Crash warning comes after high alert when space rock the size of a football pitch passed by

Nasa says around 15,000 mid-sized space rocks are unaccounted for. Photo / Adobe Stock

Astronomers do not know where thousands of “city-killer” asteroids are, Nasa’s head of planetary defence has admitted.

Dr Kelly Fast, who leads the agency’s attempts to track near-Earth objects like asteroids and comets, said around 15,000 mid-sized space rocks are unaccounted for.

These rocks, known as “city-killers”, are at least 140m in diameter and if one were to hit a populated area on Earth could “really cause regional damage”, Fast said at the American Association for the Advancement of Science annual conference in Phoenix, Arizona.

Dr Nancy Chabot, a planetary scientist at Johns Hopkins University, also warned that the planet would have no way of defending itself should one of these rocks appear to be crashing towards Earth.

Chabot led the Double Asteroid Redirection Test (Dart) mission, which deliberately crashed a specially made spacecraft into a mini-moon called Dimorphos at 14,000mph (22,530km/h) to alter its orbit.

The mission was a success and hailed as a way to protect Earth, however no other craft currently exists that could be used in a future mission.

Astronomers were put on high alert when it appeared a space rock the size of a football pitch could crash into Earth. The asteroid YR4 passed on Christmas Day, 2024, but was only seen a week later.

Analysis of its trajectory indicated it was moving away from Earth at around 61,155km/h but its orbit could put it on a collision course in 2032.

The calculated risk of collision reached around 4% before further analysis revealed the rock would miss Earth in seven years’ time.

However, there is still a chance it could hit the Moon, and studies suggested this collision would be so intense it would be visible from Earth with the naked eye.

“Dart was a great demonstration. But we don’t have [another] sitting around ready to go if there was a threat that we needed to use it for,” Chabot said.

“If something like YR4 had been headed towards the Earth, we would not have any way to go and deflect it actively right now.

“We could be prepared for this threat. And I don’t see that investment being made.”

Large asteroids, those bigger than 1km across, like the one that wiped out the dinosaurs more than 60 million years ago, are easy to spot in the sky, Fast said.

She warned that the mid-size variety - big enough to cause serious problems but small enough to avoid detection - are the main concern.

When asked what keeps her up at night, she replied: “It’s really the asteroids that we don’t know about. We’re not so much worried about the large ones from the movies, because we know where they are.

“And small stuff is hitting us all the time. [I’m] not so much worried about that. It’s the ones in-between that could do regional damage. Maybe not global consequences, but they could really cause damage.

“And we don’t know where they all are. It’s not something that even with the best telescope in the world you could find.”

Nasa is building a new space telescope to survey the night sky, to help it meet the target of identifying and tracking more than 90%of near-Earth objects larger than 140m in diameter.

But Fast warned that Nasa is “only 40%” to this goal with around 15,000 objects unaccounted for.

The Near-Earth Object Surveyor mission is expected to launch next year with the capability of detecting both bright and dark asteroids, which are the most difficult to find.

“We’re searching skies to find asteroids before they find us, and get them before they get us,” Fast concluded.

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