes talk about nuking these things, but then what you might do is just end up creating a bunch of smaller pieces that might still do a lot of damage,” McCleary says. “Nudging it into a slightly larger orbit seems to me like the safer course of action.”