Astronomers have completed a comprehensive examination of large asteroids that zoom near our planet, determining that Earth probably won't be struck by such an object for at least 1,000 years.
You may not want to breathe a sigh of relief just yet, however: The newly completed catalog looks at near-Earth objects (NEOs) at least 0.6 miles (1 kilometer) wide and therefore doesn't rule out strikes from smaller, yet still potentially dangerous, asteroids.
The team behind the new research looked at the positions and orbits of known NEOs, as cataloged by the NASA-funded Minor Planet Center at the Smithsonian Astrophysical Observatory (SAO).
"Our focus was on the objects larger than 1 kilometer, and we tried to rank them in terms of their associated risk of impact over 1,000 years, which is a longer time span than we usually analyze," study co-author Davide Farnocchia, an astrophysicist at NASA's Jet Propulsion Laboratory (JPL) in Southern California, told Space.com.
Scientists around the world — such as those at NASA's Center for Near-Earth Object Studies (CNEOS) — have been tracking NEOs for years, with the chief goal of flagging any that may pose a threat to human civilization. After all, we don't want to go the way of the (non-avian) dinosaurs, which were wiped out, along with around three-quarters of Earth's species, after the planet was struck by a roughly 6-mile-wide (10 km) asteroid around 66 million years ago.
NASA estimates that it has thus far identified around 95% of NEOs with a diameter of at least 3,300 feet (1 km) with orbits that carry them to within 30 million miles (50 million km) of Earth. Calculating the probability that any of these rocks may strike our planet over the next century by tracking their orbits, CNEOS maintains the Sentry Risk Table, which ranks the riskiest objects around Earth.
"Thanks to the Sentry impact monitoring system, we already know that none of the large NEOs that have been discovered so far has any risk of impacting Earth over the next 100 years," Farnocchia said. "We also know that none of the smaller objects already discovered is of concern in terms of impact hazard over the next 100 years."
Farnocchia and the team wanted to expand our knowledge of potential asteroid impacts far beyond the century time limit, however. They aimed to obtain similar information for the next 1,000 years for some of the larger bodies around Earth.
"We focused on objects of the 0.62 miles, or 3,300 feet (1 kilometer), range and larger because they are the ones that have the potential to cause global-scale damage if they were to reach the Earth," he said.
Why size matters when it comes to asteroid tracking
The Global Challenges Foundation estimates that asteroids more than 0.62 miles (1 km) wide are capable of inflicting the kind of global-scale damage that could not only disrupt human civilization but also, potentially, wipe out our species altogether.
During a hypothetical asteroid impact exercise conducted in April, NASA scientists led by the manager of NASA's Near Earth Object (NEO) Program Office, Paul Chodas, estimated that such an impact could release around 100,000 megatons of energy, equivalent to 6.6 million Hiroshima nuclear detonations, each one of which packed the explosive energy of 15,000 tons of TNT.
Fortunately, larger asteroid impacts are rare, with the estimated time between strikes decreasing as the diameter of those asteroids increases. In April, NASA researchers said that asteroids with diameters of at least 3,300 feet (1 km) are estimated to hit the planet around once every 700,000 years. Larger, even more, catastrophic strikes involving 3-mile-wide (5 km) asteroids are predicted to occur just once every 30 million years.
"Impact of a 1-kilometer asteroid would probably cause significant cooling on Earth due to debris ejected in and above the atmosphere blocking sunlight, subsequent die-off of vegetation and crop failure, which then leads to mass starvation of mammals and famine," Farnocchia said.
"There is no known object that will impact and cause this for 1,000 years, though there are a few we should continue to keep an eye on, how their orbits could evolve," he added.
Farnocchia and the team obtained this improved prediction by using a new approach to modeling asteroid orbits that honed in on just the points in those orbits that bring the space rocks close to Earth, thus allowing them to predict impact risks much further into the future.
"For a significant fraction of these over-3,300-feet (1 km) objects, we were able to see that, even over long time scales, their orbital ellipse or path was sufficiently far from that of the Earth," Farnocchia said. "Therefore, even if the position along the orbit becomes necessarily uncertain over time, we know that the asteroid cannot reach the Earth."
Which asteroid poses the biggest threat to Earth in the next 1,000 years?
Based on the team's investigation, the large object that presents the most risk of impacting Earth in the next 1,000 years is asteroid 7482 (1994 PC1).
This stony asteroid, which has an estimated diameter of around 3,600 feet (1.1 km) and passed within 1.2 million miles (1.9 million km) of Earth in 2022, has around a 0.0151% chance of coming within one Earth-moon distance over the next millennium, the team calculated. This makes it around 10 times more likely to impact as the next riskiest in the category, 20236 (1998 BZ7), which has a 0.001% probability of coming closer to us than the moon. (The moon orbits, on average, about 239,000 miles, or 384,600 km, from Earth.)
The team's results shouldn't be considered a complete "all clear" for Earth, however. The new study doesn't cover smaller objects, some of which could still pack quite a punch.
During the hypothetical asteroid strike exercise led by Chodas and other NASA scientists, it was said that even an asteroid about 1,000 feet (300 m) in diameter could cause devastation on a continental scale, with as much as 2,000 megatons of energy being released during impact. That's 133,000 times the estimated energy released by the bomb that devastated Hiroshima.
A strike by an asteroid just 2,000 feet (600 m) in diameter would approach the scale of a global catastrophe, releasing as much as 20,000 megatons of energy — meaning a doubling in asteroid diameter translates to a roughly tenfold increase in impact energy.
"For other objects, the orbital ellipse can be close to that of the Earth, and so we cannot yet completely rule out a collision over the next 1,000 years," Farnocchia said. "Even so, a collision from such large objects remains extremely unlikely, and our results can be useful to prioritize the large objects that will need to be more closely tracked in the future."
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Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.